Department of Biology, Colorado State University, Fort Collins, CO 80253, USA.
Proc Natl Acad Sci U S A. 2009 Nov 17;106 Suppl 2(Suppl 2):19693-8. doi: 10.1073/pnas.0901652106. Epub 2009 Sep 22.
Understanding the processes that create and maintain species' geographic range limits has implications for many questions in ecology, evolution, and conservation biology. Many expectations for the ecological and evolutionary dynamics of populations at the range margin rest on the concordance of geographic limits and the limits of a species' ecological niche. If range limits are coincident with niche limits, then marginal populations should have lower and/or more variable vital rates and population growth rates than central populations. Using data from 8 annual censuses of marked individuals, I investigated the demography of Mimulus cardinalis and Mimulus lewisii across the species' elevation ranges. Central and marginal populations exhibited striking demographic differences, but only for one species were differences in expected directions. Marginal populations from the M. lewisii lower elevation range limit had lower and more variable survival than central populations and appeared to be demographic sinks. In contrast, marginal populations from the M. cardinalis upper elevation limit had higher fecundity and higher population growth rates than central populations. Although the species differed with respect to central-marginal patterns, they were concordant with respect to elevation; that is, both species had higher fitness in higher reaches of their examined ranges. Potential explanations for these patterns include source-sink dynamics, with asymmetrical gene flow mediated by river currents, and climate change, with recent warming shifting the species' climatic envelopes to higher elevations. Hence, assessment of spatiotemporal variation in both demography and dispersal is necessary to fully understand the relationship between the niche and species' distributions.
理解形成和维持物种地理分布范围的过程对生态学、进化和保护生物学中的许多问题都具有重要意义。许多关于种群在分布范围边缘的生态和进化动态的预期都依赖于地理限制和物种生态位限制的一致性。如果分布范围与生态位范围一致，那么边缘种群的关键生活参数和种群增长率应该低于或更具变异性。本研究利用标记个体的 8 年年度普查数据，研究了拟南芥和 Mimulus lewisii 在物种海拔范围内的种群动态。中心和边缘种群表现出显著的种群动态差异，但只有一种物种的差异符合预期方向。来自 Mimulus lewisii 较低海拔分布范围限制的边缘种群的存活率较低，且更具变异性，而中央种群则表现出相反的特征，这表明边缘种群可能是种群汇。相比之下，来自 Mimulus cardinalis 较高海拔分布范围限制的边缘种群的繁殖力更高，且种群增长率更高。尽管这两个物种在中心-边缘模式上存在差异，但它们在海拔上是一致的；也就是说，两个物种在其研究范围内的较高海拔处具有更高的适应性。这些模式的潜在解释包括源汇动态，河流流动介导的不对称基因流，以及气候变化，最近的变暖将物种的气候范围转移到更高的海拔。因此，评估种群动态和扩散的时空变化对于充分了解生态位和物种分布之间的关系是必要的。
