Introduced predators and pathogens sever linkages between mountain lakes and surrounding watersheds
Introduced predators and pathogens can cause profound changes in food web structure and function. In many cases, these changes can propagate across habitat boundaries. We examine how the introduction of trout (for recreational angling purposes) and a fungal pathogen of amphibians have influenced consumer-mediated linkages between mountain lakes and surrounding watersheds. We present results from over 20 years of research in California mountain ranges, including extensive surveys, whole-lake manipulative experiments, and “natural” experiments.
We show that the introduction of trout has had profound impacts on both aquatic insect and amphibian communities in the Sierra Nevada and Klamath Mountains. Trout greatly reduced the abundance of aquatic larvae of large-bodied insects, resulting in reduced emergence of terrestrial adults; but trout tended to increase the abundance of smaller-bodied aquatic insects. Introduced trout also reduced the abundance of some amphibian species, particularly ranid frogs. These same frogs have proven to be highly susceptible to the introduced fungal pathogen Batrachochytrium dendrobatidis, which has led to significant declines and population extinctions of amphibian hosts in California mountain lakes. Collectively, the removal of large-bodied aquatic insects and amphibians – both of which have aquatic larvae and terrestrial (or semi-terrestrial) adults – diminishes the flow of energy from lakes to consumers in the surrounding watershed, such as birds and snakes. While ongoing restoration efforts have the potential to restore this pathway, more research is needed to improve our understanding of how aquatic-to-terrestrial resource flux affects the terrestrial component of mountain lake basins.