The teeming multitudes

Ecology is a science with counting at its core. In some areas of the subject, such as conservation ecology, where the aim is to keep the numbers up, or pest management, where we aim to keep numbers down, the role of numbers is clear. Even in behavioural ecology, where local density might affect a strategy, or in physiological ecology, where resource acquisition is influenced by interactions, the numbers are important. It is population ecology that provides the theoretical underpinning to allow us to understand changes in numbers of individuals through time from the past to predictions of the future.

As population ecology is so important to ecologists, it is unfortunate that there have been long periods with no new textbooks. Even blockbuster ecology textbooks from both sides of the Atlantic have traditionally devoted few pages to the subject. In the past few years, however, this trend has changed, with several texts to choose from.

Larry Rockwood is a good candidate to write such a text, as he is a very experienced US-based teacher of the subject. Unsurprisingly, we get an 11-chapter book that would fit very well with two lectures and a practical session for each. The progression is the standard one, and this is not a criticism, as it is difficult to see how the approach could be improved. We start with individuals living without restrictions on their growth and reproduction. These assumptions lead to exponential growth and predictions of vast numbers of individuals. We then consider what factors prevent the world from being overrun by the organism. There are obviously resources that are limiting: light, space, nutrients and so on. As a population expands, those resources are in shorter supply and either the death rate increases or the birth rate declines. There follows a brief chapter on regulation, including an introduction to the debate of the relative importance of density-dependent factors, such as competition for light, and density-independent factors, such as environmental catastrophe.

The next complication is to consider variation between individuals in a population. This is often achieved, as it is here, through a life table approach. For example an individual does not have equal fecundity throughout its life. In most animals, reproduction carries on until death, although humans have a long post-reproductive phase. Rockwood uses human examples to illustrate how this changing reproductive output can be combined with mortality rates to predict population size or population growth rates. We're then led through matrix algebra and calculus to extract even more from these fecundity and mortality schedules.

The final two chapters on single-species populations sit more uncomfortably in the progression. First, there is a consideration of the spatial arrangement of populations using some basic metapopulation ideas followed by a consideration of the evolutionary forces affecting life tables through life history analysis. Midway through the book/ course, we move from consideration of single-species populations to more than one species. There is a chapter each on inter-specific competition, predator-prey interactions and mutualisms, followed by a one on plant-herbivore relationships. This last makes for an unusual conclusion, as most authors would subsume these into predator-prey modelling.

One of the interesting aspects of Rockwood's presentation is that stochasticity, or random variation of various kinds, is considered at several points. I support this inclusion entirely as it moves population ecology away from the static, "balance of nature" view of ecology towards the more dynamic subject that it should be. For many of the single-species scenarios we see the results with and without stochasticity. But in the multi-species sections this is missing, which is a pity.

Rockwood does provide us with an introduction to population ecology as the title promises. It covers the ground quite well and the style is engaging with some interesting examples (many of which can be found on the Blackwell website). It's not innovative or radical, but that's not what was needed. I suspect it will be adopted as a set text by a large number of population ecology courses.