Ecology is the grammar of the whole Environment paper. Questions on protected areas, pollution or climate change all assume you know how energy moves through an ecosystem, how nutrients cycle and how species depend on one another.

What UPSC asks

Prelims: concept questions that test precision: which pyramid can be inverted, which bacteria fix or release nitrogen, what a pioneer species is, how an ecotone differs from a niche, and which interaction is which.
Mains GS III: ecology as the reasoning behind conservation: why losing a keystone species or a wetland has effects far beyond its size, and why degraded land takes decades to recover.

Key sub-topics

Sub-topic	What to master
Ecosystem structure	Biotic and abiotic components, trophic levels
Ecosystem function	Productivity, decomposition, energy flow, nutrient cycling
Food chains and pyramids	Grazing vs detritus chains; pyramids of number, biomass and energy
Ecological succession	Primary vs secondary; hydrarch and xerarch; climax
Biogeochemical cycles	Carbon, nitrogen, phosphorus; gaseous vs sedimentary
Population and community	Growth models, species interactions, niche, ecotone, keystone species

Start with the main topic page, then use the revision sheet and practice set before moving on to Biodiversity and Conservation.