Ecology (Botany): ecosystem ecology, ecological adaptation, ecological imbalances

(c) habitat like desert

Xerophytes are adapted to dry habitats — deserts are the typical example — with thick cuticle, sunken stomata and reduced leaves to cut water loss.

(a) It has the function of support

Prop roots are adventitious roots that arise from the aerial branches of plants like the banyan tree, grow downwards and fix into the soil to give mechanical support to the heavy branches.

Ecological factors in a pond ecosystem

A pond is a small, self-contained freshwater ecosystem. Its structure (the organisms and their physical setting) and its function (energy flow and nutrient cycling) are both controlled by ecological factors, which are grouped into abiotic (physical–chemical) and biotic (living) factors.

1. Abiotic factors and structure (2 marks)

• Light: Sunlight penetrates only the upper layer (euphotic zone). It determines the vertical arrangement of plants — floating and submerged plants near the surface, and the depth to which photosynthesis (hence oxygen production) can occur. In a shaded pond, phytoplankton and submerged plants decline.
• Temperature: Ponds show thermal stratification in summer — a warm upper epilimnion, a sharply cooling thermocline, and a cold, oxygen-poor hypolimnion. This layering decides where fish, zooplankton and bottom-dwelling (benthic) organisms live. Warmth also speeds up metabolism and decomposition.
• Light and temperature together control the seasonal appearance of organisms, e.g. sudden blooms of algae in spring and the dormancy of many invertebrates in winter.
• Water (its depth, pH, turbidity): Shallow, clear, slightly alkaline water supports rich plant and animal life; highly turbid or acidic water reduces light penetration and species number.
• Dissolved oxygen and carbon dioxide: Oxygen is high near the surface (from photosynthesis and diffusion) and low at the bottom. Low oxygen forces fish to the surface and favours anaerobic decomposers in the mud.
• Nutrients (N, P, K, Ca): Their amount decides whether the pond is oligotrophic (nutrient-poor, clear, few organisms) or eutrophic (nutrient-rich, green, dense life). Excess nutrients cause algal blooms and later oxygen depletion.
• Light, substratum type (mud, sand, rock) also decides which plants root and which animals burrow.

2. Biotic factors and structure (1 mark)

• Producers (phytoplankton, Hydrilla, Vallisneria, Azolla) form the base and are limited by light and nutrients.
• Consumers — primary (zooplankton, small fish, snails), secondary (frogs, larger fish), tertiary (snakes, kingfishers) — are distributed according to food and oxygen availability.
• Decomposers (bacteria, fungi) act on dead matter in the mud.
• Interactions such as competition, predation, parasitism and mutualism decide which species survive; e.g. predation by fish keeps zooplankton numbers in check.

3. Functional aspects (2 marks)

• Energy flow: Abiotic factors fix the amount of solar energy captured by producers. The food chain runs phytoplankton → zooplankton → small fish → large fish → birds, with only about 10% energy transferred at each trophic level. Nutrient-rich, well-lit ponds support longer chains and higher productivity.
• Nutrient cycling: Nutrients move between water, mud and organisms. Decomposers release N and P from dead matter; these are reused by producers. In anaerobic bottom mud, nitrogen is lost as gas, while in oxygen-rich water it is recycled — so oxygen level directly controls the cycling rate.
• Productivity and balance: A pond with optimum light, temperature and nutrients reaches a steady state; disturbance of any one factor (e.g. sewage inflow raising nutrients) upsets the balance, causing eutrophication, oxygen depletion and death of fish.

Conclusion: Ecological factors thus act as the controlling switches of the pond — they decide its structural layering (who lives where) and its functional efficiency (how energy flows and nutrients recycle). Any change in these factors is at once reflected in the pond's structure and function.

(c) Unidirectional

Energy enters from the sun, passes from producers to consumers and is lost as heat at each level; it cannot be recycled back, unlike nutrients which move in cycles.

(a) Biomass

Biomass is the total dry weight of organic matter of living organisms per unit area. Standing crop refers to the amount of living material present at a time, but the dry weight per unit area is defined as biomass.

