Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario M5 S 3B2, Canada.
Ecology. 2009 Nov;90(11):3051-61. doi: 10.1890/08-1914.1.
A species is expected to occur where the prevailing biotic and abiotic conditions fall within its fundamental niche. Geographic range limits should, therefore, occur when the survival and fitness of individuals along ecological gradients is reduced to the point at which populations are no longer self-sustaining. Abrupt limits to a species' distribution are expected to reflect abrupt changes in the ecological conditions that cause sharp declines in fitness across the limit. We investigated the correlation between geographic variation in environment and fitness across the abrupt northern range limit of a coastal dune plant, Camissoniopsis cheiranthifolia (Onagraceae). In each of 64 plots distributed across five sites along a 200-km transect spanning the species' northern range limit (four within the range plus one beyond the limit), we measured plant community composition as a proxy for variation in biotic and abiotic environmental factors and lifetime fitness of genetically standardized experimental populations of C. cheiranthifolia. There was substantial variation in plant community composition among plots and sites across the range limit, and fitness of experimental plants covaried strongly with the first principal component of plant community composition. However, we did not detect an abrupt shift in plant community or the expected decline in fitness across the range limit. In fact, fitness and recruitment increased toward the limit and were relatively high beyond the limit. This suggests that habitat beyond the range did not occur outside of the species' fundamental niche. These results challenge niche-based explanations for range limits and suggest that hypotheses involving the finer-scale distribution of habitat patches or constraints on dispersal warrant more serious consideration.
一个物种预计会出现在其基础生态位所涵盖的主要生物和非生物条件范围内。因此，地理范围的限制应该发生在个体沿着生态梯度的生存和适应能力降低到种群不再自我维持的程度时。物种分布的突然限制预计反映了导致适应能力在限制处急剧下降的生态条件的突然变化。我们研究了沿海沙丘植物 Camissoniopsis cheiranthifolia（柳叶菜科）在其北部范围突然限制处的地理环境和适应能力的地理变异之间的相关性。在沿物种北部范围限制的 200 公里长的横截线上跨越五个地点的 64 个样地中（四个在范围内，一个在范围之外），我们测量了植物群落组成，作为生物和非生物环境因素变化的代表，以及遗传标准化的 Camissoniopsis cheiranthifolia 实验种群的终生适应能力。在范围限制内的样地和地点之间，植物群落组成存在很大的差异，实验植物的适应能力与植物群落组成的第一主成分密切相关。然而，我们没有检测到植物群落的突然变化或预期的适应能力在范围限制处的下降。事实上，适应能力和繁殖在接近限制时增加，并且在限制之外相对较高。这表明，范围之外的栖息地并没有超出物种的基础生态位。这些结果挑战了基于生态位的范围限制解释，并表明涉及栖息地斑块更精细分布或扩散限制的假设值得更认真地考虑。
