Trophic Control of Fishless Ponds by Tiger Salamander Larvae

We examined the extent to which predation by larval tiger salamanders (Ambystoma tigrinum nebulosum) affected lower trophic levels in fishless ponds in montane eastcentral Arizona. An enclosure experiment manipulating salamander densities showed that effects associated with larval presence can cascade downward through three trophic levels in environments of low complexity. Enclosures with larvae contained comparatively lower herbivorous zooplankton biomasses and densities, higher chlorophyll a concentrations, and lower orthophosphate levels than those without larvae. Relative frequency of zooplankton species also varied significantly among enclosure treatments. Cyclopoid copepod frequencies were higher in treatments with salamanders than without, while ostracods were found only in treatments without salamanders. Salamander presence negatively affected cladoceran abundance and influenced cladoceran species composition. Proportions of both Ceriodaphnia quadrangula and Simocephalus vetulus were significantly greater in treatments without salamanders than in treatments with salamanders. Although Daphnia pulex frequencies were similar in enclosures with cladocerans across treatments, individuals were on average larger and biomasses were greater in enclosures without salamanders than they were in enclosures with salamanders. Trophic interactions analyzed from our survey of 14 ponds, however, were most often uncorrelated, and the few top-down effects exerted by salamanders were limited to contiguous trophic links below them. Densities of only one zooplankter, ostracods, and two taxa of macroinvertebrates, Baetis spp. mayflies and odonate larvae, were negatively related to salamander density. In contrast, benthic chironomid densities correlated positively with salamander density. However, in the two salamander-free ponds surveyed, we observed a distinct pattern of alternating responses of trophic levels that paralleled those in enclosure treatments without salamanders. Although modulating factors operate in ponds to complicate salamander impacts, salamanders have discernible direct and indirect effects on pond trophic webs.