Each species of organism – microbe, animal, or plant making up the community is obviously influenced in its existence and activities not only by the inanimate physical and chemical environment but by other species of the community as well.
This interdependence among different species is usually for food, reproduction and protection.
(A) Interdependence for Food:
Charles Elton described communities in terms of their feeding relationships. In every stable community, green plants produce their own food; herbivorous animals at the plants and carnivorous animals eat each other or herbivores.
The elimination products and the dead bodies of plants and animals replenish the soil or the ocean; and this, plus solar energy and raw materials from the environment, then make new plant growth possible. There are, in fact, chains of animals linked together by food, and all dependent in the long run upon plants. We refer w these as “food chains”, and to all the food chains in a community as the ‘food cycle’ (Elton, 1927). Now the term “food cycle” has been replaced by the term “food web”.
(B) Reproductive Interdependence:
A second link between the members of a community is reproductive interdependence. A familiar and important example of this is the pollinating activity insects. In some well-known cases of remarkable specialization a given insect visits only one m a few specific flower types for pollen and nectar. The flowers (e.g., Snapdragons) in turn are structurally adapted to facilitate entry of the insect. Such intimate reciprocity testifies to a closely co-related evolutionary development of animal and plant.
It is fairly obvious how such interdependence contributes to population balance: reduction of the insect population entails reproductive restriction of the plant, and vice-versa. Similarly significant in balancing the reproductive growth of plant population is the seed-dispersing activity of birds and mammals, and man in particular.
Other examples of reproductive interdependence are many. Birds such as cuckoos lay their eggs in nests of other birds. Insects such as gall wasps embed their eggs deep in the tissues of particular plants, where the hatching larvae find food and protection. Other insects deposit eggs on or under the skin of various animals. Certain wasps kill tarantulas and lay their eggs in them. Reproductive growth and geographical expansion of a community are intimately correlated with nutritional balances.
(C) Protective Interdependence:
The third main link among the members of a community B protective interdependence. Plants of forest and grassland usually protect animals by providing shelter against enemies and adverse weather. If the opportunities for protection are reduced, both the animal and the plant population may suffer. For example, if an overpopulation of insects makes available plant shelter inadequate, the insects will become easier prey for birds and bats. But this condition may also reduce the plant populations, for their major pollinating agents may no longer be sufficiently effective.
Animals in many cases are protected from other animals by camouflage. Such a protective device may involve body colour, or body shapes or both. Probably the most remarkable instance of colour camouflage is the phenomenon of mimicry, widespread particularly among butterflies and moths. In certain of these animals, pigmentation patterns exist which are virtually indistinguishable from those of other unrelated species.
Usually those species are mimicked which are strong and have a few natural enemies. The advantage is that an animal resembling even superficially another more powerful one will be protected too, by scaring off potential predators. Insects also display a variety of camouflages. For example, the individuals of certain species possess the detailed shape of leaves, or branches, or of thorns. A praying mantis, coloured green, resembles a thin stem and a leaf insect and leaf butterfly have a striking resemblance to a leaf on which they habitually live.
Other protective devices vary widely in type. Various birds and some mammals mimic the song and voice of other species, either defensively or as an aggressive lure. The hermit carb protects its soft abdomen in empty shells of appropriate size. Schools of small pilot fish scout ahead of large sharks, leading their protectors to likely prey. Significant protection to various animals is also afforded by man, through domestication, game laws, parks, and sanctuaries.