Ecological responses to predators and temperature in California mountain lakes
Increasing global temperatures can interact with local processes, causing indirect responses that are difficult to anticipate. Theory and experiments show that the strength of top-down forces structuring food webs increases with temperature; however little is known about these patterns vary in natural systems where community composition and species traits change along temperature gradients. To understand the combined effects of trophic cascades and temperature on lake ecosystems, we investigated the food web structure and ecosystem rates in lakes with and without introduced fish predators along an elevational gradient. We sampled the biotic community, including macroinvertebrates, zooplankton, phytoplankton, and bacteria to calculate the biomass of each trophic level and investigate changes in community composition. We measured litter decomposition and benthic production rates. A combination of linear mixed models and ANOVA were used to determine the effect of temperature and fish on trophic structure and ecosystem rates. Species turnover for communities with and without fish and along temperature gradients was compared using ANCOVA.
As predicted, zooplankton biomass increased more strongly than phytoplankton biomass with increasing temperature, and this effect was strongest in the absence of fish. The weaker effect of temperature on communities in the presence of fish is likely related to lower species turnover along the temperature gradient in lakes with fish. The fish-structured zooplankton communities had a smaller average body size, which may have made them more resistant to changes in temperature, as smaller body size is a common response to high temperature. Interestingly, the opposite patterns were found in the near-shore food web as macroinvertebrates, periphyton growth and litter decomposition were more strongly affected by fish at low temperatures, likely because benthic insects that consume periphyton and detritus were most abundant in high elevation fishless lakes. Our results indicate that top-down control increases at higher temperatures in the pelagic zone, but decreases in the benthos. Warming therefore interacts with trophic structure and species’ traits in natural systems to influence biomass pyramids and ecosystem rates by shifting the strength of top-down control.