Introduction patterns of abundance and the need for their explanation

We turn now to the effects of predation on the population dynamics of the predator and its prey, where even a limited survey of the data reveals a varied array of patterns. There are certainly cases where predation has a profoundly detrimental effect on a prey population. For example, the 'vedalia' ladybird beetle (Rodolia cardinalis) is famous for having virtually eradicated the cottony cushion-scale insect (Icerya purchasi), a pest that threatened the California citrus industry in the late 1880s (see Section 15.2.5). On the other hand, there are many cases where predators and herbivores have no apparent effect on their prey's dynamics or abundance. For example, the weevil Apion ulicis has been introduced into many parts of the world in an attempt to control the abundance of gorse bushes (Ulex europaeus), and it has often become well established. The situation in Chile, however, is fairly typical, where, despite eating on average around half, and sometimes up to 94%, of the seeds produced, it has had no appreciable impact on gorse invasiveness (Norambuena & Piper, 2000).

There are also examples that appear to show predator and prey populations linked together by coupled oscillations in abundance (Figure 10.1), but there are many more examples in which predator and prey populations fluctuate in abundance apparently independently of one another.

It is clearly a major task for ecologists to develop an understanding of the patterns of predator-prey abundance, and to account for the differences from one example to the next. It is equally clear, though, that none of these predator and prey populations exist as isolated pairs, but rather as parts of multispecies systems, and that all these species are affected by environmental conditions. These broader issues of what determines a species' abundance are taken up again in Chapter 14. However, as with any complex process in science, we cannot understand the full complexity without a reasonable understanding of the components

- in this case, populations of predators and prey. Hence, this chapter deals with the consequences of predator-prey interactions for the dynamics of the populations concerned.

The approach will be firstly to use simple models to deduce the effects produced by different components of the interactions, teasing out the separate effects before seeking to understand those effects in combination. Then, field and experimental data will be examined to see whether the deductions appear to be supported or refuted. In fact, simple models are most useful when their predictions are not supported by real data - as long as the reason for the discrepancy can subsequently be discovered. Confirmation of a model's predictions provides consolidation; refutation with subsequent explanation is progress.

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