TU BBS — BSc First Year — [40 marks]

1.1 Characteristics of Community and Measurement of Heterogeneity in an Ecosystem

Characteristics of a community

A biological community is an assemblage of populations of different species living together and interacting in a defined area. Its main characteristics are:

1. Species composition — Every community has a definite set of species; some are dominant, some rare.
2. Species diversity — Includes species richness (number of species) and evenness (relative abundance).
3. Dominance — One or a few species exert the greatest influence by virtue of size, number or biomass (e.g., Shorea robusta in a sal forest).
4. Growth form and structure — Communities show vertical stratification (herb, shrub, tree layers) and horizontal pattern (patchiness).
5. Trophic structure — Organisation into producers, consumers and decomposers, with defined food chains and food webs.
6. Ecological niches — Each species occupies a functional role; no two species occupy exactly the same niche in a stable community.
7. Periodicity — Seasonal appearance of species (flowering, migration) and diurnal rhythms.
8. Succession and dynamism — Communities are dynamic; they change over time through ecological succession until a climax.
9. Interdependence — Species interact through competition, predation, parasitism, mutualism and commensalism.
10. Productivity and stability — The community fixes energy (GPP, NPP) and resists or recovers from disturbance (resistance and resilience).

Measurement of heterogeneity in an ecosystem

Heterogeneity means the variety and unevenness of species distribution in space and time. It has two components: species richness and evenness.

(a) Species richness indices

• Margalef's index: $D_{Mg} = \frac{S-1}{\ln N}$ where S = number of species, N = total number of individuals.
• Menhinick's index: $D_{Mn} = \frac{S}{\sqrt{N}}$

(b) Diversity indices (richness + evenness combined)

• Shannon–Wiener index: $H' = -\sum_{i=1}^{S} p_i \ln p_i$ where p_i = proportion of individuals belonging to the i-th species. Higher H′ = greater heterogeneity. Usually ranges 1.5–3.5 for most communities.

• Simpson's index of dominance: $D = \sum p_i^2 \qquad \text{and} \qquad 1-D = \text{Simpson's diversity index}$ A low D (high 1−D) means high heterogeneity.

(c) Evenness indices

• Pielou's evenness: $E = \frac{H'}{H'_{max}} = \frac{H'}{\ln S}$ E ranges from 0 to 1; E close to 1 means species are evenly represented.

(d) Spatial heterogeneity measures

• Beta diversity (Whittaker): $\beta = \frac{S}{\bar{\alpha}} - 1$ comparing species turnover between habitats.
• Patchiness / dispersion indices — variance-to-mean ratio, Morisita's index, and nearest-neighbour methods to describe clumping versus random or uniform distribution.
• Rank–abundance (Whittaker) plots — geometric, log-normal and broken-stick models describe how abundance is apportioned among species.

Interpretation: A community with many species and similar abundances gives high H′, low D and high E — i.e., high heterogeneity. Heterogeneity is measured by sampling with quadrats (plants) or transects and traps (animals), then applying the above indices. High heterogeneity generally means greater stability, more complex food webs and better resource partitioning.

---

1.2 Methods of Measuring Primary and Secondary Productivity in a Terrestrial Ecosystem

Productivity is the rate of energy fixation or biomass accumulation per unit area per unit time (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹).

• Gross Primary Productivity (GPP) = total organic matter fixed by producers.
• Net Primary Productivity (NPP) = GPP − respiration of producers.
• Secondary Productivity = rate of biomass/energy accumulation at consumer levels.

A. Measurement of Primary Productivity

1. Harvest method (destructive method)

• Quadrats of known area are marked in the field.
• Vegetation is clipped at ground level at regular intervals (e.g., monthly).
• Biomass is oven-dried at 65–80 °C to constant weight and weighed.
• Productivity is calculated as the change in biomass between two successive harvests: $\text{NPP} = \frac{\Delta B}{\Delta t} + L$ where ΔB = change in standing crop, L = losses due to death and grazing.

2. Gas-exchange (CO₂) method

• An infrared gas analyser (IRGA) or closed chamber measures CO₂ uptake and release.
• Net photosynthesis (NPP) = CO₂ fixed in light − CO₂ released in dark (respiration).
• GPP = NPP + respiration.

3. Oxygen method (light and dark bottle technique)

• Paired light and dark bottles with water/plant material are incubated.
• Light bottle: net O₂ production; dark bottle: respiration.
• GPP = net O₂ in light bottle + O₂ consumed in dark bottle. Mainly used in aquatic systems but adapted for moist terrestrial microsites.

4. Radioactive tracer method (¹⁴C method)

• ¹⁴CO₂ is supplied to enclosed vegetation; labelled carbon incorporated into organic matter is measured by scintillation counting. Gives a direct estimate of photosynthetic rate.

5. Chlorophyll and remote-sensing methods

• Chlorophyll content is measured spectrophotometrically and converted to productivity using conversion factors.
• Satellite NDVI (Normalised Difference Vegetation Index) estimates large-scale NPP from canopy greenness.

B. Measurement of Secondary Productivity

1. Population dynamics method

• Changes in numbers and mean individual biomass of a consumer population are followed: $P_s = \Delta B + D + E$ where ΔB = biomass change, D = biomass lost as dead matter, E = biomass lost as excreta/egesta.

2. Removal (census) method

• Animals are sampled at intervals (traps, quadrats, netting); biomass per unit area is computed and the increment gives secondary productivity.

3. Feeding/assimilation method

• Food consumed (C), assimilated (A), respired (R), and produced as growth (P) are measured using calorimetry or labelled food: $A = C - F - U, \qquad P = A - R$ where F = faeces, U = urine.

4. Radioisotope method

• Labelled food (³²P, ¹⁴C) is fed and its incorporation into consumer tissue measured to estimate growth and production.

5. Ecological efficiency approach

• Secondary productivity is derived indirectly from NPP using trophic transfer efficiency (≈10%): $P_{n} = P_{n-1} \times \text{efficiency}$

Conclusion: Primary productivity is best measured by harvest and gas-exchange methods, while secondary productivity is measured by population biomass change, feeding experiments and tracer techniques. Combining methods improves accuracy because each has limitations.

---

1.3 Ion-Exchange Reaction in Soils and Effect of Pesticides

Ion-exchange reaction in soils

Soil colloids (clay minerals and humus) carry negative charges on their surfaces. These charges attract and hold cations (exchangeable bases) such as Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺ and Al³⁺. When the soil solution contains other cations, they replace those held on the colloid surface. This reversible replacement is called cation-exchange reaction.

$\text{Soil Colloid}^- \text{Ca}^{2+} + 2\,\text{K}^+ \rightleftharpoons \text{Soil Colloid}^- 2\text{K}^+ + \text{Ca}^{2+}$

Cation Exchange Capacity (CEC) is the total amount of exchangeable cations a soil can hold, expressed in cmol(+)/kg (or meq/100 g). Soils rich in clay and organic matter have high CEC; sandy soils have low CEC.

Laws of cation exchange:

• Exchange is reversible and reaches equilibrium.
• It is stoichiometric — one equivalent replaces one equivalent.
• Cations with higher valency and smaller hydrated size are adsorbed more strongly: Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ > H⁺ (lyotropic series).
• The reaction is rapid and does not require enzymes.

Types of exchange:

• Cation exchange — dominant in most soils; controls nutrient availability and soil fertility.
• Anion exchange — occurs on positively charged sites (e.g., iron and aluminium oxides) at low pH; anions such as H₂PO₄⁻, SO₄²⁻ and Cl⁻ are exchanged.

Importance: Ion exchange retains plant nutrients against leaching, buffers soil pH, determines base saturation and influences soil fertility and fertiliser response.

Effect of pesticides on ion-exchange reactions in soils

1. Change in soil pH: Many pesticides are acidic or basic. Acidic pesticides release H⁺, which replaces Ca²⁺, Mg²⁺ and K⁺ from exchange sites, lowering base saturation and increasing exchangeable Al³⁺.
2. Competition for exchange sites: Cationic pesticides (e.g., paraquat, diquat) are strongly adsorbed on negatively charged clay and humus surfaces, competing with nutrient cations and reducing their availability.
3. Blocking of exchange sites: Adsorbed pesticide molecules occupy exchange positions, reducing CEC available for plant nutrients.
4. Effect on microbial activity: Pesticides may kill soil microbes and earthworms, reducing organic matter decomposition and hence the formation of humus that supplies exchange sites.
5. Complex formation: Some pesticides form complexes with clay minerals and metal ions, altering the charge properties of colloids.
6. Leaching of bases: Displaced Ca²⁺, Mg²⁺ and K⁺ are lost by leaching, causing nutrient deficiency and soil acidification.
7. Long-term effect: Repeated pesticide use may reduce CEC, degrade soil structure and decrease fertility.

Conclusion: Pesticides disturb the natural cation-exchange equilibrium of soil, reduce nutrient retention and may acidify the soil; hence integrated pest management and organic amendments are recommended.

