Ecosystem and Food Chains: Components, Levels and Pyramids

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 B, Raymond Lindeman, who stated the law in 1942. It says that only about ten per cent of the energy present at one trophic level is passed on to the next, since the rest is used in respiration and other life processes and is lost to the surroundings as heat. It explains why a food chain rarely has more than four or five links.

Option A, A. G. Tansley, coined the term ecosystem in 1935. Option C, Odum, wrote the standard textbooks of ecology and is associated with the study of ecosystem structure and function rather than with this law. Option D, Ernst Haeckel, coined the word ecology itself in the nineteenth century. Learn the three names as a set: Haeckel for ecology, Tansley for ecosystem and Lindeman for the ten per cent law.

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.

The correct answer is B, detritus food chain. Detritus is dead organic matter such as fallen leaves, dead bodies and animal waste. Organisms like earthworms, termites, bacteria and fungi feed on it, and they in turn are eaten by small birds and other animals. In many ecosystems, and especially in forests, a larger share of energy flows through this chain than through the grazing chain.

Option A, the grazing food chain, starts with living green plants, as in the sequence grass, grasshopper, frog, snake. Options C and D name no standard category of this classification, although parasitism and predation are both real relationships within an ecosystem. The two food chains do not run separately: they are linked, because the waste of the grazing chain becomes the starting material of the detritus chain.

The correct answer is C, omnivores. Bears, crows, pigs and human beings eat both plant and animal food, which allows them to occupy more than one trophic level at the same time and makes them a good example of why food webs, rather than single food chains, describe nature better.

Option A, herbivores, feed only on plants and are the primary consumers of a food chain; the cow, the deer and the grasshopper belong here. Option B, carnivores, feed on other animals and form the higher trophic levels; the frog, the snake and the tiger are examples. Option D, autotrophs, are not animals at all: the word means the producers, such as green plants and algae, which make their own food by photosynthesis. Examiners often ask which trophic level a named animal occupies, so read the food chain in the question carefully before answering.

The correct answer is B, inverted. In a pond the producers are phytoplankton, tiny floating plants whose total weight at any one moment is small. They multiply very rapidly, however, so over time they can feed a far greater mass of zooplankton and fish. Measuring the biomass present at a single moment therefore gives a pyramid that stands on its head.

Option A is true of a forest, where the great mass of trees at the base supports a much smaller mass of animals. Option C describes no real pyramid. Option D is wrong because the pyramid of energy remains upright in the pond as well; energy lost as heat at each transfer ensures that it can never be inverted. Keep the exceptions ready: numbers inverted in a tree ecosystem, biomass inverted in a pond, energy never inverted.

The correct answer is B. Gross primary productivity is the total amount of energy that producers fix by photosynthesis. Plants use a part of it for their own respiration, and what remains stored in plant bodies is the net primary productivity. That remainder is the food actually available to herbivores, so it decides how much life the ecosystem can support.

Option A adds respiration instead of subtracting it and would give a figure larger than the energy actually captured. Option C is wrong because green plants capture only a small fraction of the sunlight that reaches them, the rest being reflected or passing through. Option D describes the standing biomass of consumers, which is a stock at a moment in time, not a rate of production. Productivity is always expressed per unit area per unit time.

The correct answer is B, biomagnification. DDT and mercury compounds are not broken down in the body and are not excreted, so they build up in the tissues of every organism that takes them in and are passed on to whatever eats it. The concentration therefore rises at each step, and top carnivores such as fish-eating birds and human beings receive the largest dose; in birds it caused eggshells to become thin and break.

Option A, eutrophication, is the excessive growth of algae in a water body enriched by fertilisers and sewage, which strips the water of oxygen. Option C, ecological succession, is the gradual change of a community over time until it reaches a stable climax stage. Option D, biodegradation, is the breaking down of substances by microorganisms, which is exactly what DDT resists.