Ecosystem

The second chapter of the Ecology unit and a compact, high-return topic that has appeared in every recent NEET paper, usually worth 2 to 3 marks. The NEET syllabus keeps this chapter deliberately narrow - patterns and components, productivity and decomposition, energy flow, and the pyramids of number, biomass and energy - so almost every mark is won on precision rather than breadth. The chapter covers the ecosystem as a functional unit of nature, its terrestrial, aquatic and man-made types, vertical stratification, and the pond as a self-sustaining worked example with its abiotic layer, phytoplankton and marginal producers, free-swimming and bottom-dwelling consumers and the fungi, bacteria and flagellates that dominate its floor; standing crop and the reason dry weight is the more accurate measure of biomass; primary production and the difference between production and productivity, gross primary productivity, respiratory loss and the equation GPP - R = NPP, secondary productivity, and the 170 and 55 billion tonne figures for the biosphere and the oceans; decomposition in full - detritus as raw material and the five steps of fragmentation, leaching, catabolism, humification and mineralisation, with the chemical and climatic controls that make lignin-rich litter decay slowly and warm, moist, nitrogen-rich litter decay fast; energy flow, with less than half of incident radiation arriving as photosynthetically active radiation and only 2 to 10 per cent of that captured, the grazing and detritus food chains and why the detritus chain dominates on land, trophic levels, Lindeman's ten per cent law and the unidirectional flow that the second law of thermodynamics forces; and the three ecological pyramids, the single-tree and marine cases in which number and biomass pyramids invert, the reason an energy pyramid never can, and the three standard limitations of pyramids. Six graded practice sets - two Easy, two Medium, one Hard and one NEET previous-year set - each opening with comprehensive NCERT-anchored notes.