Combined impacts of rarity and anthropogenic habitat fragmentation on genetic diversity at species range-limits

Background/Question/Methods:

The population and genetic impact of climate change at species range limits will take place in the context of a landscape altered by human land use. For many species, the forces associated with a loss of genetic diversity with range expansion (founder effects, bottlenecks) will interact with additional loss of diversity from habitat fragmentation (reduced population size, dispersal limitation). We combined long term vegetation data (historical surveys and paleoecological data) with genetic data (microsatellite variation) to quantify past impacts of climate and land-use on American beech across a gradient from central to peripheral populations. We expected reduced genetic diversity in marginal populations, but it was not clear whether secondary forests, reforesting following agricultural abandonment, would recover genetic diversity in primary forests, whether they would lose diversity due to limited gene flow, or whether they would gain diversity through colonization from differentiated source populations characteristic of marginal habitats.

Results/Conclusions:

Genetic variation in fragments of primary forests confirmed expectations from previous studies: forest fragments maintained substantial genetic diversity; this diversity was reduced in peripheral populations, which were also more strongly differentiated due to the long-term isolation of these marginal habitats. Secondary forests in central populations recovered most genetic diversity of the forest fragments from which they originated, but secondary forests in marginal habitats failed to recover genetic diversity.

As climate driven range expansions and contractions combine with habitat degradation and recovery this century, we need to understand the implications of their interactions. In this case, populations of a late successional species are slowly recovering in the century since agricultural abandonment, but the recovery of genetic diversity in these forests is not occurring at the same rate in marginal habitats.