Subtopic 1: Abiotic Factors and Organism Responses
Abiotic factors are the non-living physical and chemical components of an ecosystem. Temperature (most relevant ecologically) affects enzyme activity and metabolic rates, setting latitudinal and altitudinal limits on species distributions. Water restricts productivity in deserts. Light drives photoperiodism (day-length-triggered flowering, migration, and reproduction). Soil characteristics (pH, mineral content, texture) govern plant community composition.
Organism responses to abiotic stress: Regulators actively maintain internal homeostasis (endothermic mammals and birds thermoregulate); Conformers allow internal conditions to change with the environment (most invertebrates, fish, amphibians, reptiles); Migrants escape unfavourable seasons by relocating (migratory birds); Suspenders halt normal activity through dormancy — hibernation (winter), aestivation (summer), diapause (zooplankton, insects).
Subtopic 2: Population Attributes
Population density (N) is measured by direct census, quadrats (plants, sessile animals), or mark-recapture (mobile animals). Natality (b) and mortality (d) together determine the growth rate: r = b − d. Sex ratio and age distribution determine future population trajectory. Age pyramids: broad-based triangle = expanding; uniform column = stable; narrow-based urn = declining.
Subtopic 3: Population Growth Models
Exponential growth: dN/dt = rN; Nt = e^(rt). Occurs when resources are unlimited. J-shaped curve; no upper population limit. Logistic growth: dN/dt = rN. Occurs when resources are limited; population approaches K asymptotically. S-shaped (sigmoid) curve. Key insight: maximum growth rate at N = K/2 (inflection point).
Subtopic 4: Population Interactions (Six Types)
| Interaction | Signs | Example |
|---|---|---|
| Mutualism | +/+ | Rhizobium–legume, lichen, mycorrhiza |
| Competition | −/− | Gause's Paramecium; Abingdon tortoise displaced by goats |
| Predation | +/− | Lion–deer; chemical defence (Calotropis glycosides) |
| Parasitism | +/− | Plasmodium–human; cuckoo–crow (brood parasitism) |
| Commensalism | +/0 | Orchid on mango; cattle egret with cattle |
| Amensalism | −/0 | Penicillium inhibits Staphylococcus |
Subtopic 5: Ecosystem Productivity
GPP = total photosynthetic energy fixation. NPP = GPP − Respiration = energy available to herbivores. Most productive ecosystems: tropical rainforests > coral reefs > estuaries. Least productive: open ocean deserts.
Subtopic 6: Decomposition
Five steps — Fragmentation → Leaching → Catabolism → Humification → Mineralization (FLCHM). Detritivores (earthworms, millipedes, dung beetles) perform fragmentation. Bacteria and fungi perform catabolism via extracellular enzymes. Warm, moist, nitrogen-rich, low-lignin conditions accelerate the process.
Subtopic 7: Energy Flow
Unidirectional; not recyclable. Lindeman's 10% law: ~10% transferred per trophic level, ~90% lost as heat. Grazing food chain: plants → herbivores → carnivores. Detritus food chain: dead matter → detritivores → decomposers. Food webs provide resilience and stability.
Subtopic 8: Ecological Pyramids
Energy pyramid: always upright (invariant rule). Numbers pyramid: upright (grassland); inverted (tree ecosystem). Biomass pyramid: upright (terrestrial); inverted (aquatic — low phytoplankton standing stock despite high turnover).
Subtopic 9: Nutrient Cycling
Carbon cycle: photosynthesis → respiration/decomposition → fossil fuels → ocean (71% reservoir). Phosphorus cycle: sedimentary only — no gaseous phase. Rock weathering → soil → organisms → sediment.
Subtopic 10: Ecological Succession
Primary succession: bare substrate → pioneer lichens → mosses → herbs → shrubs → climax forest (centuries to millennia). Secondary succession: disturbed soil with seed bank → faster recovery (decades). Hydrosere: aquatic to terrestrial succession. Climax community = stable equilibrium state.