Effects of spatial subsidies and canopy cover on pond communities and multiple life stages in amphibians

Spatial subsidies are resources that move from one ecosystem to another. In aquatic systems, canopy cover often determines both light availability (a major determinant of system productivity) and subsidy input in the form of senescing leaves and terrestrial invertebrates. This creates a system where subsidies are more prevalent in areas with low primary productivity and less prevalent in areas with high primary productivity. This phenomenon has been well studied in streams, and general patterns of ecosystem production, community structure, and the reciprocal export of animals with complex life histories have been discovered. I was interested in whether these patterns also occurred in ponds. I examined these patterns using experimental pond mesocosms and supported the results on community structure using an observational study of natural ponds. For the pond mesocosm experiment, I placed mesocosms along a canopy cover gradient and manipulated spatial subsidy input. For subsidy input treatments, I had mesocosms with subsidy input (leaves), a within-system resource (an aquatic grass), and no input. I examined the ratio of primary production to community respiration (GPP/CR24) and community structure seasonally over two years and the reciprocal export of frog metamorph biomass over three years. I found that canopy cover, but not subsidy input, affected GPP/CR24, indicating a shift from net heterotrophy in closed canopy mesocosms to a balance between heterotrophy and autotrophy in open canopy mesocosms. This is concordant with that found in streams, indicating that this shift in trophic state with changes in light availability may be common for aquatic ecosystems. The macroinvertebrate community structure responded to both canopy cover and subsidy input in mesocosms. The biomass of collectors (detritivores) was highest in mesocosms with litter input and increased with canopy cover. I found a similar trend in natural ponds with collectors, but herbivores decreased with increases in litter input and canopy cover. Herbivores were rare in mesocosms and did not respond to treatments. Unlike stream systems, shredders were rare in both natural ponds and pond mesocosms. Finally, I found that litter input (both grass and leaves) increased the reciprocal export of amphibian biomass compared to no input. Amphibian biomass also decreased with increases in primary productivity, but this trend may be due changes in temperature or in the algal community from the differences in light availability. This research highlights the importance of spatial subsidies that connect different ecosystem types. I found that many of the responses that stream ecosystems have to spatial subsidies are similar to that in ponds. Conserving these ecosystem connections will help maintain biodiversity and ecosystem function.