OR — Composition of the Atmosphere and Processes Affecting its Chemistry

Composition of the atmosphere (dry air, by volume):

Water vapour varies from 0–4% by volume. The atmosphere is divided into troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Processes affecting atmospheric chemistry:

1. Photochemical reactions — solar UV radiation breaks O₂ into oxygen atoms, which form ozone (O₃) in the stratosphere; also produces photochemical smog in the troposphere.
2. Photodissociation and photoionisation — high-energy radiation splits molecules and ionises gases in the upper atmosphere, forming the ionosphere.
3. Oxidation and reduction — SO₂ oxidises to SO₃ and then H₂SO₄; NO oxidises to NO₂ and HNO₃, causing acid rain.
4. Combustion — burning of fossil fuels and biomass releases CO₂, CO, NOₓ, SO₂ and soot.
5. Volcanic activity — emits CO₂, SO₂, H₂S, HCl and ash, affecting aerosols and climate.
6. Biological processes — photosynthesis fixes CO₂; respiration and decomposition release CO₂; nitrogen fixation, nitrification and denitrification regulate nitrogen compounds.
7. Atmospheric transport and mixing — winds and convection distribute gases and pollutants globally.
8. Wet and dry deposition — rain, snow and dust remove soluble gases and particles from the atmosphere.
9. Greenhouse effect — CO₂, CH₄, N₂O and water vapour absorb infrared radiation and warm the lower atmosphere.
10. Human activities — industrialisation, vehicular emission, CFCs and aerosols deplete ozone and alter atmospheric composition.

---

1.4 Processes and Factors Affecting Soil Genesis

Soil genesis (pedogenesis) is the process by which soil develops from parent material through the combined action of climate, organisms, relief, parent material and time (Jenny's equation: S = f(Cl, O, R, P, T)).

A. Processes of Soil Genesis

1. Weathering (addition and transformation)

   • Physical weathering: temperature changes, frost action, wind and water break rock into smaller particles.
   • Chemical weathering: hydrolysis, hydration, oxidation, reduction, carbonation and solution decompose minerals and release ions.
   • Biological weathering: roots, microbes, lichens and burrowing animals break down rock and add organic matter.

2. Humification — decomposition of plant and animal residues by microorganisms forms humus, giving soil its dark colour, CEC and fertility.

3. Eluviation — downward movement of fine clay, humus, iron and aluminium oxides from the upper horizon (A horizon) by percolating water, leaving a leached, light-coloured layer.

4. Illuviation — deposition of the eluviated materials in the lower B horizon, making it finer, denser and often reddish or brownish.

5. Translocation — movement of materials within the profile, including leaching of soluble salts, carbonates and bases.

6. Podzolisation — acidic leaching under coniferous forest removes iron and aluminium, leaving a bleached horizon.

7. Laterisation — intense leaching in hot, humid tropics removes silica and bases, leaving iron and aluminium oxides (laterite).

8. Calcification — accumulation of calcium carbonate in the B or C horizon in arid and semi-arid climates.

9. Salinisation and alkalisation — accumulation of soluble salts or sodium on the surface in dry, poorly drained areas.

10. Gleisation — reduction of iron under waterlogged, anaerobic conditions, producing greyish-blue mottled soil.

B. Factors Affecting Soil Genesis

1. Parent material — determines mineral composition, texture, colour and nutrient status of the soil.
2. Climate — temperature and rainfall control the rate and type of weathering, leaching and organic matter accumulation; the most active factor.
3. Organisms (biota) — vegetation, microbes, earthworms and animals add organic matter, mix soil and recycle nutrients.
4. Relief (topography) — slope affects drainage, erosion, deposition and depth of soil; steep slopes give shallow soils.
5. Time — soil formation is a slow process; mature profiles develop only over long periods.
6. Human activities — ploughing, irrigation, fertiliser use, deforestation and urbanisation modify soil properties rapidly.

Conclusion: Soil genesis is the result of weathering, humification, eluviation, illuviation and translocation acting under the control of parent material, climate, organisms, relief and time.

2.1 Major Land-Related Issues in Nepal

Land is Nepal's most vital natural resource, but it faces several serious problems:

1. Land fragmentation: Heavy pressure on limited land and the traditional practice of dividing ancestral property among heirs has created very small, uneconomical holdings (average holding is under 0.7 ha), which discourages mechanization and reduces productivity.

2. Landlessness and unequal distribution: A large section of the population (especially landless peasants, Dalits and squatters) owns no land, while a small group holds large tracts. This inequality is a major cause of poverty and social tension.

3. Soil erosion and land degradation: Steep slopes, weak geology, deforestation, and unscientific farming (slash-and-burn, overgrazing) cause heavy soil loss, landslides, and declining fertility, particularly in the hills and the Chure region.

4. Encroachment of public land: Forests, riverbanks, roadsides, and pasture lands are illegally occupied for settlement and cultivation, reducing common property resources.

5. Unplanned land use: Rapid, haphazard urbanization and settlement expansion convert prime agricultural land into housing and industry, without proper zoning or land-use planning.

6. Landslide and flood damage: Every monsoon, landslides and floods destroy farmland, displace families, and silt up fertile land.

7. Weak land records and dual ownership: Outdated cadastral records, absentee landlordism, and the dual ownership problem in tenancy (especially after the Land Reform Act of 1964) create disputes and insecurity of tenure.

8. Land grabbing and disputes: Political instability and weak enforcement have encouraged land grabbing, illegal plotting, and long-running court cases over land ownership.

---

2.2 How Societal Development Impacts the Environment

Societal development (improvement in living standards, technology, urbanization, and consumption) affects the environment in both positive and negative ways.

Negative impacts:

• Resource depletion: Growing population and consumption increase demand for timber, fuelwood, water, minerals, and fossil fuels, leading to over-exploitation.
• Deforestation and habitat loss: Expansion of settlements, roads, agriculture, and industries clears forests, destroying biodiversity.
• Pollution: Industries, vehicles, and domestic waste release air pollutants (CO, SO₂, NOₓ, particulate matter), water pollutants (sewage, industrial effluents), and solid waste, degrading air, water, and soil quality.
• Greenhouse gas emissions and climate change: Industrialization and transport raise CO₂, CH₄, and N₂O levels, causing global warming, glacier retreat, erratic rainfall, and extreme weather.
• Land degradation: Urbanization and intensive farming cause soil erosion, salinity, and loss of fertility.
• Loss of cultural and natural heritage: Unplanned development damages forests, wetlands, and traditional landscapes.

Positive impacts:

• Environmental awareness and policy: Development raises literacy and awareness, leading to environmental laws, protected areas, and conservation programs.
• Cleaner technologies: Renewable energy (solar, hydro, biogas), electric vehicles, and efficient stoves reduce pollution.
• Better sanitation and health: Improved water supply and sanitation reduce water-borne diseases.
• Conservation institutions: National parks, wildlife reserves, and community forestry emerge from an educated, organized society.

Conclusion: Societal development is a double-edged sword. With sustainable planning, cleaner technology, and strong environmental governance, its negative impacts can be minimized.

---

2.3 Factors Regulating Population Size in Ecosystems

Population size is determined by the balance between natality, mortality, immigration, and emigration, and is regulated by density-dependent and density-independent factors.

1. Natality (birth rate): The number of births per unit time. High natality increases population size. It depends on reproductive rate, sexual maturity, and breeding frequency.

2. Mortality (death rate): The number of deaths per unit time. High mortality reduces population size. It depends on food availability, disease, predation, and age structure.

3. Immigration: Movement of individuals into a population from outside, which increases population size.

4. Emigration: Movement of individuals out of a population, which decreases population size.

Density-dependent factors (effect increases as population density rises):

• Food supply: Limited food causes starvation, reduced reproduction, and death.
• Predation: More prey density attracts more predators, increasing mortality.
• Disease and parasites: Crowding spreads disease rapidly.
• Competition: Intraspecific competition for food, space, and mates intensifies.
• Accumulation of toxic wastes: Metabolic wastes build up in crowded conditions.

Density-independent factors (effect is independent of density):

• Natural disasters: Floods, landslides, fires, droughts, earthquakes.
• Weather and climate: Temperature extremes, heavy rainfall, frost.
• Human activities: Deforestation, pollution, habitat destruction, hunting.

Conclusion: Population size is the net result of births, deaths, immigration, and emigration, modified by density-dependent and density-independent regulating factors that keep populations near the carrying capacity of the ecosystem.

---

2.4 Anthropogenic Activities Affecting the Phosphorus Cycle

Phosphorus is a sedimentary nutrient cycle; it has no gaseous phase and moves mainly through rock–soil–water–organism pathways. Human activities have greatly accelerated its movement.

1. Mining of phosphate rock: Large-scale extraction of phosphate minerals for fertilizers and detergents releases phosphorus from long-term geological storage into the active cycle.

2. Use of chemical fertilizers: Excessive application of phosphate fertilizers (DAP, TSP) on farms adds phosphorus to soil; runoff carries it into rivers and lakes.

