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 Annu. Rev. Ecol. Evol. Syst. 2009.40:415-436. Downloaded from www.annualreviews.org by Stanford University -Main Campus -Lane Medical Library on 10/08/12. For personal use only.
In plant conservation, restoration (the augmentation or reestablishment of an extinct population or community) is a valuable tool to mitigate the loss of habitat. However, restoration efforts can result in the introduction of novel genes and genotypes into populations when plant materials used are not of local origin. This movement is potentially important because many plant species are subdivided into populations that are adapted to local environmental conditions. Here we focus on genetic concerns arising from ongoing restoration efforts, where often little is known about ''How local is local?'' (i.e., the geographic or environmental scale over which plant species are adapted). We review the major issues regarding gene flow and local adaptation in the restoration of natural plant populations. Finally, we offer some practical, commonsense guidelines for the consideration of genetic structure when restoring natural plant populations.
While small‐scale studies show that more diverse native communities are less invasible by exotics, studies at large spatial scales often find positive correlations between native and exotic diversity. This large‐scale pattern is thought to arise because landscapes with favorable conditions for native species also have favorable conditions for exotic species. From theory, we proposed an alternative hypothesis: the positive relationship at large scales is driven by spatial heterogeneity in species composition, which is driven by spatial heterogeneity in the environment. Landscapes with more spatial heterogeneity in the environment can sustain more native and more exotic species, leading to a positive correlation of native and exotic diversity at large scales. In a nested data set for grassland plants, we detected negative relationships between native and exotic diversity at small spatial scales and positive relationships at large spatial scales. Supporting our hypothesis, the positive relationships between native and exotic diversity at large scales were driven by positive relationships between native and exotic beta diversity. Further, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial‐heterogeneity hypothesis but not the favorable‐conditions hypothesis.
According to theory, gene flow to marginal populations may stall or aid adaptation at range limits by swamping peripheral populations with maladaptive gene flow or by enhancing genetic variability and reducing inbreeding depression, respectively. We tested these contrasting predictions by manipulating patterns of gene flow of the annual plant, Mimulus laciniatus, at its warm range limit. Gene flow was experimentally applied by using crosses within warm-limit populations (selfed and outcrossed), between warm-limit populations, and between warm-limit and central range populations across two elevational transects. We measured the fitness of offspring in a common garden at the warm-edge species range limit. All sources of gene flow increased seedling emergence at the range limit, suggesting local inbreeding depression at both range limit populations; however, lifetime reproductive success only increased significantly when pollen originated from another warm-limit population. Center-to-warm-edge gene flow was maladaptive by delaying time to development at this warm, fast-drying range limit, whereas edge-to-edge gene flow hastened emergence time and time to reproduction. By empirically testing theory on the effects of gene flow on the formation of geographic range limits, we find benefits of gene flow among populations to be greatest when gene flow is between populations occupying the same range limit. Our results emphasize the overlooked importance of gene flow among populations occurring near the same range limit and highlight the potential for prescriptive gene flow as a conservation option for populations at risk from climate change.climate adaptation | ecological gradients | natural selection | phenology | species range limits T heory on the evolution of range limits predicts that gene flow from large, central populations to edge populations at the range limit could create a flood of maladaptive, nonlocal genes, thereby stalling adaptation and niche expansion (1-3). Alternatively, gene flow from central populations may increase effective population size and genetic variation in edge populations, thereby ultimately increasing fitness at the range limit and perhaps contributing to range expansion (4-6). Overlooked in these models is gene flow among edge populations, which may be especially beneficial because it (i) supplies both favorable alleles or gene combinations that are adaptive at range limits (7) and (ii) enhances genetic variation upon which selection may act. These ideas have not been tested empirically in natural systems at geographic range limits.With rapid climate change, low genetic variation may constrain the ability of populations to adapt quickly to warming environments (8-10). Globally, climate warming is pushing species ranges upwards in elevation, leaving rear-edge populations to adapt, migrate, or perish (9, 10). Mimulus laciniatus (cutleaved monkeyflower), an annual plant, inhabits mossy areas on granite seeps between 975 and 3,270 m on the western slope of the California Sierra Nevada Mountai...
Invasive plants can be larger and more fecund in their invasive range than in their native range, although it is unknown how often this is a result of a genetically controlled shift in traits, a plastic response to a favourable environment, or a combination thereof. Here we present data from common garden experiments that compare the size and fecundity of native and invasive California poppies, Eschscholzia californica Cham. Individuals from 20 populations, half from California (native) and half from Chile (invasive), were grown both with and without competition from other plants in a container experiment and at two field locations. There were no differences in survival between native and invasive plants at any location. We found significant increases in size and fecundity in invasive populations at two of three locations when poppies were grown without competition from other plants. Our results indicate that genetic shifts in traits have occurred in invasive populations, and that the invasive plants are better at maximizing growth and reproduction in open environments.
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