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Understanding the forms that the geographic range limits of species take, their causes and their consequences are key issues in ecology and evolutionary biology. They are also topics on which understanding is advancing rapidly. This themed issue of Proc. R. Soc. B focuses on the wide variety of current research perspectives on the nature and determinants of the limits to geographic ranges. The contributions address important themes, including the roles and influences of dispersal limitation, species interactions and physiological limitation, the broad patterns in the structure of geographic ranges, and the fundamental question of why at some point species no longer evolve the ability to overcome the factors constraining their distributions and thus fail to continue to spread. In this introduction, these contributions are placed in the wider context of these broad themes.
No species is truly cosmopolitan in its distribution. Most are confined to rather small areas, and all have limits to their geographic ranges beyond which they are not found. In some cases, the locations of these limits are apparently quite persistent over long periods, and in others they shift almost continually. In some cases, they appear to constitute abrupt spatial changes in the potential to maintain viable populations, and in others a more gradual waning of opportunities. Understanding the form that geographic range limits take, their causes and their consequences are key issues in ecology and evolutionary biology. General answers have long remained elusive, with empirical studies lagging substantially behind theory, and both theoretical and empirical work being rather piecemeal and fragmented. However, this situation is changing fast, driven partly by the research possibilities provided by much improved data (particularly from remote sensing and long-term monitoring programmes), and partly by the imperatives of predicting the consequences of rapid environmental change, particularly for the distribution of human food resources, diseases and species of conservation concern. This special issue brings together a wide variety of current research perspectives on the nature and causes of the limits to geographic ranges. In so doing, it reflects the rapid growth in understanding that is taking place.
Numerous factors have been proposed, in isolation or in combination, to limit the geographic ranges of species (Brown & Lomolino 1998; Gaston 2003). Of these, the role of dispersal is attracting much of the attention at present, with its obvious implications for the abilities of species to cope with global environmental changes, either in resisting reductions in available suitable conditions (particularly where these cause habitat fragmentation) or in exploiting expansions in those conditions. This theme is well represented in the contents of this issue. Using a spatially explicit, individual-based simulation model, Dytham (2009) explores the evolution of different dispersal strategies in different parts of the geographic ranges of species, showing how a variety of scenarios can result in changes in those strategies across a range. Dispersal is a key component of metapopulation dynamics, and Anderson et al. (2009) explore the implications of including such dynamics on the predicted responses of species to climate change, using case studies of two lagomorph species. Wilson et al. (2009) also use a metapopulation approach to model the range expansion rates of a rare butterfly species in fragmented landscapes.
The importance placed on interactions with other species for the limitation of geographic ranges has variously waxed and waned (for recent reviews see Gaston (2003) and Case et al. (2005)). Here, Price & Kirkpatrick (2009) explore how competition between species for limited resources can give rise to geographic range limits, arguing that the necessary conditions may be very common in multispecies assemblages. Looking instead at species interactions between trophic levels, arguably, species that serve as resources are presently regarded as less important in influencing the precise position of range limits of their consumers than might have been thought in the past, because while obviously the distribution of natural enemies must lie within that of the species on which they depend, there is evidence that the range limits of these species typically lie substantially within those of these resources (Gaston 2003). Conversely, while to some extent previously downplayed, the importance of natural enemies in limiting the geographic ranges of the species that they use is coming increasingly to the fore (Case et al. 2005). This latter perspective is developed in this special issue in papers on the roles of predation in range limitation (Holt & Barfield 2009), and more specifically the effects of sterilizing diseases (Antonovics 2009). In both cases, apparently counter-intuitive patterns can result from species interactions, including that predation can under some circumstances permit prey species to have larger ranges than would be the case in the absence of predation.
