Abstract
The arrangement of habitat features via historical or contemporary events can strongly influence genomic and demographic connectivity, and in turn affect levels of genetic diversity and resilience of populations to environmental perturbation. The rusty blackbird (Euphagus carolinus) is a forested wetland habitat specialist whose population size has declined sharply (78%) over recent decades. The species breeds across the expansive North American boreal forest region, which contains a mosaic of habitat conditions resulting from active natural disturbance regimes and glacial history. We used landscape genomics to evaluate how past and present landscape features have shaped patterns of genetic diversity and connectivity across the species’ breeding range. Based on reduced-representation genomic and mitochondrial DNA, genetic structure followed four broad patterns influenced by both historical and contemporary forces: (1) an east–west partition consistent with vicariance during the last glacial maximum; (2) a potential secondary contact zone between eastern and western lineages at James Bay, Ontario; (3) insular differentiation of birds on Newfoundland; and (4) restricted regional gene flow among locales within western and eastern North America. The presence of genomic structure and therefore restricted dispersal among populations may limit the species’ capacity to respond to rapid environmental change.
Highlights
The spatial organization of suitable habitat across the landscape via historical and contemporary events plays an important role in the maintenance of both genetic and demographic connectivity within plant or animal populations
These results indicate that historical and contemporary processes are shaping the distribution of genomic variation among populations of rusty blackbirds across their boreal distribution
During the LGM, the glacial ice sheets covered most of northern North America, except Alaska and areas along Pacific and Atlantic coasts [97], and may have sundered the rusty blackbird’s nesting range, isolating populations in separate western and eastern refugia, and promoting the partition in genomic variation we detected in multiple analyses of mtDNA and ddRAD loci
Summary
The spatial organization of suitable habitat across the landscape via historical and contemporary events plays an important role in the maintenance of both genetic and demographic connectivity within plant or animal populations. Discontinuities in habitat, whether from physical barriers or from natural or anthropogenic disturbances, fragment populations into smaller units and can diminish levels of connectivity when (1) the distance between suitable habitat patches exceeds the dispersal capacity of individuals or (2) dispersing individuals do not successfully reproduce In this way, spatial habitat heterogeneity influences effective dispersal (dispersal followed by reproduction) by individuals, which in turn impacts gene flow and population dynamics (i.e., potential outcome of dispersal, [1,2]). Population genomics can help identify areas where connectivity across the landscape enriches genetic diversity and enhances the resiliency of populations to environmental perturbation It can identify where contemporary or historical limitations in dispersal have led to genomic structuring and distinct populations that may require specific management strategies to remain viable. Population genomics provides a powerful approach to understanding implications of dispersal and can fill information gaps in traditional movement data, especially in migratory birds that nest in remote regions (see references within [15]) such as the vast boreal forest biome of North America
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