Southern mountain yellow-legged frog, Rana muscosa

Current Research

I am a field ecologist who uses large-scale observational studies, natural experiments, and field experiments to provide insights into how natural and anthropogenic stressors influence the structure and function of aquatic ecosystems. Addressing these often complex questions in a meaningful way requires expertise from a wide range of scientific disciplines, and most of my current research involves collaborative studies with mathematical ecologists, molecular geneticists, taxonomists, and other field biologists. I am keenly interested in applying our research findings to the management and restoration of aquatic ecosystems in the Sierra Nevada.

Effects of Introduced Fish on Lake Ecosystems

The introduction of predatory fish into naturally-fishless lakes typically results in dramatic changes to species composition and to ecosystem processes such as nutrient cycling. In 1995, I began studying the effects of nonnative trout on Sierra Nevada lake ecosystems and on the ability of these systems to recovery following fish removal. This research involved (1) surveys of amphibians, reptiles, invertebrates, and fish at more than 7,000 lakes and ponds, and (2) a whole-lake fish removal experiment designed to quantify faunal recovery in alpine lakes following removal of nonnative trout. The results of much of this research has been published (see Publications), but I continue to monitor the whole-lake fish removal experiment.

The whole-lake fish removal study was initiated in 1996, and involved the removal of nonnative trout from five lakes that were fishless historically. Changes in vertebrate and invertebrate communities in these lakes are being compared to those in three adjacent trout-containing lakes. The majority of taxa expected to have occupied these lakes prior to fish introduction recolonized within five years of fish removal, but continued monitoring indicates ongoing changes in species composition and densities.

We are now using this experiment to study how trout alter linkages between lake and terrestrial ecosystems. For example, large aquatic insects are abundant in fishless alpine lakes but are virtually eliminated by fish introductions. Research by my colleague, Peter Epanchin, shows that alpine-nesting birds forage preferentially at fishless lakes due to the increased abundance of insects relative to their abundance at fish-containing lakes.

Role of an Emerging Disease in Driving Amphibian Declines

The mountain yellow-legged frog was once one of the most abundant vertebrates in the Sierra Nevada, and occupied thousands of lakes and ponds throughout the range. During the past century, this frog has disappeared from more than 90% of its historic habitat, and is now being considered for listing under the U.S. Endangered Species Act. (For more information about the mountain yellow-legged frog, check out

Although introduced trout are undeniably a major cause of this decline, our recent research indicates that an emerging disease is also playing a critically important role. Chytridiomycosis is a disease caused by the amphibian chytrid fungus, Batrachochytrium dendrobatidis. This recently-described pathogen is causing amphibian declines worldwide, and its effects are particularly severe for amphibians living in montane habitats. In the Sierra Nevada, several amphibian species are known to be infected with chytridiomycosis, and the mountain yellow-legged frog is  particularly impacted by this disease. Our research has demonstrated that B. dendrobatidis probably arrived in the Sierra Nevada relatively recently, has now spread across most of the range, and is responsible for the recent extinction of hundreds of mountain yellow-legged frog populations.

Although most mountain yellow-legged frog populations crash to extinction following the arrival of B. dendrobatidis, some populations survive the crash and subsequently persist with the disease. We are currently using field surveys, field and laboratory experiments, molecular genetic techniques, and mathematical modeling to understand these different disease outcomes (extinction versus persistence). Possible mechanisms include differences in frog susceptibility to chytridiomycosis, differences in pathogen virulence, effects of habitat conditions on susceptibility and/or virulence, and density-dependent disease dynamics.

Taxonomy of Lake-dwelling Fauna

Despite decades of interest by both the general public and scientists in the Sierra Nevada, most of the attention has focused on terrestrial ecosystems. The fauna of aquatic habitats, especially lakes, has remained largely undescribed. Given that detecting future faunal changes requires knowing what species currently occur, we are using morphological keys and DNA markers to identify lake-dwelling taxa to the species level. This is a long-term project that requires collaboration with taxonomic experts because a significant number of collected taxa are unknown to science and will need to be described. In addition, required morphological keys are often outdated and incomplete. For a list of taxa that we have identified to date, see the Lake Fauna page.