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Declining demographic performance and dispersal limitation influence the geographic distribution of the perennial forb Astragalus utahensis (Fabaceae)

Baer, Kathryn C., Maron, John L.
Thejournal of ecology 2019 v.107 no.3 pp. 1250-1262
Astragalus, forbs, geographical distribution, habitats, models, monitoring, perennials, population growth, prediction, viability
A central goal of ecology is understanding the determinants of species’ distributions. “Metapopulation” models for the existence of distributional boundaries predict that species’ geographic ranges arise from the landscape‐scale deterioration of habitat suitability towards the range edge (i.e., niche mechanisms), which simultaneously hinders demographic performance and limits dispersal to suitable habitat beyond the edge (i.e., dispersal limitation). However, few studies have examined both of these mechanisms for the same species by examining abundance and comprehensive measures of demographic performance across the distribution and beyond its boundary. We tested the predictions of metapopulation models for range limits by contrasting abundance, demographic performance, and population growth (λ) of the perennial forb Astragalus utahensis in central and northern range edge populations. We tested for dispersal limitation by transplanting individuals at and beyond the northern range boundary and monitoring their demographic performance. Astragalus abundance and stochastic λ decreased from the range centre to the northern range edge, with stochastic λ falling to or below replacement in range edge populations. However, transplants at some sites beyond the northern range edge survived and reproduced at levels similar to transplants within the range. Thus, in addition to deteriorating conditions at the range edge, dispersal limitation appears to contribute to limiting A. utahensis’ northern distribution. Synthesis. Our results support metapopulation models for range limits by suggesting that decreased demographic performance constrains the viability of range edge populations while contributing to dispersal limitation of Astragalus utahensis’ distribution. Thus, the tandem action of niche and dispersal processes appears to play an important role in constraining this species’ northern latitudinal range.