Individual differences and the dynamics of animal populations

All population fluctuate in size from year to year. An understanding of the causes of these changes in size is helpful when managing and conserving populations and has interested biologists for more than three centuries. Until recently, understanding changes in population size, i.e. population dynamics, has concentrated on investigating the changes in numbers alone, ignoring differences between individuals. Over the last few years there has been an increasing realisation that differences between individuals in age but also in traits like body size or condition crucially affect the way populations respond to changes in the environment over time. In addition, biologists have realised that evolutionary change can happen much faster than had previously been appreciated and that ecological and evolutionary change can happen simultaneously. This means that an ability to understand population dynamics, necessary to predict and manage populations, may require understanding of the way traits change in response to ecological and evolutionary pressures. Understanding dynamics may therefore require insight into how ecological and evolutionary processes are linked. Because all ecological and evolutionary change is determined by differences in birth and death rates between groups of individuals, we can use data on the survival and fertility rates of individuals living within a population to marry changes in trait distributions to changes in population size. This is what we will do in this grant. What will the work we do deliver? Take a concrete example: harvesting large individuals from a population will affect more than just the numbers of adults: it will alter the way the animals compete for resources, allowing smaller individuals greater access, perhaps allowing them to grow or reproduce more; it is also likely to alter average reproductive rates as larger individuals may reproduce more, or give rise to higher quality offspring. Harvesting will also alter the selection pressure on individuals, making it more beneficial to mature earlier and at a smaller size. Thus harvesting's effects are more profound than simply the removal of some individuals leaving all others unchanged. We expect that our work will allow us to understand how (1) selectively removing specific individuals from a population is likely to impact the dynamics of the population, and (2) how different types environments lead to changes in the distribution of traits like body size. This will provide some information on how we might expect changes in the climate to influence both the evolution of traits like body size, as well as fluctuations in population size. The approach we will take has never been applied to animals. We will use data from four contrasting animal species / a monogamous bird, the silvereye; free-living Soay sheep; group-living meerkats; and laboratory populations of soil mites. These systems have been chosen because previous research has provided a good understanding of many aspects of their ecology, because detailed data exist and because they have very different life histories and ecologies. By investing a range of species simultaneously, we will also be able to get a feeling for the generality of our conclusions and the degree to which we need to develop joint understanding of the way numbers and traits vary.