3. Detergents and industrial effluents: Phosphate-based detergents and industrial discharges add phosphorus to water bodies.

4. Sewage and animal waste: Untreated domestic sewage and livestock manure are rich in phosphorus and enter water systems.

5. Deforestation and land-use change: Clearing forests and converting land to agriculture increases soil erosion, washing phosphorus into rivers.

6. Agricultural runoff and soil erosion: Intensive tillage and irrigation accelerate phosphorus loss from land to water.

7. Urban runoff: Storm water from cities carries phosphorus from streets, lawns, and construction sites into water bodies.

Consequences:

• Eutrophication: Excess phosphorus in lakes and rivers causes algal blooms, oxygen depletion, fish kills, and loss of aquatic biodiversity.
• Water quality deterioration: Bad taste, odour, and increased treatment cost.
• Soil imbalance: Over-fertilization disturbs nutrient balance and can pollute groundwater.

Conclusion: Mining, fertilizer use, sewage, detergents, deforestation, and runoff are the main anthropogenic activities disturbing the natural phosphorus cycle, causing eutrophication and water pollution.

---

2.5 Use of Volumetric and Gravimetric Analysis in Water Quality Assessment

Volumetric (titrimetric) analysis measures the volume of a standard solution (titrant) required to react completely with a known volume of the sample. It is used to determine:

• Alkalinity: Titration with standard acid (HCl or H₂SO₄) using methyl orange or phenolphthalein indicator.
• Acidity: Titration with standard alkali (NaOH).
• Chloride: Mohr's method — titration with standard AgNO₃ using K₂CrO₄ indicator.
• Hardness of water: EDTA titration using Eriochrome Black T indicator.
• Dissolved oxygen (Winkler method): Iodometric titration with sodium thiosulphate.
• Chemical oxygen demand (COD): Titration of excess dichromate with ferrous ammonium sulphate.

Gravimetric analysis measures the mass of a substance by precipitation, filtration, drying, and weighing. It is used to determine:

• Total dissolved solids (TDS) and total suspended solids (TSS): Evaporation of a filtered sample and weighing the residue.
• Sulphate: Precipitation as BaSO₄, filtration, drying, and weighing.
• Phosphate: Precipitation as magnesium ammonium phosphate and ignition to Mg₂P₂O₇.
• Oil and grease: Extraction and weighing of the residue.

Advantages:

• Volumetric methods are fast, simple, and suitable for routine field and laboratory analysis.
• Gravimetric methods are highly accurate and reliable, and are used as reference methods to standardize other techniques.

Conclusion: Both methods are fundamental in water quality assessment — volumetric analysis for quick determination of alkalinity, hardness, chloride, and oxygen demand; gravimetric analysis for accurate measurement of solids, sulphate, and phosphate.

---

2.6 Extreme Weather Events and Tornado Formation

Extreme weather events are unusual, severe, and often rare weather phenomena that differ significantly from normal conditions and cause serious damage to life and property. Examples:

• Tornadoes — violent rotating columns of air.
• Tropical cyclones/hurricanes/typhoons — intense low-pressure storms with heavy rain and wind.
• Floods — overflow of water due to heavy rainfall or cloudbursts.
• Droughts — prolonged dry periods with water shortage.
• Heat waves and cold waves — prolonged extreme temperature.
• Hailstorms, blizzards, thunderstorms, and lightning.

These events are becoming more frequent and intense due to climate change.

Process of tornado formation:

1. Warm, moist air meets cold, dry air: A strong contrast between a warm, humid air mass and a cold, dry air mass creates instability in the atmosphere.
2. Updraft develops: The warm air rises rapidly, forming a thunderstorm cloud (cumulonimbus).
3. Wind shear: Winds at different heights blow at different speeds and directions, causing the rising air to rotate horizontally.
4. Rotation tilts vertical: The horizontal rotating column of air is tilted upward by the strong updraft, forming a rotating mesocyclone inside the storm.
5. Funnel cloud forms: The rotating air extends downward from the cloud base as a funnel cloud.
6. Tornado touches ground: When the funnel reaches the ground, it becomes a tornado, with very low pressure at the centre and extremely high wind speeds (up to 500 km/h).
7. Movement and dissipation: The tornado moves along the ground causing destruction, and dissipates when the supply of warm moist air is cut off or friction weakens the vortex.

Conclusion: Tornadoes form from severe thunderstorms with strong updrafts and wind shear; they are among the most violent and destructive extreme weather events.

---

2.7 Stability Conditions in the Atmosphere

Atmospheric stability describes the tendency of an air parcel to rise, remain, or sink. It is determined by comparing the environmental lapse rate (ELR) with the adiabatic lapse rate (ALR).

1. Absolute stability:

• Condition: ELR < ALR (environmental lapse rate is less than the adiabatic lapse rate).
• A rising air parcel cools faster than its surroundings, becomes denser, and sinks back.
• Vertical motion is suppressed; clouds are flat and layered (stratus), with little or no precipitation.
• Associated with subsidence and high pressure.

2. Absolute instability:

• Condition: ELR > ALR.
• A rising parcel cools slower than its surroundings, remains warmer and lighter, and continues to rise.
• Strong vertical motion; tall cumulonimbus clouds, thunderstorms, heavy rain, hail, and tornadoes.
• Associated with low pressure and strong heating.

3. Conditional instability:

• Condition: ALR (dry) > ELR > ALR (saturated).
• Stability depends on whether the air is unsaturated or saturated. Unsaturated air is stable; once it becomes saturated (releases latent heat), it becomes unstable.
• Common in the atmosphere; produces cumulus clouds and showers when lifting is sufficient.

4. Neutral stability:

• Condition: ELR = ALR.
• A displaced parcel neither accelerates upward nor sinks back; it stays at its new level.
• Vertical motion is neither encouraged nor suppressed.

Conclusion: Stability depends on the lapse rate comparison. Stable air gives fair, layered clouds; unstable air gives towering clouds and storms; conditional instability gives showers; neutral air gives neither strong rise nor sink.

---

2.8 Mountain Building Processes

Mountain building (orogeny) is the process by which large-scale mountain ranges are formed, mainly through plate tectonics and related crustal deformation.

1. Plate convergence (collision):

• Continental–continental collision: When two continental plates collide, neither subducts easily; the crust is compressed, folded, and thickened, raising high mountains. Example: the Himalayas, formed by the collision of the Indian and Eurasian plates.
• Oceanic–continental convergence: An oceanic plate subducts beneath a continental plate, forming volcanic mountain arcs and fold mountains. Example: the Andes.

2. Folding: Compressional forces buckle rock strata into anticlines (upfolds) and synclines (downfolds), producing fold mountains.

3. Faulting and block mountains: Tension or compression causes the crust to break along faults; uplifted blocks form horsts (block mountains) and down-dropped blocks form grabens (rift valleys). Example: the Vosges and Black Forest around the Rhine graben.

4. Volcanism: Magma rising from the mantle builds volcanic mountains (e.g., Mount Fuji, Andes volcanoes).

5. Isostasy: Thickened crust is buoyantly supported; erosion reduces load and causes isostatic rebound, while deposition causes subsidence.

6. Accretion and terrane collision: Smaller crustal fragments (terranes) are welded onto continents, adding to mountain belts.

Conclusion: Mountain building results mainly from plate convergence, folding, faulting, and volcanism, with isostasy and erosion modifying the uplifted crust over time. The Himalayas are a classic example of ongoing orogeny.

---

2.9 Abundance of Minerals in the Earth

Minerals are naturally occurring inorganic solids with definite chemical composition and crystal structure. Their abundance in the Earth's crust is uneven.

1. Most abundant elements in the crust (by weight):

• Oxygen (O) — about 46.6%
• Silicon (Si) — about 27.7%
• Aluminium (Al) — about 8.1%
• Iron (Fe) — about 5.0%
• Calcium (Ca) — about 3.6%
• Sodium (Na) — about 2.8%
• Potassium (K) — about 2.6%
• Magnesium (Mg) — about 2.1%
• Others (Ti, H, P, Mn, etc.) — remaining small percentage.

2. Most abundant minerals (rock-forming minerals):

• Silicates dominate the crust — about 90% of crustal minerals.
• Common silicates: feldspar (most abundant mineral group, ~60%), quartz (~12%), amphibole, mica, olivine, pyroxene.
• Feldspar and quartz together make up about 72% of the crust.

3. Distribution:

• Continental crust: rich in granitic (sial) rocks — quartz, feldspar, mica.
• Oceanic crust: rich in basaltic (sima) rocks — olivine, pyroxene, plagioclase.
• Mantle: rich in ultramafic minerals — olivine, pyroxene.
• Core: mainly iron and nickel (metallic).

4. Economic minerals: Though less abundant, ores of iron, copper, gold, zinc, lead, and aluminium are economically important and unevenly distributed.

Conclusion: The Earth's crust is dominated by oxygen and silicon, and by silicate minerals — especially feldspar and quartz — while economically valuable ore minerals are relatively scarce and unevenly distributed.

