Population genetic analysis of the intertidal limpet Lottia scabra and inference of the causes and mechanisms of range limits
Range limits have been described for many species, and the interest in range limits has increased in the wake of climate change, but few researchers attempt to document the causes and mechanisms of these limits and empirical tests of range limit theory remain sparse. Three principle mechanisms have been proposed to limit species’ range in models incorporating environmental heterogeneity and evolution: genetic impoverishment, migration load, or a physical barrier to dispersal. Other possibilities include a “leaky” barrier and secular migration. Here, I describe the distribution, abundance, genetic variation in partial 16S and cytochrome c oxidase subunit I mitochondrial loci, and environment of the low to medium dispersal species Lottia scabra, an intertidal limpet with a range from Oregon, USA, to Baja California, Mexico, that crosses a known biogeographic break at the region on Point Conception, CA. The environmental variables describe latitudinal geographic variation of air temperature, precipitation, vapor pressure deficit, solar radiation, salinity, sea surface temperature, and concentration of chlorophyll-a in sea water. I found L. scabra is comprised of two, not one, evolutionary lineages: the North Clade from 43.3°N to 34.5°N, and the South Clade from 35.7°N to 29.9°N. I found high gene flow and low to medium genetic diversity within both the North and South Clades, high upwelling and cold temperatures at the northern limit of the North Clade, and a pronounced decrease in chlorophyll-a with an increase in sea surface temperature at the southern limit of the North Clade. I found the likely cause of the northern range limit of the North Clade was migration load with recruitment limitation, and two plausible causes for the southern range limit of the North Clade and the northern range limit of the South Clade: migration load and/or a “leaky” barrier.