Abstract
Fine-scale spatial variation in genetic relatedness and inbreeding occur across continuous distributions of several populations of vertebrates; however, the basis of observed variation is often left untested. Here we test the hypothesis that prior observations of spatial patterns in genetics for an island population of feral horses (Sable Island, Canada) were the result of spatial variation in population dynamics, itself based in spatial heterogeneity in underlying habitat quality. In order to assess how genetic and population structuring related to habitat, we used hierarchical cluster analysis of water sources and an indicator analysis of the availability of important forage species to identify a longitudinal gradient in habitat quality along the length of Sable Island. We quantify a west-east gradient in access to fresh water and availability of two important food species to horses: sandwort, Honckenya peploides, and beach pea, Lathyrus japonicas. Accordingly, the population clusters into three groups that occupy different island segments (west, central, and east) that vary markedly in their local dynamics. Density, body condition, and survival and reproduction of adult females were highest in the west, followed by central and east areas. These results mirror a previous analysis of genetics, which showed that inbreeding levels are highest in the west (with outbreeding in the east), and that there are significant differences in fixation indices among groups of horses along the length of Sable Island. Our results suggest that inbreeding depression is not an important limiting factor to the horse population. We conclude that where habitat gradients exist, we can anticipate fine-scale heterogeneity in population dynamics and hence genetics.
Highlights
It is clear that over continuous distributions, spatial variability in individual behaviour can lead to small-scale spatial heterogeneity in movements [1]
Shrub species like Empetrum nigrum, junipers, and Calluna vulgaris were only observed in plots of area 2, which is the widest region of the island and most protected from coastal stress
Lucas et al [13] quantified genetic substructure in the Sable Island horses across four arbitrary divisions of the island, we believe that availability of water and food resources leads to three defensible groupings divided in longitudinal fashion
Summary
It is clear that over continuous distributions, spatial variability in individual behaviour can lead to small-scale spatial heterogeneity in movements [1]. Where this occurs, fine-scale genetic substructure may arise, which has importance for understanding kin relationships and genetic diversity of populations [2,3,4,5,6]. The processes that cause dispersal to be limited or lead to variation in mating structures have not been well addressed in the context of fine-scale population substructure, though social structure, limited dispersal, and spatial heterogeneity in resources are common in wild vertebrate populations [6]. Demonstrating why a population might vary in demography, and genetic relatedness and inbreeding, may further the development of new theory (e.g., evolution of specialists and generalists in heterogeneous habitats [12]) and bring to attention potential problems for conservation and management (e.g., confounding effects on population and quantitative genetics [2,6])
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