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Updated amphibian population trends at Rainbow Bay, South Carolina: Natural and human influences.
Pechmann, Joseph*,1, Scott, David2, Gibbons, J. Whitfield2, Greene, Judith2, Metts, Brian2, Dixon, Philip3, 1 University of New Orleans, New Orleans, LA2 Savannah River Ecology Laboratory, Aiken, SC3 Iowa State University, Ames, IA
ABSTRACT- Evaluating the extent and causes of declines in amphibian populations requires long-term census data. We have counted amphibians migrating to and from Rainbow Bay, SC daily since September 1978 using a drift fence with pitfall traps. Numbers of breeding adults and metamorphosed juveniles of five focal species varied among years by several orders of magnitude. Breeding population sizes of Ambystoma tigrinum, Pseudacris nigrita, and P. ornata declined to near zero over the study. Breeding female A. opacum increased from zero to >1000 during the period, whereas A. talpoideum exhibited no overall trend. Population declines were related to recurrent intense droughts, especially those associated with strong La Niña events, which reduced or eliminated breeding activity and juvenile recruitment. Although we have viewed drought effects as natural fluctuations, there are suggestions that global warming may have increased drought frequency and severity. Drought and fire suppression allowed trees to invade the pond basin over the study, which may have reduced hydroperiods. The effects of canopy closure on temperature, food resources, and other factors may have made the habitat less suitable for Pseudacris. Ambystoma opacum was less affected by droughts than the other Ambystoma because of its life history. Predation by the increased numbers of A. opacum larvae may have reduced juvenile recruitment of other species. Population trends at Rainbow Bay have many causes which are difficult to distinguish because many are temporally confounded.
Key words: global warming, population dynamics, amphibian decline, canopy closure