Abstract
Biotic interactions between herbivores and primary producers are believed to play a major role in determining community composition within many ecological communities. How differences in herbivore abundance alter the relative importance of these interactions is not well understood, especially in seasonal wetland systems. This study investigated the effects of varying Pseudacris regilla (pacific chorus frog) larval densities on California vernal pool aquatic communities. It was hypothesized that with increasing tadpole density, macrophyte abundance, biomass and richness would decrease; while algae and zooplankton abundance, biomass and richness increase. The experimental mesocosm design consisted of five tadpole density treatments, and was arranged in a 5 x 5 randomized block design. Each density treatment was replicated five times. Macrophytes, algae and invertebrates were identified to the lowest possible taxonomic unit, and the biomasses of each community were quantified. Turbidity, dissolved oxygen, nitrate, orthophosphate, temperature, conductivity and pH were measured monthly throughout the experiment. Treatment effects were determined using both multivariate and univariate analysis of variance techniques. To detect differences in community composition, multivariate ordination analyses (ANOSIM, SIMPER, nMDS) were conducted. Increasing densities of P. regilla tadpoles had a large impact on the vernal pool macrophyte community. Plant abundance, diversity, biomass, percent cover and species richness were all negatively impacted by increasing tadpole densities. Macrophyte community structure was also found to be significantly different in the presence of tadpoles due to the fact that Downingia bicornuta, Ranunculus aquatilis, and Gratiola ebracteata were predominately isolated to mesocosms were P. regilla was absent. In addition, periphyton biomass was shown to be facilitated by increasing tadpole densities in the lower treatment ranges. Aquatic macroinvertebrates and phytoplankton communities, on the other hand, were not significantly affected by increasing tadpole densities. This study demonstrates the impact of P. regilla larvae on the primary producers within California vernal pool systems. Understanding the impact of these abundant consumers, and the mechanisms that drive the importance of their direct and indirect effects are essential for the conservation of vernal pools and the threatened species associated with them.