The ten per cent law of energy transfer in a food chain was given by

The correct answer is A, A. G. Tansley. The British botanist Arthur George Tansley used the word ecosystem in 1935 for a unit in which living organisms and their non-living surroundings are treated together as one system exchanging energy and matter. The idea is the foundation of modern ecology.

Option B, Charles Darwin, is remembered for the theory of evolution by natural selection, set out in his work on the origin of species. Option C, Raymond Lindeman, gave the ten per cent law of energy transfer in 1942 and is the name to remember for energy flow, not for the term itself. Option D, Charles Elton, worked on food chains and ecological pyramids, and the pyramid of numbers is often named after him. These four names are regularly offered together in this question.

The correct answer is A, producers. Green plants make their own food from carbon dioxide and water by photosynthesis, using the energy of sunlight, so they are also called autotrophs. They form the first trophic level, and all other organisms in the ecosystem depend directly or indirectly on the food they prepare.

Option B, primary consumers, are the herbivores that feed on plants, such as the grasshopper, the deer and the cow. Option C, decomposers, are bacteria and fungi, which break down dead bodies and waste and return the nutrients to the soil. Option D, secondary consumers, are the carnivores that eat herbivores, such as the frog in the chain grass, grasshopper, frog, snake, hawk. Fix the order of that chain in your mind, since questions often ask for the level of one organism in it.

The correct answer is C, the pyramid of energy. Energy is lost as heat at every transfer from one trophic level to the next, so a higher level can never hold more energy than the level below it. For that reason the energy pyramid is upright in every ecosystem without exception.

Option A, the pyramid of numbers, is inverted in a single tree ecosystem, where one large tree supports thousands of insects and those insects support many birds. Option B, the pyramid of biomass, is inverted in a pond or in the sea, because the phytoplankton present at any moment weigh far less than the fish they support, though they multiply fast enough to feed them. Option D is therefore wrong. This is the most frequently asked point in the chapter.

The correct answer is B, decomposers, also called saprotrophs. They feed on dead bodies, fallen leaves and waste matter, break the complex organic substances into simple ones and release nutrients back into the soil, water and air, where plants can take them up again. Without them dead matter would pile up and the supply of nutrients would stop.

Option A, producers, are the green plants that prepare food by photosynthesis. Option C, herbivores, are the plant-eating animals that form the second trophic level. Option D, omnivores, are animals such as the bear, the crow and human beings, which eat both plants and animals. Remember that decomposers also start the detritus food chain, which begins with dead organic matter rather than with living plants. They are the reason nutrients circulate while energy does not, since only they can return carbon, nitrogen and phosphorus to the soil and the air.

The correct answer is A, unidirectional. Energy enters an ecosystem as sunlight, is fixed by green plants, and then moves from producers to herbivores, to carnivores and finally to decomposers. At every step a large part of it is lost to the surroundings as heat, and none of it returns to the Sun, so the flow runs in one direction only.

Option B confuses energy with nutrients. Carbon, nitrogen and other nutrients do move in cycles, because decomposers release them from dead matter for plants to use again. Option C reverses the direction: consumers depend on producers, never the other way round. Option D contradicts the ten per cent law, since only about a tenth of the energy at one level reaches the next. A short way to remember it: nutrients cycle, energy flows.