---

2.10 Distribution of Soil Types in Nepal

Nepal has diverse soil types due to variation in altitude, climate, parent rock, and vegetation. Major soil types and their distribution:

1. Alluvial soil (Entisols/Inceptisols):

• Found in the Terai plains, river valleys (Kathmandu, Pokhara), and floodplains.
• Formed by deposition of sediments by rivers.
• Fertile, rich in potash and lime, but poor in nitrogen and organic matter.
• Major crop-growing soil (paddy, wheat, sugarcane).

2. Sandy and gravelly soil:

• Found along riverbanks and floodplains (Terai and inner Terai).
• Coarse-textured, low water-holding capacity, low fertility.

3. Clay soil:

• Found in Terai and valley basins where fine sediments settle.
• Heavy, sticky, poor drainage, but fertile when managed well.

4. Loamy soil:

• Found in Terai and lower hills.
• Balanced mixture of sand, silt, and clay; highly fertile and ideal for agriculture.

5. Red and brown soil:

• Found in the mid-hills and Mahabharat range.
• Formed from crystalline and sedimentary rocks; rich in iron and aluminium oxides.
• Acidic, low in nitrogen and phosphorus; suitable for maize, millet, and citrus.

6. Residual soil:

• Found on hill slopes and ridges where soil forms in situ from weathered parent rock.
• Shallow, stony, and prone to erosion.

7. Mountain and glacial soil:

• Found in the Himalayan region above 3,000 m.
• Thin, immature, stony, and covered with snow or glacial deposits.
• Poor for agriculture; supports only grazing and hardy crops.

8. Peaty and marshy soil:

• Found in wetlands, marshes, and swampy areas (e.g., Terai wetlands, high-altitude bogs).
• Rich in organic matter, waterlogged, acidic, and unsuitable for most crops.

9. Arid and desert soil:

• Found in rain-shadow areas like Mustang, Manang, and upper Karnali.
• Dry, sandy, low in organic matter, and poor in fertility.

Conclusion: Nepal's soils range from fertile alluvial soils of the Terai to thin mountain soils of the Himalayas. Their distribution is controlled mainly by altitude, climate, and topography, and they are increasingly threatened by erosion, landslides, and unscientific land use.

3.1 Agrarian and industrial society

3.2 Clumped and random distribution

3.3 Alpha and beta diversity

3.4 Spatial and hypervolume niche

3.5 Linear and Y-shaped energy flow model

3.6 Production and decomposition

3.7 Potentiometric and conductometric titration

3.8 Lapse rate and temperature inversion

3.9 Turbulence and diffusion

3.10 Crust and mantle

What is an Ecosystem? Structural and Functional Components of a Lentic Ecosystem

Ecosystem

An ecosystem is a self-regulating, interacting unit consisting of a biotic community (living organisms) and its abiotic environment (non-living physical and chemical factors), through which energy flows and matter cycles. The term was coined by A. G. Tansley (1935). Examples: a pond, a forest, a grassland, a cropland.

---

Structural Components of a Lentic Ecosystem

A lentic ecosystem is a standing-water ecosystem (pond, lake, swamp). Its structure has two major divisions:

A. Abiotic (Non-living) Components

1. Physical factors — light (penetration depth), temperature (thermal stratification: epilimnion, metalimnion, hypolimnion), water currents, turbidity, depth, pressure, and dissolved gases (O₂, CO₂).
2. Chemical factors — dissolved inorganic nutrients (nitrates, phosphates, sulphates, carbonates, calcium, magnesium, iron), salinity, pH, and organic matter in solution or suspension.

B. Biotic (Living) Components

Organised by mode of nutrition and habitat zone:

1. Producers (Autotrophs) — mainly phytoplankton (diatoms, Chlorella, Scenedesmus), floating and submerged macrophytes (Hydrilla, Vallisneria, Nymphaea, Lemna), and rooted emergent plants (Typha, Phragmites) in the littoral zone. They fix solar energy by photosynthesis.
2. Consumers (Heterotrophs)
   • Primary consumers (herbivores) — zooplankton (Daphnia, Cyclops), snails, tadpoles, small fishes.
   • Secondary consumers (carnivores) — small fishes, insects, frogs feeding on herbivores.
   • Tertiary consumers (top carnivores) — large fishes, water snakes, kingfishers, herons.
   • Decomposers / saprotrophs — bacteria and fungi acting on dead organic matter in the profundal zone and bottom sediments.
3. Habitat zones — littoral zone (shallow, rooted plants), limnetic zone (open water, plankton), profundal zone (deep, dark, no producers), and benthic zone (bottom).

---

Functional Components of a Lentic Ecosystem

1. Energy flow — unidirectional flow of energy from the sun → producers → consumers → decomposers, following the 10 % law of energy transfer (Lindeman). Energy is lost as heat at each trophic level.
2. Food chain and food web — grazing food chain (phytoplankton → zooplankton → small fish → large fish) and detritus food chain (dead matter → bacteria → detritus feeders).
3. Nutrient cycling (biogeochemical cycles) — cycling of carbon, nitrogen, phosphorus, and oxygen between the biotic and abiotic components; in ponds, nutrients are recycled from the bottom sediments to the surface by mixing.
4. Energy budget / productivity — gross primary productivity, net primary productivity, and secondary productivity of the pond.
5. Ecological succession and homeostasis — the pond passes through hydrarch succession (pond → marsh → meadow → forest) and maintains stability through self-regulation.

---

OR

Types of Productivity

1. Gross Primary Productivity (GPP) — total organic matter synthesised by producers through photosynthesis per unit area per unit time.
2. Net Primary Productivity (NPP) — GPP minus respiration losses of producers: NPP = GPP − R. Only NPP is available to consumers.
3. Secondary Productivity — rate of formation of organic matter by consumers (herbivores and carnivores) at the heterotroph level.
4. Net Community Productivity — NPP minus respiration of consumers and decomposers.

---

Cropland vs. Heavily Grazed Grassland — Can Higher Standing Crop Prove Higher Productivity?

No, the conclusion is not valid. Standing crop biomass is only a snapshot of the organic matter present at one moment; it is not a measure of productivity.

Reasons:

1. Standing crop ≠ productivity. Productivity is a rate (energy fixed per unit area per unit time), while standing crop is a stock (mass present at a given instant). A large stock may simply mean that material is accumulating and not being removed.
2. Removal by grazing. In a heavily grazed grassland, herbivores continuously consume the plant biomass. Much of the NPP is transferred to consumers or lost, so the standing crop remains low even though productivity may be high. In cropland, harvest is delayed until maturity, so biomass accumulates and appears large.
3. Harvesting pattern. Crops are grown for a single harvest season and the entire biomass is removed at once, giving a high standing crop at that time. Grasslands are grazed repeatedly, so biomass is removed continuously.
4. Turnover rate. Grassland plants have a high turnover rate — they grow and are consumed rapidly. Crops have a low turnover within a season. High turnover keeps standing crop low despite high productivity.
5. Comparison of NPP. Actual measurements show that heavily grazed grasslands often have NPP comparable to or even higher than many croplands, because grasses are fast-growing and rapidly replace the grazed tissue.

Conclusion: Standing crop biomass alone cannot be used to compare productivity. To compare productivity, one must measure the rate of biomass production (NPP) over time, not the biomass present at a single moment. Hence the given information is insufficient to conclude that the cropland ecosystem has higher productivity.

Density-dependent population regulation is the process by which the growth rate of a population is affected by its own density (number of individuals per unit area or volume). As density increases, factors such as competition for food and space, predation, disease, and accumulation of toxic wastes become more intense. These factors lower the birth rate and raise the death rate, so the per capita growth rate declines as the population becomes crowded. Such regulation tends to keep the population near the carrying capacity of the environment. It is contrasted with density-independent regulation, in which factors like fire, flood, drought or pesticides affect the population regardless of its density.

1. Exponential (geometric) growth model

Proposed by Malthus (1798) and expressed in continuous form by Verhulst, this model assumes an unlimited supply of resources, so that every individual reproduces at its maximum (biotic) potential. There is no competition, no predation and no environmental resistance.

Let $N$ = population size at time $t$, $r$ = intrinsic rate of natural increase (per capita growth rate), $b$ = birth rate and $d$ = death rate.

$r = b - d$

The rate of change of population is:

$\frac{dN}{dt} = rN$

Integrating between $N_0$ (population at $t = 0$) and $N_t$:

$\int_{N_0}^{N_t}\frac{dN}{N} = r\int_0^t dt$

$\ln N_t - \ln N_0 = rt$

$N_t = N_0 e^{rt}$

Nature of the curve: The graph of $N$ against $t$ gives a J-shaped curve. Growth is slow at first, then accelerates rapidly and continues to rise without limit. The population doubles in a constant time, called the doubling time:

$t_d = \frac{\ln 2}{r} = \frac{0.693}{r}$

Example: A population of 100 bacteria with $r = 0.5$ per hour would reach $N_t = 100\,e^{0.5t}$; after 10 hours, $N = 100\,e^{5} \approx 14{,}841$. Such unlimited growth is only possible in the laboratory or in a newly colonised habitat; in nature it cannot continue for long because resources are finite.

