The longleaf pine ecosystem was once the dominant vegetation of the Coastal Plain, and embedded within longleaf pine forests were numerous geographically isolated wetlands that supported a high proportion of regional biodiversity including many amphibian species. Historically, habitat conversion for agriculture and development was the primary threat to these wetlands, but today remnant isolated wetlands are often overgrown and hydrologically altered due to fire exclusion or incompatible, cool season fire regimes. In the absence of warm season fires when wetlands are dry, shrubs and trees succeed herbaceous plants, which alters wetland productivity via effects on light and detritus quality. In turn, wetland productivity determines larval amphibian performance including survival and size at metamorphosis, which are large determinants of amphibian population growth.
Therefore, the objectives of our work are to quantify the effects of hardwood encroachment on the growth, development, and survival of gopher frog (Rana [Lithobates] capito) and ornate chorus frog (Pseudacris ornata) tadpoles. Tadpoles were reared in outdoor aquatic mesocosms containing either leaf litter of an encroaching hardwood or a grass/sedge with and without a shading treatment. The results of this research will be utilized to inform management and restoration of longleaf pine wetlands.
Results/Conclusions
Survival of P. ornata was minimally affected by shading and litter treatments (proportion survived, 0.56-0.27). In contrast, survival of L. capito was significantly decreased by shading and hardwood litters (proportion survived, 0.08-0.95). Larval periods for both species were significantly increased by shading and in hardwood litters with higher phenolic levels (p < 0.001). Shading decreased metamorphic mass in both species (p < 0.05); litter effects on mass were variable for P. ornata while only pine litter decreased mass of L. capito (p < 0.05).
Here we show direct and indirect effects on survival due to shading and detritus quality; prolonged development results in recruitment failure when individuals fail to complete metamorphosis before wetlands dry. Reduced mass at metamorphosis, a significant driver of amphibian population growth, can also impact population viability through effects on adult survival and fecundity. Further, deleterious effects of hardwood litters in the absence of shading suggests that restoration of longleaf pine wetlands to open canopies may not yield immediate results. Overall, our results suggest that successional changes within geographically isolated longleaf pine wetlands due to fire exclusion may negatively impact P. ornata and L. capito populations through reduced survival, prolonged development and reduced mass.