Geographical gradients in species diversity are almost universal across taxa. Understanding this is no longer an academic matter: given the current rapidity of global climate and land use change, we urgently need to find a working understanding of what controls diversity (Lawton et al. 1996). Despite the obviousness of patterns in diversity, this is not easy. Many explanations have been suggested over the years for diversity patterns (reviewed by Ricklefs 1987; Rohde 1992; Rosenzweig 1995; Turner, Lennon & Greenwood 1996) and it is questionable if these have lead to understanding or confusion: most of them are very difficult to test. The explanation we consider in this paper, namely the species diversity–energy hypothesis, is something of an exception, as the predictions are relatively clear and the necessary data are available: the strongest determinant of diversity should be some measure of energy.
The species–energy hypothesis
Perhaps for these reasons of relative simplicity, the species diversity–energy hypothesis has attracted much interest (Wright 1983; Currie & Paquin 1987; Turner, Gatehouse & Corey 1987; Turner, Lennon & Lawrenson 1988; Adams & Woodward 1989; Currie 1991; Currie & Fritz 1993; Wylie & Currie 1993; Wright, Currie & Maurer 1994; Fraser & Currie 1996; Turner et al. 1996). Although it has found a measure of support, there is some evidence against it, particularly as an explanation for continental-scale trends in tree diversity, where long-term historical factors may be paramount (Latham & Ricklefs 1993; McGlone 1996).
The main prediction is that local species diversity increases with energy availability. This ‘energy availability’ is usually taken (rather crudely) to mean average temperature. For example, the latitudinal temperature gradient is suggested to be the cause of the latitudinal diversity gradient shown by most taxonomic groups. But even if there is a causal relationship between climate and diversity, it is not known how this works. In the diversity literature, we can identify two broad categories of possible mechanisms: the indirect and the direct. In the indirect route, temperature influences diversity through complex pathways involving effects on resource levels, population densities, competition, predation and other biotic interactions. Most potential mechanisms fall into this indirect category. For example, temperature may control the quantity and seasonal availability of food and so influence species population dynamics (this may in turn produce cascading changes in the outcome of species’ interactions). These net effects on population dynamics may alter local extinction risk, and so temperature may indirectly influence local species diversity. In contrast, a single direct mechanism has been proposed (Turner et al. 1988, 1996), in which the temperature directly experienced by individuals is of fundamental importance. Here, diversity is controlled by the relatively simple effects of temperature on the energy budgets of homeotherms; these effects on individuals determine local population densities and hence control local extinction probabilities. Because cold locations impose greater thermoregulatory loads, an individual has to devote relatively more energy to thermoregulation. As a result, less energy is available for growth and reproduction, so colder areas will tend to have lower population densities than warmer areas. This lower density will tend to make local populations more susceptible to extinction. Spatial variation in diversity therefore occurs as a consequence of climatically driven variation in local extinction probability.
Testing the hypothesis using british birds
Because temperature and latitude are correlated, it can be difficult to disentangle the effect of temperature from that of latitude. However, temperature is predicted to be more strongly associated with diversity than latitude alone. While earlier workers have relied heavily on this prediction, the task is complicated by the fact that most climatic and environmental variables are correlated, and this correlation structure can make it challenging to disentangle the causal factors.
Because many species of British birds are migratory, and additionally some species move within the country between the seasons, the spatial pattern of diversity changes in time. This opens up the possibility of seeing if diversity tracks seasonal change in climate. Usefully, oceanic influences mean that the spatial distribution of climatic factors in Britain changes dramatically from summer to winter. Summer is dominated by a south to north decline in temperature, but in winter the gradient swings round to a south-west to north-east direction. We can use this ‘natural experiment’ to predict that the changing pattern of climate between seasons should change the spatial pattern of diversity, if there is a direct link between energy input and diversity.
In earlier work, we found some associations between climate and bird diversity (Turner et al. 1988; Lennon 1990; Turner et al. 1996). Here we update and improve upon this work by using more recent bird survey data, better climatic data, most of the British bird fauna and a much larger number of quadrats. We simultaneously assess the effects of climate, habitat (as land use), topography and geographical location. We also introduce analytical methods that have several advantages over those usually used.