(a) Producers

Producers capture solar energy and have the largest energy content. Only about 10% is passed on at each higher level (the ten percent law), so energy decreases at higher levels.

(b) Xerosere

Succession that begins on dry areas such as bare rock is called xerarch succession, and the series of communities is a xerosere (lithosere on rock). Hydrosere starts in water and halosere in saline conditions.

(c) Halophytic (mangrove) regions

Pneumatophores are upward-growing aerial roots with pores (pneumathodes) in mangroves such as Rhizophora, helping gas exchange in waterlogged, saline, oxygen-poor mud.

Ecological succession and hydrarch succession (hydrosere) (5 marks)

Ecological succession is the gradual, orderly and predictable replacement of one community by another in an area over time, until a stable climax community is reached.

Hydrarch succession begins in a water body such as a pond and ends in a forest on land. Its stages are:

1. Phytoplankton stage (pioneer): algae, diatoms and bacteria colonise the water. When they die they form the first organic sediment.
2. Submerged stage: rooted submerged plants (Hydrilla, Vallisneria, Chara) grow; the pond becomes shallower as silt and dead matter accumulate.
3. Floating (rooted) stage: plants such as Nymphaea and Nelumbo with floating leaves and Trapa shade out the submerged plants and speed up filling.
4. Reed-swamp (amphibious) stage: Typha, Sagittaria and Phragmites grow in the shallow water, filling it with roots.
5. Sedge-meadow (marsh) stage: sedges and grasses (Carex, Cyperus) colonise the soil; water is nearly gone.
6. Woodland stage: shrubs and trees (Salix, Populus) establish, drying the soil.
7. Climax forest: a stable mesophytic forest forms, according to climate.

Thus a water body gradually changes into dry land.

Ecological succession and primary succession on bare rock (xerosere) (5 marks)

Ecological succession is the orderly, gradual replacement of one community by another in the same area, until a stable climax community is reached.

Xerarch (primary) succession begins on a dry, bare area such as rock with no soil. Stages:

1. Crustose lichen stage (pioneer): crustose lichens grow on the rock, secrete acids and slowly weather it, forming the first thin soil.
2. Foliose lichen stage: leafy lichens trap more dust and organic matter and add more soil.
3. Moss stage: mosses grow in the soil pockets, hold water and make a thicker humus layer.
4. Herbaceous stage: annual and then perennial grasses and herbs establish, deepening the soil and shading the rock.
5. Shrub stage: shrubs replace the herbs.
6. Climax forest: trees form a stable community suited to the climate (mesophytic forest).

At each stage the community modifies the habitat and makes it less suitable for itself but more suitable for the next stage, so the dry area gradually becomes a forest.

Ecological significance of lichens as pioneer species (5 marks)

Lichens are a symbiotic association of a fungus (mycobiont) and an alga or cyanobacterium (phycobiont). They are the pioneer species in primary succession on bare rock because:

1. They tolerate harsh conditions: the fungus gives attachment and absorbs water, while the alga makes food by photosynthesis, so lichens can survive on bare rock, drought and extreme temperature.
2. Rock weathering: they secrete lichen acids (e.g. oxalic acid) that dissolve and break down the rock surface.
3. Soil formation: dead lichen remains mix with rock particles to form the first humus and soil.
4. Trapping dust and water: the thallus holds dust, moisture and organic matter, making a base for mosses.
5. Nitrogen enrichment: cyanobacterial lichens (e.g. Peltigera) fix atmospheric nitrogen and enrich the soil.

By preparing soil, lichens allow the later stages, mosses, herbs and finally forest, to establish in the succession.

Lichens as pollution indicators and types of lichens (8 marks)

Lichens as biological indicators of pollution

Lichens absorb water and minerals directly over the whole thallus from the air, and they have no cuticle or roots to filter them. As a result:

1. They readily accumulate pollutants, especially sulphur dioxide ($\text{SO}_2$), which destroys chlorophyll of the algal partner.
2. They grow slowly and are very sensitive to air pollution, so they disappear from polluted areas (for example, near industry and cities).
3. Their presence, abundance and species type shows air quality: Usnea (fruticose) shows clean air, while crustose forms such as Lecanora tolerate some pollution.