2. Logistic growth model

Proposed by Verhulst (1838) and popularised by Pearl and Reed (1920), this model is more realistic because it includes environmental resistance. As the population grows, resources become limiting, so the growth rate slows down and finally stops when the population reaches the carrying capacity $K$ — the maximum number of individuals the environment can support.

The logistic equation is:

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$

Here $\dfrac{K-N}{K}$ is the environmental resistance term. When $N$ is small, $\dfrac{K-N}{K} \approx 1$ and growth is nearly exponential. As $N$ approaches $K$, the term approaches zero and growth stops. If $N > K$, the term becomes negative and the population declines.

The integrated form is:

$N_t = \frac{K}{1 + e^{a - rt}} \quad \text{or} \quad N_t = \frac{K}{1 + \left(\dfrac{K - N_0}{N_0}\right)e^{-rt}}$

Nature of the curve: The graph gives a sigmoid (S-shaped) curve with three phases:

• Lag phase: growth is slow because $N$ is small.
• Log (exponential) phase: growth is rapid, at the maximum rate, when $N \approx K/2$.
• Stationary (plateau) phase: growth slows and levels off at $K$, where births equal deaths.

The point where the growth rate is maximum, $N = K/2$, is called the point of inflexion.

Comparison

Conclusion: Density-dependent factors act as a brake on population growth. The exponential model describes growth only under ideal, unlimited conditions, while the logistic model describes the more realistic situation in which growth is checked by environmental resistance and the population stabilises at the carrying capacity.

Chemical and Mineralogical Composition of Soil

Soil is a three-phase system (solid, liquid, gas) whose solid fraction consists of mineral matter (about 45%) and organic matter (about 5%). The chemical and mineralogical composition of the inorganic fraction is described below.

1. Chemical Composition of Soil

The chemical composition is expressed in terms of the elements and oxides present in the soil solids.

A. Elemental composition (by weight of the mineral fraction):

B. Oxide composition (typical analysis of a soil):

C. Colloidal (exchange) composition: The clay and humus colloids carry exchangeable cations — Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺ — and exchangeable anions such as H₂PO₄⁻, SO₄²⁻, NO₃⁻. The cation exchange capacity (CEC) depends on the amount and type of clay and on organic matter.

2. Mineralogical Composition of Soil

Soil minerals are grouped into primary minerals and secondary minerals.

A. Primary minerals — inherited from the parent rock, largely unchanged:

B. Secondary minerals — formed by weathering and re-crystallisation; these dominate the clay fraction:

C. Organic (humic) matter: Humic acid, fulvic acid and humin — colloidal, high CEC (150–300 cmol/kg), responsible for structure, water holding and nutrient retention.

Significance: The mineralogical make-up decides CEC, plasticity, swelling, water holding capacity, nutrient reserve and fertility of a soil. Kaolinite-dominated soils are low in CEC; montmorillonite soils swell and shrink; quartz-rich sandy soils are infertile.

---

Soil Types of Nepal

Nepal's soils are young, derived from the Himalaya, and vary with altitude, climate, parent material and topography. The main types are:

1. Alluvial soils (Entisols / Fluvisols) — Found in the Terai and river valleys (Koshi, Gandaki, Karnali flood plains). Formed from transported sediments. Light to medium texture, high fertility, good for paddy, sugarcane, jute and pulses. Occupy the largest cultivated area.

2. Sandy and gravelly soils (Regosols) — Along river banks and the Chure/Bhabar zone. Coarse, porous, low water holding capacity and low fertility; prone to erosion.

3. Red and brown forest soils (Alfisols / Cambisols) — In the mid-hills and Mahabharat range (1,000–2,500 m). Formed from phyllite, schist and gneiss. Acidic to slightly acidic, rich in iron and aluminium oxides, moderate fertility; support maize, millet, citrus and tea.

4. Hill and mountain soils (Cambisols, Podzols) — On the Higher and High Himalaya (above 2,500 m). Shallow, stony, immature, low in bases; podzolic in coniferous belts. Used for pasture, apple and potato.

5. Residual soils of the Siwalik and Chure — Derived in situ from sandstone and conglomerate; coarse, weakly developed, highly erodible.

6. Lacustrine soils (Kathmandu and Pokhara valleys) — Formed from lake deposits. Fine-textured, poorly drained, with black clay (Kalimati) rich in organic matter; used for paddy and vegetables.

7. Acidic soils — Common in the eastern hills and in tea-growing areas; pH below 5.5, high Al³⁺, need liming.

8. Calcareous soils — In pockets of the Terai and inner Terai where limestone parent material occurs; free CaCO₃, pH above 7.5.

9. Saline and alkaline (Usar) soils — In parts of the eastern and central Terai (e.g. Jhapa, Saptari, Bara). High soluble salts or exchangeable sodium; poor crop growth; need gypsum and drainage.

10. Peaty and marshy soils — In waterlogged pockets of the Terai (e.g. Koshi Tappu area) and valley floors; high organic matter, very poor drainage.

Distribution summary:

Conclusion: Nepal's soils range from fertile alluvium of the Terai to thin, stony mountain soils. Their mineralogy is dominated by quartz, feldspar, mica and secondary layer-silicates, while chemically they are rich in Si, Al and Fe but generally deficient in nitrogen, phosphorus and, in the hills, in bases — hence the widespread need for liming and balanced fertilisation.

Temperature Lapse Rate and Its Effect on Pollutant Dispersion

Temperature Lapse Rate

The temperature lapse rate is the rate at which atmospheric temperature decreases with increasing altitude. In the lower atmosphere (troposphere), temperature normally falls with height. The average or normal lapse rate is about 6.5 °C per kilometre of rise in altitude (approximately 6.5 K/km).

The environmental lapse rate (ELR) is the actual rate of decrease of temperature with height at a given place and time. It varies with location, season, and weather conditions. The dry adiabatic lapse rate (DALR) is about 9.8 °C/km, and the wet adiabatic lapse rate (WALR) is about 5–6 °C/km.

Types of Lapse Rate and Atmospheric Stability

The relation between the environmental lapse rate and the adiabatic lapse rate determines the stability of the atmosphere, which in turn controls how pollutants mix and disperse:

Effect on Dispersion of Pollutants

1. Unstable (super-adiabatic) conditions: When the environmental lapse rate is greater than the adiabatic lapse rate, warm air near the ground rises rapidly. This causes strong vertical mixing and turbulence, so pollutants are carried upward and diluted quickly. Dispersion is good, but pollutants may be carried to higher levels.

2. Neutral conditions: The environmental lapse rate equals the adiabatic lapse rate. Vertical mixing is moderate, and pollutants disperse at a steady, average rate.

3. Stable conditions: When the environmental lapse rate is less than the adiabatic lapse rate, vertical motion is suppressed. Pollutants do not rise easily and remain close to the ground, leading to poor dispersion and higher ground-level concentrations.

4. Temperature inversion: When temperature increases with altitude instead of decreasing, a temperature inversion forms. A warm layer of air sits above cooler air near the ground, acting like a lid. Vertical mixing stops, and pollutants are trapped in the lower layer. This causes severe air pollution, smog, and health problems, especially in valleys and cities during winter mornings.

Conclusion

The temperature lapse rate determines whether the atmosphere is stable or unstable. A steep lapse rate promotes good dispersion of pollutants, while a small lapse rate or an inversion traps pollutants near the ground, causing serious air pollution. Thus, the lapse rate is a key factor in the dispersion and dilution of atmospheric pollutants.

Environmental Worldviews

An environmental worldview is the set of basic beliefs, values and assumptions that a person or society holds about the natural world and humanity's place in it. It determines how we perceive environmental problems and what solutions we consider right. The main types are:

1. Anthropocentric (Human-Centred) Worldview This view places humans at the centre of the universe and treats nature as a resource existing for human use. Nature has only instrumental value — it is valuable because it benefits people. It supports unlimited economic growth and technological optimism. Examples: frontier worldview, cornucopian view.

2. Biocentric (Life-Centred) Worldview Here all living organisms are considered to have intrinsic value, regardless of their usefulness to humans. Humans are just one species among many, and every species has an equal right to exist. It stresses the protection of biodiversity. Example: the views of Albert Schweitzer and Paul Taylor.

3. Ecocentric (Ecology-Centred) Worldview This worldview places the entire ecosystem, including non-living components (air, water, soil), at the centre. The whole Earth is seen as a community of interdependent parts in which humans are only one member. It emphasises ecological integrity, sustainability and the carrying capacity of the Earth. Examples: deep ecology, Gaia hypothesis, land ethic of Aldo Leopold.

4. Stewardship Worldview A middle path which holds that humans are the caretakers or managers of nature, entrusted by God or by ethics to use resources wisely and preserve them for future generations. It combines concern for human welfare with responsibility towards nature.

