The main characteristics of organisms from the various kingdoms of life were selected through evolution because they adapt the organisms to their physical and biotic environments. As biologists Marc Johnson and John Stinchcombe point out, we now realise that genetic variation and evolution within a species influence not only that species but also others with which it interacts and the surrounding environment. Thus the study of the interactions within and between species and their environment, collectively called ecology, is also the study of evolution in action. This is the topic of this theme.

We consider interactions between organisms of the same species as they build up a population and then examine the interactions between different species living together and forming a community. Finally, we cover the main characteristics of marine, freshwater and terrestrial environments and how the interactions between the communities within them and the physical environment form characteristic ecosystems. We need different techniques to study the microbes, plants and animals in the ecosystems and we describe some of them, as well as some of the instruments available and the ways of analysing and presenting data.