A hump-shaped relationship between isolation and abundance of Notonecta irrorata colonists in aquatic mesocosms

We examined the relationship between the isolation of experimental aquatic mesocosms and the abundance of an aquatic insect colonist, Notonecta irrorata, over two years. We used a curve-fitting approach to assess whether linear or quadratic models better describe the relationship between isolation and abundance. For two measures of mesocosm isolation, distance to nearest source and distance to the largest source population, there was a significant quadratic relationship between isolation and abundance. Abundance of colonizing N. irrorata was not found to be significantly related to a third measure of isolation, mesocosm connectivity. These results indicate that the relationship between habitat isolation and colonist abundance may not be a monotonic decline across all spatial scales, a finding that contradicts the usual assumption incorporated in measures of habitat connectivity. Our results suggest that under some circumstances individuals that have undertaken dispersal may bypass patches they encounter early in this process and preferentially settle in patches encountered later. This behavioral preference in conjunction with decreased numbers of potential colonists at sites far from the source environment could lead to the "hump-shaped" colonist abundance by habitat isolation relationship we observed in this study. We suggest that simple assumptions about the relationship between habitat isolation and the probability a site is colonized need to be reexamined and alternative possible forms of this relationship tested.