Mind the gap: genetic distance increases with habitat gap size in Florida scrub jays

Abstract

Habitat gap size has been negatively linked to movement probability in several species occupying fragmented landscapes. How these effects on movement behaviour in turn affect the genetic structure of fragmented populations at local scales is less well known. We tested, and confirmed, the hypothesis that genetic differentiation among adjacent populations of Florida scrub jays--an endangered bird species with poor dispersal abilities and a high degree of habitat specialization--increases with the width of habitat gaps separating them. This relationship was not an artefact of simple isolation-by-distance, as genetic distance was not correlated with the Euclidean distance between geographical centroids of the adjacent populations. Our results suggest that gap size affects movement behaviour even at remarkably local spatial scales, producing direct consequences on the genetic structure of fragmented populations. This finding shows that conserving genetic continuity for specialist species within fragmented habitat requires maintenance or restoration of preserve networks in which habitat gaps do not exceed a species-specific threshold distance.

Figure 1.

Genetic distances between groups of Florida scrub jays (FSJs) living in fragmented areas, as a function of (a) the ln of the Euclidean distance between the centroids of the groups; (b) the ln of the width of the gap between the patches occupied by each group. Distances in metres are provided between brackets under their corresponding logarithmic values. The dotted line is the linear regression of genetic distances as a function of ln(gap size).

Figure 2.

Genetic distances between groups of FSJs inhabiting continuous tracts of habitat, as a function of the ln of the Euclidean distance between the centroids of the groups. Distances in metres are provided between brackets under their corresponding logarithmic values.