✅ Answer: A) 1 and 2 onlyStatements 1 and 2 are correct. A food web is more stable than a food chain because it has multiple pathways — if one species disappears, energy can flow through alternative routes. Pyramid of energy is always upright because energy always decreases up the food chain (10% law). Statement 3 is incorrect — decomposers ARE part of food chains (detritus food chain). They break down dead organic matter and recycle nutrients.

✅ Answer: B) A.G. TansleyA.G. Tansley coined the term “Ecosystem” in 1935. Ernst Haeckel coined “Ecology” in 1866. R.L. Lindeman proposed the 10% law of energy transfer in 1942. E.P. Odum is called the “Father of Modern Ecology” for his comprehensive study of ecosystems in his book “Fundamentals of Ecology” (1953). The ecosystem concept is fundamental to understanding how nature works as an integrated system.

✅ Answer: C) Aquatic (pond/ocean) ecosystemThe Pyramid of Biomass is inverted in aquatic ecosystems because phytoplankton (producers) have very small individual biomass but reproduce rapidly. At any given time, the biomass of zooplankton (primary consumers) may exceed that of phytoplankton. However, phytoplankton produce more biomass over time. The Pyramid of Numbers is inverted in tree ecosystems (one tree supports many insects). Pyramid of Energy is always upright.

✅ Answer: B) AzotobacterAzotobacter is a free-living (non-symbiotic) aerobic nitrogen-fixing bacterium found in soil. Rhizobium is a symbiotic nitrogen fixer (lives in legume root nodules). Clostridium is a free-living anaerobic nitrogen fixer. Cyanobacteria (Anabaena, Nostoc) are free-living nitrogen fixers in water and soil. Nitrosomonas converts ammonia to nitrites (nitrification). Pseudomonas converts nitrates to N₂ (denitrification).

✅ Answer: D) PseudomonasDenitrification (converting nitrates back to N₂ gas) is carried out by Pseudomonas and Thiobacillus denitrificans. Nitrogen cycle steps: N₂ fixation (Rhizobium, Azotobacter) → Ammonification (decomposers) → Nitrification: Nitrosomonas (NH₃ → NO₂⁻) + Nitrobacter (NO₂⁻ → NO₃⁻) → Assimilation (plants absorb NO₃⁻) → Denitrification (Pseudomonas: NO₃⁻ → N₂). Denitrification returns N₂ to atmosphere.

✅ Answer: A) Lichens → Mosses → Herbs → Shrubs → Trees (Climax)In primary succession on bare rock: Lichens (pioneer species) colonise first — they secrete acids that break down rock and create soil. Then Mosses grow, followed by Herbs, Shrubs, and finally Trees (climax community). This process takes hundreds to thousands of years. Lichens are the pioneer species because they can survive on bare rock without soil. They are a symbiosis of fungi and algae.

✅ Answer: B) OceansOceans are the largest carbon sink on Earth, absorbing about 25-30% of CO₂ emitted by human activities. Phytoplankton in oceans fix CO₂ through photosynthesis. CO₂ also dissolves directly in seawater. Tropical rainforests are the largest terrestrial carbon sink. Soil is also a significant carbon store. Permafrost contains large amounts of frozen organic carbon — its melting due to climate change could release massive amounts of CO₂ and methane.

✅ Answer: D) SunlightSunlight is an abiotic (non-living) component of an ecosystem. Biotic components include: Producers (plants, algae), Consumers (herbivores, carnivores, omnivores), and Decomposers (bacteria, fungi). Abiotic components include: sunlight, temperature, water, soil, air, minerals, pH, humidity. The interaction between biotic and abiotic components defines an ecosystem. Sunlight is the primary energy source for most ecosystems.

✅ Answer: B) The functional role and position of an organism in its ecosystemAn ecological niche is the functional role and position of an organism in its ecosystem — including what it eats, what eats it, where it lives, and how it interacts with other species. “Habitat” is where an organism lives (its address). “Niche” is what it does (its profession). No two species can occupy the same niche in the same habitat (Gause’s Competitive Exclusion Principle). Species with similar niches compete for resources.

✅ Answer: B) Dead leaves → Earthworm → Robin → HawkA detritus food chain starts with dead organic matter (detritus). Dead leaves → Earthworm (detritivore) → Robin → Hawk is a detritus food chain. Detritivores (earthworms, millipedes, woodlice) feed on dead organic matter. Decomposers (bacteria, fungi) break down organic matter into inorganic nutrients. Detritus food chains are important in forest ecosystems and recycle nutrients. Options A, C, D are grazing food chains (starting from living producers).