Thus lichens are used to monitor air quality, and a "lichen desert" indicates heavy pollution.

Types of lichens based on thallus morphology

1. Crustose lichens: thallus is flat and crust-like, tightly attached to the substrate (rock, bark) so it cannot be removed without damage. Examples: Graphis, Lecanora, Rhizocarpon.
2. Foliose lichens: thallus is leaf-like and lobed, loosely attached to the substrate by rhizines. Examples: Parmelia, Physcia, Peltigera.
3. Fruticose lichens: thallus is branched, shrub-like or hair-like, standing upright or hanging, attached only at the base. Examples: Usnea (old man's beard), Cladonia, Ramalina.

Pyramid of energy and why it is always upright (5 marks)

A pyramid of energy shows graphically the amount of energy (kcal/m²/year or J/m²/year) available at each trophic level of a food chain, with producers at the base and top carnivores at the apex.

Example: producers $\to$ herbivores $\to$ carnivores $\to$ top carnivores shows, say, 10,000 $\to$ 1,000 $\to$ 100 $\to$ 10 units.

Why it is always upright

1. Ten percent law (Lindeman): only about 10% of the energy at one trophic level passes to the next. The rest is lost as heat in respiration, as undigested waste and in the energy used for body activities.
2. Energy flow is unidirectional: energy cannot be recycled, and each successive level therefore receives less than the one below.
3. Since the base always has the most energy and each higher level has less, the pyramid is always upright, whatever the ecosystem (grassland, pond or forest). It cannot be inverted, unlike pyramids of number and biomass.
4. The loss of energy also explains why food chains are short (usually 4 to 5 levels).

Ecological adaptation and adaptations of hydrophytes (5 marks)

Ecological adaptation is the structural, physiological or behavioural modification that enables organisms to survive and reproduce in a particular habitat.

Hydrophytes are plants that grow in water or in very wet soil (e.g. Hydrilla, Eichhornia, Nymphaea).

Morphological adaptations

1. Roots: poorly developed or absent in submerged plants (Ceratophyllum); roots are fibrous and have a root cap-like root pocket in Eichhornia.
2. Stem: long, thin, flexible and spongy, which helps floating.
3. Leaves: submerged leaves are thin, narrow and ribbon-like or dissected (Hydrilla); floating leaves are broad and flat with stomata on the upper surface (Nymphaea). Petioles of some are swollen for floating (Eichhornia).

Anatomical adaptations

1. Cuticle is thin or absent; the epidermis absorbs water, minerals and gases directly.
2. Aerenchyma: large air cavities in cortex store air, provide buoyancy and help gas exchange.
3. Mechanical tissue (sclerenchyma) is poorly developed, and vascular tissue, especially xylem, is reduced.
4. Stomata are absent in submerged leaves and only on the upper surface of floating leaves.
5. Chlorenchyma is present in the epidermis in submerged leaves.

Ecological adaptation and structural adaptations of xerophytes (5 marks)

Ecological adaptation is a heritable structural or functional modification that helps an organism survive and reproduce in its particular habitat.

Xerophytes are plants of dry habitats (deserts), such as Opuntia, Euphorbia, Nerium and Casuarina. Their structural adaptations help to absorb more water and reduce water loss.

Root and stem

1. Roots: very long, deep and extensively branched tap roots reach water deep underground; some have shallow, spreading roots.
2. Stem: in succulents (Opuntia), the stem becomes fleshy, green and photosynthetic (phylloclade) and stores water in mucilaginous parenchyma.

Leaves

1. Leaves are reduced to spines or scales (Opuntia, Euphorbia), or small and rolled (Ammophila), which reduces transpiration.
2. In Nerium, stomata are sunken in cavities (stomatal crypts), lined with hairs.

Anatomical features

1. Thick cuticle and a multilayered epidermis, with wax deposits.
2. Dense hairs on the surface trap moisture.
3. Sunken stomata, fewer in number.
4. Well-developed sclerenchyma and a thick vascular system; water storage tissue present in succulents.