Explaining spatial heterogeneity in population dynamics and genetics from spatial variation in resources for a large herbivore

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.

Figure 1. Study area and longitudinal gradient in sources of fresh water for feral horses on Sable Island, Canada, 2008–2010.

(a) Sources of water (n = 122) grouped according to hierarchical cluster analysis, with area 1 containing only permanent ponds (triangles); area 2 containing permanent ponds and horse-excavated wells (circles); and area 3 containing only excavated wells. (b) Boundaries for areas 1, 2, and 3 that include clusters of water sources and the exclusive movements of females using those water sources.

Figure 2. Three-group dendrogram showing average clustering of Cartesian locations of water sources (permanent water ponds and horse-excavated wells, n = 122) on Sable Island, Canada, 2008–2010.

Boxes separate groups; each node represents a single location of a water source. The height of lines connecting water sources represents the distance between groupings. The left-most box contains sources located in area 2; the centre box contains sources located in area 1; the right-most box contains sources in area 3.

Figure 3. Graphical results of a K-means analysis indicating the appropriate number of clusters of water source-groupings, based on within-groups sum of squares as a function of the number of clusters from hierarchical cluster analysis of points of water use (n = 122), for the analysis of water availability to the Sable Island horses, 2008–2010.

The first obvious bend indicates the appropriate number of groups (i.e., 3).

Figure 4. Comparison of the mean abundance of sandwort Honckenya peploides (right-side-up triangle) and beach pea Lathyrus japonicas (upside-down triangle) with percent cover for vegetated plots (open circle) in areas 1, 2, and 3 (west to east) on Sable Island, Canada, 2010.

Error bars are ±1 SE.

Figure 5. Comparison of the annual count in summer (August) of females of all age classes for areas 1, 2, and 3 (west to east) from 2008 to 2010, Sable Island, Canada.