A unified approach to studying animal abundance: integrating evolution, ecology and scale dependency

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Questions about the abundance of animal populations are as old as the science of ecology. Answers include 'tigers are rare because they are big' and 'Dartford warblers are rare because they are not suited to the British climate'. These are actually answers to different questions: the first is about the relative abundance of different species; the second is about variation among populations of one species. However, the two are fundamentally linked because the pattern of relative abundance varies from place to place. For example, Dartford warblers may be rare in England but are relatively common in France. Biologists aim to understand both types of variation but have been forced to consider each in isolation. In my research, I use new tools that allow both types of question to be addressed within a single framework. For the first time it is possible to model in the same analysis the variation in abundance of multiple species in many places. It is now possible to compare the importance of evolution and ecology in determining which species should be common or rare and why abundance varies from place to place. My research takes advantage of these new methods to ask questions about the abundance of mammals and British butterflies. Mammals are an excellent group on which to conduct studies of this type, because they are well known, they cover the entire globe, they include both small and large animals and they display a great variety of diets and ways of life. Butterflies are also an ideal group for studying abundance, due to the systematic records from across the UK that date back over 20 years. These data were collected by a large volunteer network and have become one of the most valuable datasets in biology. Analysing abundance data in a single framework means that it becomes possible to ask several types of questions at the same time. We can ask whether large-bodied species always less common and which other evolutionary factors are important. At the same time, we can ask whether there more animals are found in warm places compared with cold ones. Also, we can tackle more complicated issues, like why some species respond well to human intervention, whereas others respond poorly. Evolutionary and ecological questions like these have rarely been addressed together. Finally, the new approach allows me to address questions about spatial scale. For example, are species that are locally sensitive to humans more likely to be at risk of global extinction? Relatively little is known about whether processes operating at small scales can account for patterns at larger scales, so the results should be of interest to a wide group of scientists.