No living organism in this world can live by itself. In any habitat one organism is dependent upon the other for its welfare. Even man influences, and is influenced by, other organisms. Animals and plants depend on one another. This is known as interdependence. Due to this, a delicate equilibrium is maintained in nature. Green plants (producers) produce food with the help of abiotic components. All animals (consumers) depend upon this food directly or indirectly. The dead bodies of animals and plants are decomposed by decomposers like bacteria and fungi. The decomposers convert the dead bodies into simple organic substances and return them to the soil.
Thus there is a cyclic passage of key elements (such as carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur) between living organisms and the nonliving (abiotic) components of the environment. This is called biogeochemical cycle.
There are different types of animal associations ranging from casual interactions to deep seated ones.
1. Symbiosis or mutualism
When two organisms live together in such a way that both of them are benefited by the mutual exchange from each other, it is known as symbiosis or mutualism. Lichens are very good examples of mutualism. Each lichen has two partners i.e., green algae and nongreen fungi. Green algae produce food and the fungal partner fives mineral, nitrogen and water.
Rhizobium, a nitrogen fixing bacterium, lives in the root nodules of leguminous plants. In the roots of plant, bacteria get various nutrients and food for their growth. In return bacteria fix atmospheric nitrogen and convert it into nitrates. The nitrates act as a good fertilizer and help the plant in us growth and development similarly, some nitrogen fixing blue green algae like Nostoc and anabaena live in the cells of the leaves of fern, Azolla. The fern provides organic substances to algae, whereas, the algae supply nitrogen to the fern.
Parasitism is a mode of life. A parasite is an organism with a peculiar lifestyle. It lives in the body of another organism (host) for food, shelter and reproduction. The host does not get any benefit from the parasite. On the contrary, harm is caused to the host. This harm may be reflected in the form a disease. Hence, parasites are called pathogens. For example, Plasmodium is a single celled parasite which grows and multiplies in the liver and red blood cells (RBC) of man. Plasmodium is infected into the human blood by a female mosquito when the latter inserts in mouth parts in to a persons body for sucking blood. Plasmodium multiplies in the persons red blood cells causing malaria. The tapeworm is a multicellular parasite. It enters the body of man along with food. The larvae of the tapeworm are present in his meat of cattle or pig. When their semi cooked meat is eaten, the larvae are brought to the intestine of man where they become adult and begin to reproduce, resulting in harm to man. Cattle mites and dog mites are e3xamples of parasites which live on outer parts of the exterior parts of the host’s body are called ectoparasites and those living inside the host’s body are called endoparasites.
Viruses, bacteria, some fungi, higher plants and different animals show the phenomenon of parasitism. An angiosperm plant Cuscuta grows as a parasite on shrubs and trees. Due to lack of chlorophyll it cannot synthesize food. It produces haustoria which penetrate the stem tissues of the host plant and develop direct contact with host’s vascular tissue for drawing nourishments.
Protocooperation is a type of interdependence similar to symbiosis but it is not obligatory. In this association each species tends to depend on the other for some advantages. The partners in this relationship are not so intimate and do not appear as a single entity observed in symbiosis or mutualism. For example, an erget bird takes rides on a water buffalo and eats lice, ticks and other insects found on the skin of the water buffalo. Here the bird gets food in the form of insects and the buffalo is freed from irritating insect bites. Similar association can be seen between tick birds and rhinoceros.
In Africa, ostriches accompany herds of zebras and antelopes. These birds inform the zebras about the forthcoming danger and in return they get food from the herds. Let us also take the example of crab and sea anemones (a coelenterate). Sea anemones grow on the body of crabs. The stinging cells of the sea anemone scare the enemies of the crabs. Sea anemone, a fixed coelenterate, is shifted from place to place with the movement of the crab and thus gets better opportunities to find food.
In this type of interaction a big species regularly consumes or eats another species. For example, a tiger eats deer, a snake eats rat, an eagle eats rabbit and mouse are known as prey. The predator is usually the larger animal.
In this type of association, one organism is benefited while the other remains unaffected and there is no involvement of food. For example, woodpeckers peck holes for nesting in tree trunks. After they have abandoned the holes, some other small birds such as bluebirds use them for nesting. A crab gets protection from the empty shell of a moll use. The association of soft bodied crab (Hermit crab) and an empty mollusc shell is a good example of commensalism.
It is a type of interaction between two organisms in which both try to obtain the same benefits in any habitat. The competitors may belong to the same species or to different species. For example, in the tropical evergreen forests, the thickly populated plants compete for light. The competition among plants mainly occurs for water, nutrients and light, and among animals it is for food, shelter and mating. This controls the population of a species.
Living organisms interact with climatic conditions in a habitat. If there are more trees in a region, such a habitat would be darker and cooler. The soil conditions of this habitat become favorable for the growth of a large number of life forms such as mosses, ferns, herbs, shrubs, insects, earthworms, etc. actually the trees and their leaves influence the temperature and humidity in a habitat. If the trees and plants are reduced in a habitat, there would be less moisture in the area. The soil would become barren and there would be less rainfall. Atmospheric temperature would increase due to shortage of water. This habitat would no longer be suitable for living forms because of such depleting changes.