All animals derive their food either directly or indirectly from plants. Carnivorous animals feed on other animals which them- selves feed on smaller animals but sooner or later in such a series we come to an animal which feeds on vegetation. For example, guppies eat mosquito larvae, and mosquito larvae feed on microscopic plants in the pond or lake. This kind of relationship is called a food chain. The basis of food chains on land is vegetation in general, but particularly grass and other leaves. In water, the basis is the phytoplankton the millions of microscopic plants living near the surface of the sea, ponds and lakes. These need only the water round them, the dissolved carbon dioxide and salts and sunlight to make all their vital substances. Feeding on these microscopic plants are tiny animals, zooplankton such as crustacea and the larvae of many kinds of animal.
The small animals of the zooplankton are eaten by surface feeding fish, which themselves are eaten by larger fish or by man.
diatoms>crustacea>fish>man
A comparable food chain on the land might be
grass>insect>lizard>bird
The green plants at the beginning of a food chain are some- times referred to as the producers, while the animals which eat them are called consumers. A first order consumer such as an antelope eats vegetation; a second order consumer such as a lion eats animals which feed on vegetation. Consumers Such as fungi which are involved in the process of decay are classed as decomposers.
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The plants at the start of a food chain are frequentlý small in size and very numerous. At the end of a food chain the animals are often large and relatively few in numbers. The food chain can thus be represented as a pyramid of numbers in which the horizontal width of the bands represents the numbers or, sometimes, the mass (biomass) of the organisms.
In reality, food "chains" are not so straightforward as described since a given animal, particularly a predator, does not live exclusively on one type of food, e.g. lions may hunt gazelle or zebra, young crocodiles eat water bugs, while older be crocodiles eat fish. This more complex relationship can be shown as a food "web" but even this is greatly simplified and generalized.
If the population of one of the animals in a food web is altered, all the others are affected. When the rabbits in Britain were almost exterminated by the disease myxomatosis in 1954-5, the vegetation in what had been rabbit-infested areas changed; sheep could graze where rabbits had previously eaten all the available grass and trees that were hitherto nipped off as seedlings began to grow to maturity, with the result that what had once been grassland, e.g. chalk downs, started to become scrub and eventually woodland. Foxes ate more voles, beetles and blackberries than before and attacked more lambs and poultry. Similarly, in Uganda, protection of the hippopotamus resulted in their multiplying to a point where they began to destroy their habitat and threaten their Source of food.