Autecological effects of habitat alteration: trophic changes in mangrove marsh fish as a consequence of marsh impoundment

The key to predicting, counteracting, and managing environmental change is understanding the mechanisms through which habitat change affects populations of organisms. The mangrove ecosystem of the Indian River Lagoon (IRL), east-central Florida, has been impounded for the purpose of mosquito control and is representative of such large-scale alterations. Sailfin mollies Poecilia latipinna in this system exhibit significant interpopulation differences in adult body size in relation to habitat alteration. One likely hypothesis for this pattern is the apparent change in diet experienced as a result of impounment, as has been documented in earlier studies. To address the question of whether these dietary changes would have been sufficient to cause trophic differences in P. latipinna populations relative to impoundment, a comparative study of trophic position in 2 size-divergent populations of P. latipinna using stable isotopes was employed. Results suggested that P, latipinna from an unimpounded site had a slightly higher trophic position than those from an impounded site and may constitute a partial explanation of differences in adult body size between the 2 populations. Results from the present study also showed that there was a fundamental difference in energy source between the study sites, which suggests greater assimilation of mangrove-based carbon in impoundments. Specific population- and community-level effects of impoundment on mangrove habitats have been demonstrated, indicating a mechanistic link between anthropogenic habitat alteration and life history of P. latipinna.