Species range limits (SRLs) are defined as the spatial boundaries beyond which no living individuals of a given species occur. Populations occurring near or at SRLs are often referred to as “marginal,” “peripheral,” “edge,” or “border” populations. SRLs may represent areas beyond which individuals cannot physiologically tolerate ecological conditions or areas where they have not yet dispersed. SRLs may be stable (i.e., at equilibrium) or may represent areas where range expansion through migration or population growth is in the process of occurring. SRLs are significant to ecology, evolution, and conservation for several reasons. They provide opportunities to understand the conditions under which populations expand or contract, and the conditions under which populations may evolve new forms. Additionally, SRLs can provide clues about how species may respond in the face of rapid environmental change.
Characteristics of Species Range Limits
Biologists have long been interested in whether populations near SRLs possess predictable properties, such as differences in abundance or genetic variation when compared to more central populations. Two general patterns have emerged by reviewing patterns of DNA variation across a broad set of species, mostly from the Northern Hemisphere. Genetic differentiation levels (defined as the degree to which populations differ in their genes to one another) among nearby populations are higher approaching SRLs than genetic differentiation levels among populations more centrally located within species ranges. Also, genetic diversity levels (defined as the diversity or variety of genes within a population) are generally lower near SRLs. These patterns are not especially strong, and their general causes are still not understood, but the following factors may contribute: stronger natural selection near SRLs, increased isolation or genetic drift near SRLs, or greater fluctuations in environmental suitability near SRLs that translate into unstable population dynamics. Importantly, despite the general genetic signature of decreased diversity in the majority of species examined, exceptions to this pattern are common. Additionally, populations near SRLs, even those having lower genetic diversity than interior populations, can host unique genes and adaptations that can have high conservation value.
Causes of species range limits
SRLs can be maintained by a variety of interacting factors. Barriers may cause range limits, such as a rapid transition in ecological suitability (e.g., a land to sea transition) that prevents individuals from establishing elsewhere. Non-biological environmental factors, termed abiotic factors (e.g., climate, soil type, etc.), explain large biogeographic divisions such as biomes (deserts, tropical forests, etc.) and often determine the occurrence of SRLs. Species interactions, often termed biotic factors, include predation, parasitism, mutualisms, competition, and hybridization and can also cause SRLs. Abiotic and biotic factors can interact, and any one range-limiting factor, or set of interacting factors, may change gradually or abruptly across space to determine SRLs. Evolutionary constraints, or the failure of a given species to adapt to all conditions on Earth, are the ultimate cause of SRLs.
Adaptation at species range limits
Why do species fail to adapt to conditions at SRLs? The main reason for evolutionary constraints at SRLs, or anywhere, is a lack of genetic variation to respond to natural selection. This lack of genetic variation to adapt to novel conditions may be a characteristic of populations approaching SRLs or a characteristic of the species as a whole. In this vein, species with very limited levels of genetic variation (contained within their populations) may have smaller range sizes.
Rainforest specialist flies such as these (Drosophila birchii) have been shown to have low levels of genetic variation related to cold-climate tolerance, which likely explains their narrow species ranges in the tropics. Source: Science.au.dk Adaptation to conditions at SRLs can result in a species range expansion as newly adapted populations spread into new areas. This process of species range expansion through adaptation at SRLs is essentially niche evolution, or the evolutionary broadening of niche breadth (e.g., an increase in the range of growing degree days in which a plant can grow and produce viable seeds). As genetic mutations arise within a species’ range, and as populations trade these new genes through gene flow, genetic variation may accumulate and increase over time, allowing a gradual increase in range size known as secular migration. When genetic variation is not limited at range limits, beneficial traits can evolve quickly. This phenomenon has been observed at the spatial margins of recent biological invasions. For example, dispersal speed (through increased leg length) has rapidly evolved in edge populations of the cane toad invasion in Australia.
Cane toad, Bufo marinus, Queesland National Park, Australia. Source: CC-sa-3.0 Gene flow between populations can have negative and positive consequences on adaptation to novel conditions, and these consequences can depend on the environmental source of gene flow. Gene flow between very different environments may increase genetic variation within populations, but this gene flow may also disrupt the process of adaptation by natural selection by mixing adapted genes with maladapted genes. In this vein, gene flow could stall adaptations occurring near SRLs, thereby contributing to the maintenance of SRLs. Alternatively, gene flow may introduce beneficial genes to stressful environments near SRLs, especially if originating from similar environments where beneficial adaptations already exist. Gene flow between similar environments of ecological gradients may be an important force in adaptation at species range limits. In this vein, populations occupying similar environments should be considered collectively for their potential to share genes with adaptive results, and not just as a set of populations that are individually high or low in adaptive and conservation value.
Shifting limits under climate change?
The extent to which SRLs are at equilibrium is unknown for most species. If SRLs are in tight equilibrium with current climates, we may expect a rapid shift in range limits with rapid climate change. This would result in leading-edge and rear-edge portions of species ranges where adjustments to climate change are occurring. Leading-edge areas are those that become more favorable during climate shifts and where populations will migrate towards through a process of niche tracking. Rear-edge areas are expected to become unsuitable during climate shifts as the species niche moves away from populations inhabiting environmental extremes that are becoming even more extreme (e.g., warming of the current hottest areas inhabited by a species). Indeed, many species have already responded to recent human-linked climate warming during the past few decades by expanding their geographic ranges towards the poles. An important question is, which species will be most vulnerable to range shifts under modern climate change? The challenge of conservation efforts during climate shifts is to assist populations in tracking their species niche as it moves away from them, or to promote rapid climate adaptation in cases where individuals are unable to migrate due to habitat requirements unrelated to climate (e.g., species restricted to certain soils or bodies of water).
The low-elevation species range limit of the alpine chipmunk (Tamias alpinus) from the Sierra Nevada of California has retracted upwards over the last century, resulting in reduced genetic diversity. Photo courtesy of Mammalia. http://www.faculty.ucr.edu/~pdeley/omnialex/california-mammals.htm.
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The low-elevation species range limit of the alpine chipmunk, Tamias alpinus, from the high Sierra Nevada of California, has retracted upwards over the last century, resulting in reduced genetic diversity. Photo courtesy of Risa Sargent.