Evolutionary ecology at the extremes of species’ ranges

Abstract

The nature of species at the extremes of their ranges impinges fundamentally on diverse biological issues, including species’ range dynamics, population variability, speciation and conservation biology. We review the literature concerning genetic and ecological variation at species’ range edges, and discuss historical and contemporary forces that may generate observed trends, as well as their current and future implications. We discuss literature which shows how environmental, ecological and evolutionary factors act to limit species’ ranges, and how these factors impose selection for adaptation or dispersal in peripheral populations exposed to extreme and stochastic biotic and abiotic stressors. When conditions are sufficiently harsh such that local extinction is certain, peripheral populations may represent temporary offshoots from stable core populations. However, in cases where peripheral populations persist at the range edge under divergent or extreme conditions, biologically significant differences can arise from historical and contemporary ecological and evolutionary forces. In many such cases reviewed herein, peripheral populations tended to diverge from the species’ core, and to display lower genetic diversity or greater stress-adaptation. We conclude that while such populations may be of particular conservation value as significant components of intraspecific biodiversity or sources of evolutionary innovation and persistence during environmental change, small and greatly variable population size, especially combined with low genetic variability, can result in elevated extinction risk in harsh and stochastic peripheral environments. As a result, while peripheral populations should not be dismissed as evolutionary dead-ends destined for local extinction, neither should they be uncritically granted inherently superior significance based only on their peripheral position alone.

Thermal tolerance breadths among groundwater crustaceans living in a thermally constant environment

“…Second, most populations of a widely distributed species colonizing deep groundwater habitats would typically encounter little seasonal variation of temperature, although they may live under different mean annual temperatures. Although this situation should favor divergent selection, thermal niche narrowing among populations may be counteracted by maladaptive gene flow among populations (Räsänen and Hendry, 2008;Sexton et al, 2009;Geber, 2011;Hardie and Hutchings, 2010). Finally, populations of narrowly distributed species colonizing deep groundwater habitats would be expected to have a narrow thermal niche breadth because they experience little seasonal or spatial variation of temperature.…”

Plant genetic effects on soils under climate change

“…Geographical position of plants within a species distribution may affect trait-based responses to climate change (Hardie and Hutchings 2010;Wang et al 2010;Fournier-Level et al 2011;Hancock et al 2011). For instance, for some northern latitude species such as Pinus and Larix species growth is expected to decrease on the warm edge (either southern or low elevation locations) of their distribution, but increase on the cold edge (northern or high elevation locations; Rehfeldt et al 1999;Wang et al 2010).…”

“…For instance, for some northern latitude species such as Pinus and Larix species growth is expected to decrease on the warm edge (either southern or low elevation locations) of their distribution, but increase on the cold edge (northern or high elevation locations; Rehfeldt et al 1999;Wang et al 2010). Similarly, populations are known to differ in local adaptation and genetic variation at the core versus the margins of a species' distribution due to gene flow and local climate niche-based selection processes (Hardie and Hutchings 2010;Fournier-Level et al 2011). For example Solidago genotypes from northern and southern parts of their ranges differ in their ability to tolerate heat stress, and low genetically-based variation in heat-tolerance in northern populations may lead to a reduction in their ability to respond to warming in the future (Souza et al 2011;Breza et al 2012).…”

Clonal structure and dynamics of peripheral Populus tremula L. populations

“…Peripheral populations are often characterized by low genetic diversity due to isolation, genetic drift, natural selection and inbreeding (Lammi et al 1999, Eckert et al 2008, Hardie & Hutchings 2010. However, the diversity levels found in the studied populations are higher than expected.…”