The niche, limits to species' distributions, and spatiotemporal variation in demography across the elevation ranges of two monkeyflowers

Abstract

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. geographic range limit ͉ lambda ͉ population dynamics ͉ vital rate ͉ projection matrix T he geographic range is a fundamental unit of biogeography (1), yet our understanding of the ecological and evolutionary determinants of the placement, occupancy, extent, and limits of geographic ranges remains largely idiosyncratic and systemspecific. In particular, the processes that lead to evolutionarily stable range limits remain poorly understood for most organisms. Many investigations of range limits begin by associating distribution boundaries with potentially limiting environmental variables (e.g., refs. 2 and 3). This approach has a venerable history (4-7) and was central to the early development of the niche concept (8, 9). Thus, in the broadest sense, species' distributions can be viewed as a spatial manifestation of the niche (10, 11), where the geographic range represents a mapping of fitness as a function of the abiotic and biotic environment onto a geographically varying environmental landscape and range edges arise at points along environmental continuums where births no longer exceed deaths (12, 13). Hence, quantifying variation in demographic performance across the geographic range can provide a fundamental first step toward understanding the exten...

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