Evolution and Ecology of Species Range Limits

Species range limits involve many aspects of evolution and ecology, from species distribution and abundance to the evolution of niches. Theory suggests myriad processes by which range limits arise, including competitive exclusion, Allee effects, and gene swamping; however, most models remain empirically untested. Range limits are correlated with a number of abiotic and biotic factors, but further experimentation is needed to understand underlying mechanisms. Range edges are characterized by increased genetic isolation, genetic differentiation, and variability in individual and population performance, but evidence for decreased abundance and fitness is lacking. Evolution of range limits is understudied in natural systems; in particular, the role of gene flow in shaping range limits is unknown. Biological invasions and rapid distribution shifts caused by climate change represent large-scale experiments on the underlying dynamics of range limits. A better fusion of experimentation and theory will advance our understanding of the causes of range limits. 415 A nn u. R ev . E co l. E vo l. Sy st . 2 00 9. 40 :4 15 -4 36 . D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g by U ni ve rs ity o f C al if or ni a M er ce d on 0 7/ 15 /1 4. F or p er so na l u se o nl y. ANRV393-ES40-20 ARI 1 October 2009 12:18 Species range limit: point in space beyond which no living individual occurs Niche conservatism: conservation of ecological requirements (niche characteristics) among closely related taxa Range boundary disequilibrium: mismatch between a species’ current distribution and potential geographic distribution caused by limitations in dispersal or lags in tracking appropriate conditions Isolation by distance: decrease in gene flow with increased geographic distance resulting in increased genetic differentiation Selection regime: local ecological conditions by which natural selection acts INTRODUCTION Species range limits are essentially the expression of a species’ ecological niche in space. The challenge is to identify the environments within which births are greater than deaths, how those environments are distributed across the landscape, and how they are connected by dispersal. This requires understanding how spatial variation in fitness results from the fit between phenotype and environment and how differences in fitness translate into population-level differences in abundance. Focusing on fitness variation helps to link ecological limitations and evolutionary processes. A central goal in evolutionary biology is understanding conditions that facilitate adaptive diversification versus those that promote niche conservatism. Adaptation to novel habitats at the range margin is akin to niche evolution (see Holt & Gomulkiewicz 1997 for a classic introduction to this issue). Thus, range limits can serve as testing grounds to understand the conditions by which populations can adapt—or fail to adapt—to novel conditions. A long-standing interest in the ecology of range limits is reflected in seminal works (e.g., Darwin 1859, MacArthur 1972). The study of geographic ranges has enjoyed a recent resurgence, spurred by the need to accurately forecast responses to large-scale, anthropogenic alterations to climate and habitat. This renewed interest has been facilitated by the development of extensive online databases compiling species’ occurrences and environmental variables. However, many questions remain about the ecological and evolutionary processes that give rise to range limits. Here we review the major hypothesized causes of range limits and the empirical approaches used to test them. We discuss the theory of range limits and review the predictions and hypotheses generated by theory for natural systems. We end by highlighting important frontiers in range limits research, including biological invasions and climate change responses. NATURE OF SPECIES RANGE LIMITS Species ranges are highly mobile, often shifting, expanding, and contracting over time (Brown et al. 1996, Davis & Shaw 2001, Gaston 2003). Range mobility could reflect gradual niche evolution over time or spatial tracking of the environmental niche in response to changing environmental conditions (Pfenninger et al. 2007). Some studies suggest that spatial tracking of the niche is more common (Davis & Shaw 2001, Pease et al. 1989), although range shifts can lag behind environmental changes (Svenning et al. 2008). Range boundary disequilibria with environmental niches is often attributed to dispersal limitation, where populations cannot expand as quickly as the environment becomes favorable (e.g., recolonizing formerly glaciated areas; Fang & Lechowicz 2006). Species differ markedly in their extent of climatic disequilibrium; this may reflect differences in dispersal (Tinner & Lotter 2006), life history (Araujo & Pearson 2005), recolonization history (Stewart & Lister 2001), or stochasticity (Clark 1998). Although there are clear examples of range disequilibrium, it is certainly not the rule. Range equilibrium is suggested by transplant experiments beyond species’ current range boundaries; many species have low fitness and exhibit negative population growth in areas beyond present distribution limits (e.g., Angert & Schemske 2005, Geber & Eckhart 2005, Griffith & Watson 2006, but see Carter & Prince 1985, Van der Veken et al. 2007). Despite centuries of interest in range limits, little is known about basic differences in the evolutionary ecology of edge and central populations. Such differences could have important effects on range limit dynamics and could arise by several mechanisms. First, edge populations may exhibit reductions in the diversity and number of immigrants by virtue of spatial arrangement alone, resulting in an isolation-by-distance effect. At range edges spanning ecological gradients, gene flow from different selection regimes may be limited to nearby populations from just inside, or at, the 416 Sexton et al. A nn u. R ev . E co l. E vo l. Sy st . 2 00 9. 40 :4 15 -4 36 . D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g by U ni ve rs ity o f C al if or ni a M er ce d on 0 7/ 15 /1 4. F or p er so na l u se o nl y. ANRV393-ES40-20 ARI 1 October 2009 12:18 Ecological gradient Current limit Past limit Past limit