All creatures great and small

The animal kingdom includes many large and beautiful creatures. It is easy to engage the public's sympathy for the plight of endangered animals such as tigers and pandas, or, closer to home, Tasmanian devils and numbats, and these animals are often the focus of intense conservation efforts. Parasites, on the other hand, live in or on other organisms. They are typically small, hard to see and often harm their hosts, so they are usually either ignored or regarded as a nuisance in practical conservation programs. This is a simplistic view. Parasites have vital roles to play in the functioning of natural ecosystems. In recent years it has been realised that many parasites are valuable indicator species, and their disappearance from an ecosystem is often a symptom of a deeper, underlying problem resulting from pollutants or other human impacts.

Particularly important as indicators of environmental quality are parasites with complex life cycles, that travel through a range of different host species on their way from egg to adult. Figure 13.1, for example, shows the life cycle of a species of flatworm that lives in freshwater ecosystems in Australia. A parasite such as this may be a very sensitive indicator of environmental quality. The parasite and its various hosts are bound in a complex web of feeding interactions. They all have very different body structures and very different lifestyles and the environment in which they live must support all of these.