Relative to the expanses of land and sea, the great majority of species are absent from most places and restricted in occurrence to rather few. The fundamental importance of determining what limits their geographic ranges has long been recognized in many research fields, including ecology, evolution, epidemiology and physiology. However, for no single species do we yet have a comprehensive understanding. The need to resolve this situation has become yet more pressing given that the knowledge bears on some of the most significant environmental challenges presently facing humankind (e.g. biological invasions, habitat loss, climate change, emerging diseases, food security).
One reason for the failure to provide a comprehensive answer to the question of what limits the geographic ranges of species has been the rather poor integration of different strands of research. While answers have been proffered at different levels of biological organization (e.g. genes, populations, communities) and from different perspectives (e.g. genetics, physiology, population dynamics, forestry, fisheries), these have tended to take place in relative isolation, and the associated literatures have tended to remain surprisingly distinct (Gaston 2003
). Likewise, something of a disjunction has persisted between the literature addressing the theory of range limits and empirical studies of those limits. The latter have lagged substantially behind developments in the former, and continue essentially to constitute a large set of case studies conducted in a heterogeneous fashion and with only limited synthesis.
A number of frameworks have been suggested to integrate the wide variety of perspectives on range limits. These variously distinguish between, for example, constraints posed by abiotic and biotic factors; intrinsic and extrinsic factors; ultimate (historical) and proximate (functional) factors; unmodifiable environmental factors, modifiable environmental factors and natural enemies; and physiological/ecological adaptation and extinction/colonization (e.g. Caughley et al. 1988
; Brown & Lomolino 1998
; Hochberg & Ives 1999
). These have undoubtedly helped to crystallize thinking on key topics in range limitation. However, there are two recurrent problems with such schemes. First, while often very explicitly recognizing that this is not the case, they tend nonetheless to encourage a view of the different possible factors that may limit ranges as mutually exclusive alternatives. Second, and in a related vein, they can sometimes be seen to confound the differentiation of different kinds of constraints on ranges with the differentiation of different levels of explanation of those constraints. For example, although genetic, physiological and population dynamic constraints are often distinguished, the first may shape the second, and the first two are played out through their effects on the third.
Here, I provide a broad overview of the determinants of range limits, attempting to draw together much of the disparate material. While far from entirely resolving previous problems, in so doing I take a different approach to both integrating the various perspectives on range limits and drawing together theoretical insights and empirical findings. Rather than focus on attempting to distinguish between different kinds of factors that may limit geographic ranges, I consider, in turn, how influences on the different terms of the simple single-population equation Nt+1
can determine range limits, where N
is population size; t
is time; i
is the number of immigrants; b
is the number of births; d
is the number of deaths; and e
is the number of emigrants. Throughout, I attempt to extract some broad generalizations, acknowledging that the factors determining the limits to geographic ranges can vary between species, between different parts of the range of any given species and between different times (Porter et al. 2002
; Gaston 2003
; Morin et al. 2007