This chapter builds on the previous one by cataloguing the world's ecosystems, describing major types of terrestrial and aquatic environments and how these are shaped by prevailing environmental conditions. For reasons of space, the coverage of some subjects in this chapter is rather superficial and you may already be familiar with parts and wish to skip over them. Guidance is given to further reading for those who wish to study particular aspects in more detail.

By reading this chapter, students will gain an understanding of some major terms used in describing the natural world; of the major types of ecosystems that presently exist and how major environmental factors dictate the distribution of ecosystem types.

The ecosystem concept

Anecosystem is a community of living organisms together with the physical processes that occur within an environment. Allorganisms are faced with environmental variables to cope with. These are usually divided into abiotic factors, including the broad climate and geology as well as specificfactors such as temperature,water (rainfall and humidity),light, salinity, pressure and soil and water chemistry (pH and mineral content), and biotic factors, which are interactions with other organisms, including competition, predation, parasitism and symbiosis. Thus there are abiotic (non-living) and biotic (living) components of an ecosystem, all potentially interacting to forma functioning unit, distinguishable, although not isolated, from other ecosystems. The concept of the ecosystem is central to our understanding of the natural world. Ecological studies have shown how energy flows through ecosystems, from the capture of light energy by plants and conversion to the chemical energy in sugars, to its passage through successive trophic levels and constant escape back into the environment (Fig. 2.1).