5. Planetary Management Worldview Humans are seen as separate from and superior to nature, and the Earth is viewed as a resource bank to be managed for human benefit. It encourages maximum resource exploitation and is the root cause of most present-day environmental crises.

6. Environmental Wisdom Worldview This view holds that nature exists not just for humans but for all species, that resources are limited and should be used sustainably, and that human success depends on learning from and cooperating with nature rather than dominating it.

Conclusion: Among these, the ecocentric and environmental wisdom worldviews are considered most suitable for achieving sustainable development, because they balance human needs with the long-term health of the Earth.

Multidisciplinary approach to environmental problems — need and justification

Meaning. A multidisciplinary approach means studying and solving environmental problems by drawing on knowledge, methods and tools from many disciplines at once — natural sciences (biology, chemistry, physics, geology, ecology), social sciences (economics, sociology, political science, law), and applied fields (engineering, agriculture, public health, management, education).

Why a single discipline is not enough. Environmental problems are complex, multi-causal and multi-dimensional. They arise from the interaction of natural processes with human activities, so no one discipline can describe them fully, let alone solve them. The following points establish the need:

1. Complex and interlinked nature of problems. An issue such as air pollution in Kathmandu involves emission sources (technology, fuel use), meteorological dispersion (physics), health effects (medicine), transport policy (public administration), and household behaviour (sociology). Each aspect needs its own expert.

2. Cause–effect chain crosses disciplines. Deforestation causes soil erosion (geology), floods (hydrology), loss of biodiversity (ecology), and displacement of people (sociology). Understanding the whole chain requires all these fields together.

3. Identification of problems needs different lenses. A biologist detects species loss, a chemist detects water contamination, a sociologist detects who bears the cost. Without all three, the problem is only partly seen.

4. Integrated solutions. A solution such as a hydropower project must be technically feasible (engineering), economically viable (economics), socially acceptable (sociology), and legally permitted (law). A remedy from one discipline alone often creates a new problem elsewhere.

5. Trade-offs and conflicting interests. Environment versus development, conservation versus local livelihood, upstream versus downstream users — resolving such conflicts needs economic valuation, legal frameworks, ethical reasoning and political negotiation, not just ecological data.

6. Policy, planning and management. Environmental Impact Assessment (EIA), land-use planning, climate adaptation plans and pollution standards require inputs from natural scientists (baseline data), economists (cost–benefit), lawyers (compliance) and administrators (implementation).

7. Awareness, education and behaviour change. Sustainable use of resources ultimately depends on people's attitudes; this needs pedagogy, communication and psychology alongside technical knowledge.

8. Global and transboundary issues. Climate change, ozone depletion, acid rain and biodiversity loss cross national boundaries and involve international law, diplomacy, trade and finance — again beyond any single discipline.

9. Uncertainty and risk. Environmental systems are non-linear and uncertain; assessing risk needs statistics, modelling and social perception studies together.

10. Sustainability as a goal. Sustainable development balances ecological, economic and social objectives; this balance is by definition multidisciplinary.

Conclusion. Environmental problems cannot be solved by ecologists, engineers or economists working in isolation. Only a multidisciplinary (and at the planning stage, interdisciplinary) approach can diagnose the problem correctly, weigh alternatives, and deliver solutions that are ecologically sound, economically efficient and socially just. Hence it is not merely useful but essential.

Comparison of Environmental Issues: Rural vs Urban Nepal

Similarities

• Both face pollution and degradation: Rural areas suffer from indoor air pollution (biomass burning) and urban areas from vehicular and industrial air pollution; both experience water pollution and deforestation.
• Both are linked to poverty and weak enforcement: Inadequate enforcement of environmental laws, low public awareness, and poverty-driven resource overuse are common to both.
• Both are affected by national/global problems: Climate change, landslides, floods and biodiversity loss affect rural and urban Nepal alike.
• Both show inadequate waste management: Rural areas lack proper disposal of agricultural and household waste; urban areas lack proper solid waste and sewage systems.

Contrasts

Conclusion

Rural environmental problems are largely poverty- and resource-dependence-driven, while urban problems are growth- and consumption-driven. Both are interlinked — rural degradation pushes migration to cities, worsening urban pollution. Hence, Nepal needs integrated policies: rural electrification, alternative energy and afforestation on one hand, and urban waste treatment, public transport and planned settlement on the other.

Types of Competition

Competition is the interaction between two or more organisms (or species) that require the same limited resource, so that the presence of one reduces the availability of that resource to the other. It is classified in several ways:

1. On the basis of taxonomic relationship

• Intraspecific competition: Competition between individuals of the same species. Since members of a species have identical resource requirements, this competition is usually intense. It is a major agent of natural selection (survival of the fittest) and is the basis of density-dependent population regulation. Example: two deer of the same herd grazing the same patch of grass; overcrowded maize plants competing for light, water and nutrients.

• Interspecific competition: Competition between individuals of different species for a shared resource. Example: goats and sheep grazing the same pasture; weeds and crop plants competing for soil nutrients. This may lead to competitive exclusion (one species eliminates the other) or to resource partitioning / niche differentiation, allowing coexistence.

2. On the basis of the mechanism of interaction

• Exploitation (scramble) competition: Competitors do not interact directly; each simply consumes the resource, and the one that harvests faster or more efficiently gets the larger share. Example: two filter-feeding mussels removing plankton from the same water.

• Interference (contest) competition: Competitors directly harm or exclude one another by aggression, territorial defence, allelopathy or overgrowth. Example: a male lion driving off a rival; a large tree shading out seedlings; Calamagrostis releasing allelopathic chemicals that suppress other plants.

3. On the basis of the outcome / symmetry

• Contest (winner-takes-all): The stronger individual monopolises the resource; the loser gets little or nothing. Common where the resource is patchy or defensible (territory, mate, nest site).

• Scramble: The resource is shared, but at high density every individual gets too little, so all may suffer (often leading to population crash).

• Symmetric vs. asymmetric competition: In symmetric competition the competitors are equally affected; in asymmetric competition one species or individual is affected far more than the other (e.g. a tall tree vs. a seedling).

4. Other special types

• Direct vs. indirect competition: Direct competition involves immediate contact or active exclusion; indirect competition occurs when one species alters the resource or habitat used by another (e.g. a beaver changing stream flow).

• Apparent competition: Two species do not compete for a resource but each suffers because a shared predator or herbivore increases in abundance.

Ecological Significance of Competition

1. Regulation of population size: Intraspecific competition acts as a density-dependent check on population growth, preventing unlimited increase and keeping populations near the carrying capacity of the habitat.

2. Natural selection and evolution: Competition is a major selective force. Individuals with better adaptations (for obtaining food, light, space or mates) survive and reproduce more, driving adaptation and speciation.

3. Determination of species distribution and abundance: Interspecific competition restricts the realised niche of a species, so a species may be absent from a habitat where it could otherwise live (competitive exclusion principle — Gause's principle).

4. Structuring of communities: Competition shapes community composition, relative abundance of species and patterns of zonation (e.g. barnacle zonation on rocky shores; different bird species feeding at different heights in a forest).

5. Resource partitioning and coexistence: To reduce competition, species evolve differences in niche (different food, time or place of feeding), which increases species diversity and stability of the ecosystem.

6. Energy flow and productivity: Competition influences which species capture the available energy and nutrients, thereby affecting primary productivity, food-chain efficiency and trophic structure.

7. Maintenance of ecosystem balance: By preventing any one species from dominating and monopolising resources, competition helps maintain equilibrium, biodiversity and the smooth functioning of the ecosystem.

Changes in Ecosystem Characteristics During Ecological Succession

Ecological succession is the gradual, directional and predictable change in the structure and composition of a community over time. As succession proceeds from a pioneer stage to a climax stage, the following ecosystem characteristics change:

1. Species diversity and composition Species diversity increases from the pioneer to the climax stage. Early stages have few, simple, hardy species (lichens, mosses, grasses), while later stages have many species with complex composition. The number of niches also increases.

2. Biomass and productivity Total biomass increases steadily as succession advances. Gross primary productivity (GPP) rises, but net primary productivity (NPP) is highest in the early-intermediate stages and declines near the climax, because respiration losses increase as biomass accumulates.

3. Trophic structure and food webs Food chains and food webs become longer and more complex. The number of trophic levels increases, and energy flow becomes more diversified and stable.

4. Soil and nutrient status Soil depth, organic matter (humus) and nutrient content increase. Soil moisture and water-holding capacity improve. Early stages have poor, thin soil; the climax stage has deep, fertile soil.

5. Niche specialization Niches become narrower and more specialized. Early species are generalists (broad niche), while climax species are specialists (narrow niche), reducing competition.

6. Stability and resilience Ecosystem stability increases. The climax community is more resistant to disturbance and recovers faster (higher resilience) than pioneer communities.

