Meeting Abstract
An unresolved question in integrative biology is why natural selection does not favor the evolution of greater environmental tolerance and the expansion of geographic ranges. Common hypotheses often invoke a role for competition on range limits, but few l examples exist in nature. Here we test how salinity and competition impact life history traits in two closely related euryhaline fish; Poecilia reticulata and Poecilia picta. Both species can tolerate and reproduce in brackish and fresh water, yet on Trinidad the species segregate along a salinity gradient. P.reticulata is found alone in upstream freshwater sites, while P. picta is found alone in downstream brackish water sites. Both species coexist in lowland freshwater streams near the transition zone with brackish water. To test how competition and salinity might interact to prevent P.picta from moving upstream and P. reticulata from expanding its range into brackish water, we measured how fecundity, offspring size, and reproductive allocation change where the species do and do not co-occur. We found that offspring size increases for both species where they coexist. Larger offspring sizes have been suggested to be a strategy that can ameliorate the fitness costs of competition. However, the increase in offspring size came with no apparent cost in the other traits measured, as reproductive allocation and offspring number remained unchanged for both species. These results suggest competitive interactions between species may interact with salinity to shape range limits. Further, our results highlight why a broad, integrative approach incorporating both abiotic and biotic factors is needed to explain range limitations.
