Niche width contraction induced by aquatic ecosystem fragmentation

Ecosystem fragmentation is one of the core drivers of biodiversity loss at local, regional, and global scales.  Whereas much research has focused on the organism traits (diet, home range, etc.) that determine a population’s susceptibility to extirpation (or extinction), fewer have examined the ecology of those organisms that are seemingly resilient to the adverse effects of fragmentation.  I use a new quantitative measure to assess how niche width of an ecologically and economically important fish species (gray snapper Lutjanus griseus) is affected by ecosystem fragmentation.  This measure is based on position of individuals within standard carbon and nitrogen isotope bi-plots, and is calculated as the convex hull area encompassing all sampled individuals from a population.  My model study systems are Bahamian tidal creeks, which are characterized by varying degrees of hydrologic connectivity (between interior wetlands and the adjacent marine environment) due to fragmentation by human (road construction) and natural (hurricanes) events.  I demonstrate that niche width contracts significantly with increasing degree of fragmentation.  As quantified by the novel stable isotope measure, populations of gray snapper in unfragmented sites had a niche width more than 11 times greater than populations from completely fragmented sites.  This niche width contraction is likely due to significant declines in the trophic diversity of common prey items in the more fragmented sites.  Niche width contraction has critical implications for the continued persistence of populations in fragmented habitats.  These results offer new insight into our understanding of the effects of aquatic ecosystem fragmentation on food web structure, as well as providing a new measure of niche width that can be used to examine populations of any organism amenable to stable isotope analyses.