Organisms and the Environment Around

Ecology is the study of the interactions among organisms, and between organisms and their physical (abiotic) surroundings. The word was first used by Reiter, but it was E. Haeckel who gave the term its real substance. Ecologists organise the living world into a hierarchy of four sequentially more complex levels: organism, population, community and biome. The individual organism is the basic unit of this hierarchy. Organisms of the same kind sharing a geographical area constitute a population. Several populations of different species occupying a particular area, and interacting with one another, constitute a community. A biome is a large regional terrestrial unit delimited by a specific climatic zone, carrying a characteristic major vegetation type (plant community) and its associated animal life -- there are six major groups of terrestrial biomes on Earth. This chapter deals with the first two levels of the hierarchy, the organism and the population.

At the level of the individual organism, ecology is essentially the study of physiology -- it helps us understand how organisms are adapted to their environment, not only to survive but also to reproduce and multiply. Earth's rotation around the Sun and the tilt of its axis cause the seasons, and the resulting annual variation in temperature and in precipitation (rain and snow) gives rise to the formation of the major biomes of the Earth -- desert, rain forest, grassland, tundra, and so on (Fig. 13.1). Within any one biome, further regional and local variation leads to the formation of a whole variety of habitats. On Earth, life exists even in extreme and harsh habitats -- the scorching deserts of Rajasthan, the perpetually rain-soaked forests of the north-eastern states, and the high mountain tops of the Himalayas are all inhabited.