Although there is ample evidence of important interplays between resource availabilities and physiological tolerances and capacities, perhaps the most common and most enduring explanation of geographic range limits has simply been that those tolerances and capacities are constrained, and thus that species cannot persist in areas where environmental demands exceed these (Spicer & Gaston 1999). Indeed, the assumption of such a link, and a tight one at that, underlies the majority of attempts to predict the responses of species to global environmental, and particularly climate, change. Rather surprisingly, however, few studies have tested these physiological tolerances and capacities at range limits. Duncan et al. (2009) do so indirectly, determining whether climate envelope models developed in the native range can predict distribution in the introduced range for South African dung beetle species introduced to Australia. The results do not encourage the wide application of such models in their current form. Lee et al. (2009) provide a more direct test, exploiting the introduction of a slug species to a remote island to examine the factors determining the limits in different parts of its distribution, and particularly the role of physiological tolerances.
Given the interrelatedness of numerous population and life-history phenomena, it is not surprising that differences in many ecological and evolutionary traits have been documented at or towards the geographic range limits of particular species (Brown et al. 1996; Gaston 2003). In most cases, the extent to which these are general patterns, or the circumstances under which they do and do not emerge, remain poorly understood. In this special issue, in exploring the case of spatial variations in body size using the carnivores as a study system, Meiri et al. (2009) document the complexity that can arise and the importance of examining large numbers of species. Likewise, drawing on a vast dataset for species of trees in the eastern USA, Purves (2009) examines the, from an empirical perspective surprisingly neglected, pattern of change in demographic rates between the core and limit of geographic ranges. Roy et al. (2009) test for phylogenetic conservatism in the position of range limits that might arise as a consequence of trait similarities. Finally, there are few studies that look at changes in multiple disparate traits across the ranges of individual species, and Kunin et al. (2009) provide a useful example, documenting variation in microclimates, genes and metabolomics of northern rock cress.
Whatever the factors that limit the geographic ranges of species, the question arises as to why they have not evolved the ability to overcome these constraints and continued to spread. In some cases, the changes required are obviously so marked that they seem virtually impossible to envisage, e.g. where terrestrial species are limited by the extent of land. However, in perhaps the majority of species the changes required to enable significant range expansion would seem to be much more subtle. The notion has long persisted that the movement of individuals from larger more central populations into those towards range limits could restrict local adaptation at, and thus beyond, those limits (Mayr 1954; Hoffmann & Blows 1994). However, this model has increasingly been criticized, being sensitive to the levels of immigration into populations at range limits, any tendencies for dispersing individuals to move preferentially to areas for which they were pre-adapted, and whether phenotypic plasticity allows adaptive adjustment to local conditions (Case & Taper 2000). Here, Bridle et al. (2009) provide empirical evidence from rainforest Drosophila that the effect of gene flow on evolution varies at different spatial scales. van Heerwaarden et al. (2009) show that the range limits in the three Drosophila species that they study are not constrained by low overall genetic variation but in some cases at least reflect patterns of selection and genetic variability in key traits. As geographic ranges shift in response to environmental change, the evolutionary dynamics at the limits are also expected to change. McInerny et al. (2009) investigate the effects on neutral evolution, using a metapopulation model, and showing how this structure can alter the opportunities for mutation occurrence and subsequent population survival.
Many of the gaps in understanding the form that the geographic range limits of species take, their causes and their consequences are being filled through new research. Nonetheless, drawing all of the different perspectives on range limitation together into a single coherent framework constitutes a huge and ongoing challenge. Gaston (2009) provides one attempt down this path, providing a broad overview, drawing together many of the disparate threads, and considering in turn how influences on the terms of a simple single population equation can determine geographic range limits.
Drawing all of the different papers in this special issue together has also provided its own challenges, and to conclude, I would like to thank all of those who have been involved in its preparation and production. First, I am grateful to all of the authors for accepting the invitation to participate, and finding time in busy schedules to take on demanding deadlines. Each of the contributions has been formally peer reviewed, and the referees have provided stimulating and thoughtful input for which I and the authors are much indebted. Finally, I would like to express my thanks to the editorial and production staff of Proc. R. Soc. B who have as ever been a delight to work with.
One contribution of 17 to a Special Issue ‘Geographic range limits of species’.