Population structure of state threatened Jefferson salamanders in an agricultural landscape

Background/Question/Methods

Amphibians are generally thought to have poor dispersal abilities and because of this, even low levels of habitat fragmentation may result in the isolation of previously connected populations. The subdivision of a large panmictic population into several smaller, isolated populations makes populations (and species) more vulnerable to extirpation. Therefore, successful conservation of species in highly fragmented landscapes requires information on patterns of gene flow (i.e. connectivity) among populations. Jefferson salamanders (Ambystoma jeffersonianum) are considered a species of conservation concern over portions of their range and are listed as a state threatened species in Illinois. We used 11 polymorphic microsatellite loci to determine the genetic population structure of Jefferson salamanders breeding at 16 ponds in east-central Illinois, which is a landscape fragmented by agriculture; sites were selected that spanned a range of geographical distances to allow us to test predictions about the effects of distance and habitat fragmentation on genetic structure. Our objectives were to estimate the amount of genetic differentiation among populations, determine the extent to which landscape features affect gene flow among breeding ponds, and estimate effective population sizes.

Results/Conclusions

Employing a variety of analyses, we found a significant amount of population-level genetic structure for Jefferson salamanders. Most pair-wise F_ST values were significantly different from 0 between populations and STRUCTURE analyses indicated nine distinct genetic clusters. Breeding ponds that were not significantly differentiated were located within 1 km of one another and connected by continuous forest. Effective population size estimates (programs MIGRATE and OneSamp) were 22-51 individuals. Our results generally corroborate previous empirical measures of Jefferson salamander dispersal, showing that genetic homogenization is prevalent at distances less than 1 km in continuous habitat. The significantly reduced gene flow and reduced effective population sizes found in this study indicate that agricultural fragmentation restricts dispersal ability of Jefferson salamanders and likely interferes with normal rescue-recolonization processes found in amphibian metapopulations, which ultimately makes populations more susceptible to local extinction.