Light, nutrients and community affect amphibian development and growth in experimental pond systems
In ponds, canopy cover influences the energy and nutrients available in the food web through leaf litter and by altering light. The majority of canopy-cover studies on pond-breeding amphibians have focused on anurans and concluded open canopy is better; the relatively few studies on predatory salamanders have mixed results. To our knowledge, no one has separated the effects of light and nutrients and compared how they individually and interactively influence amphibian larval development and growth. In summer 2014, we conducted an experiment in 1000 L replicated experimental ponds. We manipulated light supply (low/high), nutrients (low/high), southern leopard frog tadpoles (Lithobates sphenocephalus; presence/absence) and larval spotted salamanders (Ambystoma maculatum; presence/absence). These species are often found in the same ponds and occupy different feeding niches, so comparing the effect of each individual species with the two together provides insights into food-web dynamics. We hypothesized that nutrients would be the most important factor in determining development of both species and that co-occurrence of tadpoles and salamander larvae would increase growth and development through indirect food-web effects.
Our results suggest the components of canopy cover differentially affect anurans and salamanders. For frogs, nutrients were the most important predictor; for salamanders, light was more important. We observed strong bottom-up effects of light and nutrients on algal biomass, but also top-down effects of tadpole presence on periphyton biomass and chironomid abundance. Both salamanders and tadpoles significantly, indirectly influenced phytoplankton biomass. This research shows that separating out light and nutrients from ‘canopy cover’ can explain more variation in amphibian response; it also highlights the need for better understanding of trophic dynamics within ponds.