Species traits, habitat dynamics, and community interactions determine amphibian responses to changing water availability
Of the myriad of issues facing amphibian populations worldwide, the direct and indirect effects of climate change are among the most difficult to isolate and predict. The effects of climate change on species distribution do not occur for individual species in isolation but instead depend on interactions between climate, other co-occurring species, and the physical environment in which interactions occur. However, current models do not consider community dynamics or complex interactions between climate and the physical environment, making it difficult to predict how species distributions and community assemblages will be affected. For wetland communities, in particular, recent shifts in regional climate are likely to have profound and ongoing affects on site suitability and as a result, species composition. We examined how patterns of amphibian occurrence along the hydroperiod gradient responded to extreme climate events at St. Marks National Wildlife Refuge from 2009-2014, where fluctuations in temperature and precipitation have resulted in periods of extreme drought as well as expansive flooding. We developed a set of dynamic multispecies occupancy models that incorporate habitat dynamics and spatial structure (i.e. wetland distribution across the landscape) to investigate the occurrence dynamics of three representative amphibian species in response to changing water availability.
During periods of drought from 2009–2011, occurrence of the Ornate Chorus Frog (Pseudacris ornata) increased from 20% to 40% of surveyed wetlands. Occurrence of the Mole Salamander (Ambystoma talpoideum) declined from 22% to 10% due to wetland drying and reduction of wetland hydroperiod during this time. A different mechanism led to observed changes during extreme wet events in the later months of 2012. High rainfall was associated with extensive flooding, which allowed colonization of predatory fish species to many of the wetlands. Presence of fish was associated with a decline in the occurrence of Ornate Chorus Frogs in short hydroperiod wetlands (40% to 25%) and Mole Salamanders in long hydroperiod wetlands (25% to 15%). Occurrence of Pig Frogs (Rana grylio) remained fairly constant throughout the study period, and was not influenced by the introduction of fish. Projecting community composition under various climate scenarios (i.e. future increase in frequency of drought and/or flood events), we determined that responses are linked to species traits, wetland characteristics and the interaction between climate and the physical environment. These results demonstrate the mechanisms by which extreme events and changing water availability can destabilize wetland amphibian communities and lead to a loss of amphibian diversity.