Ecosystems

The English botanist A.G. Tansley (1871-1955) was interested in the concept of the 'super-organism'. This concept suggested that a community of individuals, such as a termite nest, containing several million individual termites, could itself be regarded as an individual, a super-organism. There could then be evolutionary competition between super-organisms. This concept had a clear implication of systems levels, with each level being called a population. For example, a forest is a population of trees, a tree is a population of leaves, a leaf is a population of cells, a cell is a population of organelles, and so on. The concept also had a clear implication of group selection.

Tansley liked the concept but not the term, and he made a memorable contribution when he coined the entirely new term 'ecosystem'. An ecosystem is a community of individuals of many different species interacting with each other and their environment, at all systems levels. An ecosystem is self-organising. It is stable, resilient, and self-sustaining.

Of particular relevance to this book is the ecological concept of r-strategists and K-strategists (MacArthur &Wilson, 1967). An r-strategist is a 'quantity breeder', which reproduces very quickly, with very many, small, cheap individuals. Aphids are an obvious example. An r-strategist is a species whose population size is governed by r, the intrinsic rate of population increase during a favourable season. An r-strategist is able to exploit an ephemeral food supply very quickly, and very effectively, by producing a population explosion. As we have just seen, this population explosion is the result of positive feedback. With the end of the favourable season, the positive feedback stops, and there is then a population extinction. Only a few individuals survive, usually in a special state of dormancy, sufficient to initiate the next population explosion. Most crop parasites are r-strategists. It is their population explosions that can be so very damaging, and so very difficult to control.

Conversely, a K-strategist is a 'quality' breeder, which reproduces only slowly, with only a few individuals, which are very valuable. K-strategists have a population size that is more or less constant and is governed by 'K', the carrying capacity of the environment. Elephants are an obvious example.

However, it would be a mistake to think that all species can be classed firmly into one or the other of these categories. There is, in fact, a continuum, a spectrum, with all degrees of difference between the extreme K-strategist and the extreme r-strategist.

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