7. Energy and nutrient cycling Nutrient cycling becomes more closed and efficient. Energy flow becomes more complex but the proportion of energy lost as respiration increases.

8. Microclimate The community modifies its own environment — light penetration decreases, humidity rises, temperature fluctuations are buffered, and wind speed at ground level falls.

9. Net community productivity Net community productivity (NPP minus respiration of the whole community) is positive in early stages and approaches zero at climax, where production roughly equals respiration (steady state).

Summary table

Thus, succession moves the ecosystem from a simple, unstable, low-biomass state toward a complex, stable, high-biomass climax state in which energy flow and nutrient cycling are balanced.

Fate of Pesticides in the Environment

Once pesticides are applied, only a small fraction reaches the target pest; the rest enters the environment and undergoes several processes that determine their fate:

1. Adsorption — Pesticide molecules bind to soil particles (especially clay and organic matter). Strongly adsorbed pesticides are less mobile and less available to plants and organisms, but persist longer in soil.

2. Volatilization — Volatile pesticides (e.g., some insecticides) escape into the atmosphere as vapour from soil, water or plant surfaces. They may travel long distances and return to the earth through rainfall (wet deposition) or dust fall (dry deposition).

3. Leaching — Water-soluble pesticides are carried downward through the soil profile by percolating water. This can contaminate groundwater, making it unfit for drinking and irrigation.

4. Surface runoff — Rain or irrigation water carries pesticides over the soil surface into streams, ponds and rivers, causing contamination of surface water and harm to aquatic life.

5. Spray drift — Fine droplets and dust particles are carried by wind away from the target area during and after application, polluting neighbouring fields, water bodies and settlements.

6. Microbial and chemical degradation — Soil microorganisms, sunlight (photodegradation), hydrolysis and oxidation break pesticides into simpler compounds. Some break down into metabolites that may be more toxic or more persistent than the parent compound.

7. Bioaccumulation and biomagnification — Fat-soluble, non-degradable pesticides (e.g., DDT) accumulate in the tissues of organisms and become more concentrated at each higher trophic level of the food chain. This causes poisoning in birds, fish and humans at the top of the chain.

8. Persistence (half-life) — Depending on their chemical nature, pesticides may remain in the environment for days to years. Persistent pesticides such as organochlorines remain active for a long time and cause long-term soil and water pollution.

Conclusion: Pesticides ultimately accumulate in soil, water and air, enter the food chain, and disturb the ecological balance. Their safe and judicious use, along with integrated pest management, is essential to minimise environmental harm.

Stratospheric Ozone Chemistry and Ozone Layer Depletion

1. The ozone layer and its formation

The ozone layer lies in the stratosphere, roughly 15–35 km above the Earth's surface, where ozone (O₃) concentration peaks at about 10 ppm. Ozone is formed when molecular oxygen absorbs solar ultraviolet radiation of wavelength below 240 nm and dissociates into oxygen atoms:

$\text{O}_2 + h\nu \;(\lambda < 240\ \text{nm}) \rightarrow \text{O} + \text{O}$

Each oxygen atom then combines with an oxygen molecule in the presence of a third body (M, usually N₂ or O₂) to form ozone:

$\text{O} + \text{O}_2 + \text{M} \rightarrow \text{O}_3 + \text{M}$

Ozone is destroyed when it absorbs UV radiation in the 200–310 nm range and splits back into O₂ and O:

$\text{O}_3 + h\nu \;(\lambda < 320\ \text{nm}) \rightarrow \text{O}_2 + \text{O}$

This continuous cycle of formation and destruction (the Chapman cycle) maintains a steady-state ozone concentration and absorbs harmful UV-B and UV-C radiation, shielding life on Earth. The ozone layer is measured in Dobson Units (DU); the normal average is about 300 DU.

2. Catalytic destruction of ozone

Certain trace gases released by human activity carry chlorine, bromine, nitrogen or hydrogen atoms that destroy ozone catalytically — one atom can destroy thousands of ozone molecules before being removed. The general cycle is:

$\text{X} + \text{O}_3 \rightarrow \text{XO} + \text{O}_2$ $\text{XO} + \text{O} \rightarrow \text{X} + \text{O}_2$ $\text{Net: } \text{O}_3 + \text{O} \rightarrow 2\text{O}_2$

where X = Cl, Br, NO, or OH.

3. Chlorofluorocarbons (CFCs) and the chlorine cycle

CFCs (e.g. CFCl₃, CF₂Cl₂), used as refrigerants, aerosols and foam-blowing agents, are chemically inert in the troposphere and slowly diffuse into the stratosphere. There they are photolysed by UV radiation to release chlorine atoms:

$\text{CFCl}_3 + h\nu \rightarrow \text{CFCl}_2 + \text{Cl}$ $\text{CF}_2\text{Cl}_2 + h\nu \rightarrow \text{CF}_2\text{Cl} + \text{Cl}$

The chlorine atom then enters the catalytic cycle:

$\text{Cl} + \text{O}_3 \rightarrow \text{ClO} + \text{O}_2$ $\text{ClO} + \text{O} \rightarrow \text{Cl} + \text{O}_2$

The Cl atom is regenerated, so a single chlorine atom can destroy up to about 100,000 ozone molecules. Chlorine is finally removed as HCl or ClONO₂ (reservoir species), but these can be re-photolysed, releasing Cl again.

4. Role of polar stratospheric clouds (PSCs)

In the extreme cold of the Antarctic winter (below −78 °C), stratospheric clouds of ice and nitric acid crystals form. Heterogeneous reactions on these surfaces convert reservoir species (HCl, ClONO₂) into photolytically active forms:

$\text{HCl} + \text{ClONO}_2 \rightarrow \text{Cl}_2 + \text{HNO}_3$

When sunlight returns in spring, Cl₂ is photolysed, releasing chlorine atoms that rapidly destroy ozone. This is why the ozone hole appears over Antarctica each spring (September–November) and why it is far more severe than over the Arctic.

5. Consequences of ozone depletion

• Increased UV-B radiation reaching the surface, causing skin cancer, cataracts, and immune suppression in humans.
• Damage to phytoplankton, crops and forest ecosystems.
• Global warming contribution, since ozone is also a greenhouse gas.
• The Antarctic ozone hole has reached over 25 million km² in some years.

6. Control measures

The Montreal Protocol (1987) and its amendments phased out CFCs, halons, carbon tetrachloride and methyl chloroform. Substitute chemicals such as HCFCs and HFCs were introduced, and later the Kigali Amendment (2016) targeted HFCs. As a result, stratospheric chlorine levels are slowly declining and the ozone layer is expected to recover by the middle of this century.

Cation Exchange Capacity (CEC)

Cation Exchange Capacity is the total capacity of a soil to hold exchangeable cations (positively charged ions such as Ca²⁺, Mg²⁺, K⁺, Na⁺, H⁺, Al³⁺) on the negatively charged surfaces of clay minerals and organic matter, and to release them into the soil solution in exchange for other cations. It is expressed in centimoles of charge per kilogram of soil (cmol(+)/kg), which is numerically equal to the older unit milliequivalents per 100 g (meq/100 g).

$CEC = \frac{\text{sum of exchangeable cations (cmol(+))}}{\text{mass of soil (kg)}}$

Mechanism: Clay minerals (especially 2:1 types like montmorillonite) carry permanent negative charges from isomorphous substitution, and organic matter carries pH-dependent negative charges from dissociation of carboxyl and phenolic –OH groups. These negative sites attract and adsorb cations from the soil solution. When the soil solution becomes richer in a particular cation (e.g. from fertilizer or lime), that cation displaces the adsorbed one, which is released to the plant — hence "exchange."

Typical values:

Significance:

1. Nutrient holding capacity — High CEC soils retain applied K⁺, Ca²⁺, Mg²⁺ and NH₄⁺ against leaching, acting as a nutrient reservoir; low CEC sandy soils lose nutrients readily and need frequent, light fertilizer doses.
2. Nutrient availability — Adsorbed cations are held in an exchangeable (plant-available) form and are released gradually, giving steady supply rather than a sudden flush.
3. Fertilizer and lime recommendation — CEC is used to decide fertilizer rates and lime requirement; a soil with high CEC needs more lime to raise pH by a given amount.
4. Buffer capacity against pH change — Soils with high CEC resist sudden changes in soil reaction, keeping pH stable.
5. Soil structure and fertility — High CEC is associated with Ca²⁺ and Mg²⁺ dominance, which promotes flocculation, good aggregation, porosity and better infiltration.
6. Indicator of clay and organic matter — CEC serves as an index of the clay and humus content, and hence of the general fertility status of the soil.
7. Management of problem soils — The ratio of exchangeable Na⁺ to total CEC (exchangeable sodium percentage) diagnoses sodicity and guides gypsum application.
8. Retention of pollutants — High CEC soils adsorb heavy metal cations and some pesticides, reducing groundwater contamination.

