Peripheral Populations and Climate Change: Implications for the Spotted Gar

Peripheral populations occupy the edge of a species’ range and are considered to be exceptionally important in terms of a species’ ecology, biogeography, evolution, and conservation.  Peripheral populations often persist under different environmental conditions from the species’ central or “core” populations, and may exhibit genetic and morphological adaptations to potentially “harsher” marginal environments.  Peripheral populations also often experience low gene flow and high degrees of genetic drift, further increasing likelihood of divergence from core populations.  Because of differing environmental conditions related to geographical factors such as latitude, populations may also exhibit different life history characteristics such as size and age at maturity, growth rate, and mortality schedules.  Further, peripheral populations may be more vulnerable to effects of climate change.  Conserving distinct peripheral populations is therefore an integral and important component of conserving global biodiversity.

To further explore and better understand these issues, we studied the ecology and biogeography of the spotted gar (Lepisosteus oculatus) from core and peripheral populations.  Although relatively common in drainage basins of the southern United States, the spotted gar is poorly studied and its ecology and status comparatively unknown in the Great Lakes basin.  We used common garden experiments, life history analyses, and phylogeography to address the overall hypothesis that spotted gars from peripheral, Great Lakes Basin populations exhibit different life history characteristics and genetic diversity than spotted gars from core populations.  Our findings suggest Great Lakes Basin populations have adapted to life at higher latitudes (shorter growing season) and exhibit countergradient variation in growth as well as lower mortality rates.  Analysis of mitochondrial DNA from core and peripheral populations indicated genetic diversity was highest in the Mississippi River Basin, lowest in the Great Lakes Basin, and most divergent in the western Gulf Coast Basin.  Genetic structure and low diversity in the Great Lakes Basin were likely related to recent post-glacial colonization from Mississippian refugia, founder effects, and low gene flow.  We address the implications of climate change for spotted gars and peripheral populations in general.  The Great Lakes Basin population of spotted gars was shown to be a unique component of the overall species, and can serve as a case study to inform conservation strategies and better understand the evolution and maintenance of vertebrate life history patterns and genetic diversity.