Ecology: Organisms, Populations, Ecosystems and Biodiversity
Ecology in the DHSE paper rewards quantitative handling of population growth and energy flow, plus correct classification of interactions and conservation categories. Answers should name the principle, then apply it to the given data.
Organisms and populations
- Exponential growth follows dN/dt = rN; logistic growth follows dN/dt = rN(K - N)/K, where r is the intrinsic rate and K the carrying capacity.
- Interactions: mutualism (+,+), commensalism (+,0), parasitism and predation (+, -), competition (-, -). Gause's competitive exclusion principle states that the superior competitor eliminates the other.
- Adaptations include xerophytic leaves, CAM photosynthesis, endothermy versus ectothermy, dormancy and migration.
Ecosystem
- Gross primary productivity minus respiration equals net primary productivity.
- Decomposition steps: fragmentation, leaching, catabolism, humification, mineralisation.
- The 10 per cent law governs energy transfer; the pyramid of energy is always upright, while pyramids of number and biomass can invert.
- Carbon cycles as a gas; phosphorus is sedimentary and has no gaseous phase.
- Succession may be hydrarch or xerarch, moving from pioneer to climax community.
Biodiversity and conservation
- Species-area relation: log S = log C + Z log A; Z is about 0.1 to 0.2 for small areas and 0.6 to 1.2 for very large regions.
- Indian hotspots: Western Ghats and Sri Lanka, Himalaya, Indo-Burma and Sundaland.
- The evil quartet: habitat loss and fragmentation, over-exploitation, alien species, co-extinctions.
- In situ conservation includes biosphere reserves, national parks and sacred groves; ex situ includes zoos, seed banks and cryopreservation.
Worked example
Producers fix 20,000 kJ. Applying the 10 per cent law: primary consumers 2,000 kJ, secondary consumers 200 kJ, tertiary consumers 20 kJ. Three successive transfers leave only 0.1 per cent of the original energy.
Traps that cost marks
- Do not call the energy pyramid invertible; only number and biomass pyramids can invert.
- GPP and NPP are not interchangeable.
- The phosphorus cycle has no gaseous phase, so it is not atmospheric.
- Standing crop is biomass; standing state is the nutrient quantity.