Climate change is the long-term, significant change in the average weather patterns of the Earth (temperature, rainfall, wind, and seasons) lasting for decades or centuries. It is caused mainly by the increase of greenhouse gases such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) in the atmosphere due to human activities like burning fossil fuels, deforestation, and industry. These gases trap heat and cause global warming, which drives climate change.

Two impacts of climate change:

1. Rise in temperature and melting of glaciers: Global average temperature has increased by about 1.1 °C since pre-industrial times. Himalayan glaciers, including those in Nepal, are melting rapidly. This causes glacier lake outburst floods (GLOFs), reduces fresh water supply in rivers, and threatens the water security of millions of people.

2. Change in rainfall pattern and extreme events: Climate change makes rainfall irregular — long droughts followed by heavy floods and landslides. This damages crops, reduces agricultural production and food security, and increases the frequency of disasters such as floods, landslides, and heat waves. In Nepal, monsoon shifts have already affected paddy and maize yields.

Wind as a Geological Agent of Change (5 marks)

Wind is an important geological agent, especially in arid and semi-arid regions where vegetation is sparse and the surface is dry and loose. It erodes, transports, and deposits rock material, shaping the land surface.

1. Erosion by wind

• Deflation: Removal of loose, dry, fine particles (sand, silt, dust) from the surface, leaving behind a stony desert surface called desert pavement or reg.
• Abrasion: Sand-blasted by wind-borne particles hitting rock surfaces, producing smooth, polished, or faceted surfaces (e.g., ventifacts).
• Attrition: Wind-borne particles collide with one another and become smaller and more rounded.

2. Transportation by wind

• Suspension: Fine dust and silt carried high in the air over long distances.
• Saltation: Sand grains bounce along the surface in short hops — the dominant mode of sand movement.
• Surface creep: Coarser grains roll or slide along the ground.

3. Deposition by wind

• Sand dunes: Mounds of sand formed where wind velocity drops; classified as barchan, transverse, longitudinal (seif), parabolic, and star dunes.
• Loess: Extensive deposits of wind-blown silt, forming fertile, thick, homogeneous soils (e.g., Chinese Loess Plateau).

4. Resulting landforms

• Erosional: Mushroom rocks, yardangs, zeugen, inselbergs, deflation hollows, and desert pavements.
• Depositional: Dunes and loess sheets.

Significance: Wind action produces characteristic desert landscapes, transports fertile loess to distant regions, and causes soil erosion and dust storms in dry areas. Thus, wind is a powerful geological agent of change, particularly in arid climates.

Simple (single-chain) energy flow model

• It is a linear, unbranched model: producers → herbivores → carnivores → top carnivores, in a single straight chain.
• It shows energy transfer along one grazing food chain only (e.g. grass → grasshopper → frog → snake → hawk).
• It is unrealistic in nature, because in real ecosystems one organism is eaten by many others and feeds on many others.
• It ignores the role of decomposers and does not show the interconnection of food chains.
• It is useful only for simple, theoretical explanations of energy transfer.

Y-shaped energy flow model

• It is a branched model in which the food chain splits into two arms — a grazing (predator) chain and a detritus (decomposer) chain — both drawing from the same primary source.
• The Y is formed because the energy of dead organic matter goes one way (to decomposers) and the energy of living producers goes the other way (to grazers).
• It shows that both living plants and dead organic matter are energy sources for the ecosystem.
• It is more realistic, as it includes decomposers and links the grazing and detrital pathways.
• It explains energy flow in natural ecosystems far better than the simple model.

Key difference in one line: The simple model shows energy flow in a single straight line, while the Y-shaped model shows energy flow through two separate but interconnected pathways (grazing and detritus).

Sympatric speciation is the formation of new species from a single ancestral population while all individuals remain in the same geographic area, without any physical barrier. Reproductive isolation arises through factors such as polyploidy (common in plants), habitat differentiation within the same region, or disruptive selection based on resource use. Example: apple maggot flies (Rhagoletis pomonella) diverging on apple and hawthorn trees in the same locality.

Allopatric speciation is the formation of new species when a population is divided by a physical (geographic) barrier such as a mountain range, river, or ocean. The separated populations are exposed to different selection pressures and accumulate genetic differences; when they can no longer interbreed even if they meet again, they become distinct species. Example: Darwin's finches on different Galapagos islands.

Key difference: sympatric speciation occurs without geographic separation, whereas allopatric speciation requires geographic separation of populations.

Sedimentary cycles — Nutrients are stored in the Earth's crust (rocks and soil) rather than the atmosphere. They are recycled through weathering, erosion, sedimentation and geological uplift, so the cycle is slow and its reservoir is sedimentary rock. Examples: phosphorus cycle, sulphur cycle, calcium cycle.

Gaseous cycles — Nutrients are stored mainly in the atmosphere or ocean as gases. They are recycled quickly through biological processes such as photosynthesis, respiration and decomposition, so the cycle is rapid and its reservoir is the atmosphere. Examples: carbon cycle, nitrogen cycle, oxygen cycle, water cycle.

Volumetric analysis (titrimetric analysis) is a quantitative method in which the concentration of an unknown solution is determined by measuring the volume of a standard solution (titrant) of known concentration that reacts completely with a measured volume of the analyte. The reaction is carried to the equivalence point, usually detected with an indicator or by a pH/conductivity change. It requires accurate volume measurement (burette, pipette) and a reaction that is fast, complete, and stoichiometrically well-defined. Example: titrating NaOH against standard HCl using phenolphthalein.

Gravimetric analysis is a quantitative method in which the amount of a constituent is determined by measuring the mass of a substance of known composition that is chemically related to the analyte. The analyte is usually converted into a pure, stable, insoluble precipitate (or a volatile compound), which is filtered, washed, dried or ignited, and weighed on an analytical balance. The mass is then converted to the amount of analyte using stoichiometric factors. Example: determining chloride by precipitating it as AgCl and weighing the dried precipitate.

Key difference: volumetric analysis measures volume of a reacting solution of known concentration, whereas gravimetric analysis measures mass of a pure substance obtained from the analyte.

Resistance is the ability of an ecosystem to remain relatively unchanged (to withstand disturbance) when subjected to a stress or disturbance such as drought, fire, or pollution. A resistant ecosystem keeps its structure and function close to the original state.

Resilience is the ability of an ecosystem to recover and return to its original state (structure and function) after being disturbed or damaged. A resilient ecosystem recovers quickly after a disturbance.

In short: resistance = "staying the same" during a disturbance; resilience = "bouncing back" after a disturbance.

Mohorovicic discontinuity (Moho): The boundary between the Earth's crust and the mantle, at a depth of about 35 km below continents and about 5–10 km below the ocean floor. Seismic P-wave velocities jump from about 6–7 km/s in the crust to about 8 km/s in the mantle across it.

Gutenberg–Weichert discontinuity: The boundary between the mantle and the outer core, at a depth of about 2,900 km. It marks the sharp drop of S-wave velocity to zero (S-waves cannot pass through the liquid outer core) and a fall of P-wave velocity from about 13 km/s to about 8 km/s.

Birth rate and growth rate [2 marks]

Birth rate (CBR): The number of live births per 1,000 people in a given population during a year.

$\text{CBR} = \frac{\text{Number of live births in a year}}{\text{Mid-year total population}} \times 1000$

Growth rate: The rate at which a population increases or decreases per year, expressed as a percentage of the base population. It combines natural increase (births − deaths) and net migration.

$\text{Population growth rate} = \frac{P_2 - P_1}{P_1} \times 100$

Relation: Birth rate is a component of growth rate. A high birth rate with a low death rate gives a high growth rate.

Weather and climate

Weather is the short-term atmospheric condition of a place at a particular time — its temperature, pressure, humidity, cloud, rainfall and wind. It changes frequently, even within a day or a few hours.

Climate is the average or long-term atmospheric condition of a place over a long period, usually 30 years or more. It is relatively stable and changes very slowly.

Key difference: Weather is short-term and variable; climate is long-term and stable. "Weather" tells what it is like today; "climate" tells what it is usually like over many years.

El Niño is the abnormal warming of surface waters in the eastern equatorial Pacific Ocean off the coast of Peru and Ecuador, occurring every 2–7 years and lasting about 12–18 months. It weakens or reverses the normal trade winds.

ENSO (El Niño–Southern Oscillation) is the coupled ocean–atmosphere phenomenon that combines both phases: the oceanic part (El Niño = warm phase, La Niña = cold phase) and the atmospheric part (Southern Oscillation = the see-saw of air pressure between Tahiti and Darwin). So El Niño is only one component of ENSO; ENSO describes the full interaction between the Pacific Ocean and the overlying atmosphere.

Metamorphic rock: A rock formed when pre-existing igneous or sedimentary rock is altered by heat, pressure, or chemically active fluids (e.g., marble from limestone, slate from shale).

Sedimentary rock: A rock formed by the compaction and cementation of sediments deposited in layers (e.g., sandstone, limestone, shale).

Key difference: Metamorphic rocks form under high temperature and pressure within the Earth, while sedimentary rocks form at or near the surface through deposition and lithification.