Current Research: Cherie Briggs

I am a theoretical ecologist who combines mathematical and statistical models with laboratory or field observations and experiments to understand the factors affecting the dynamics of animal populations. I am specifically interested in how stage-specific processes, life-history traits and behaviors of individuals translate into effects on dynamics at the population level. My lab is currently working on three main projects involving either disease-host or parasitoid-host interactions, all in natural systems in California.

The Frog-killing Chytrid Fungus in the California Sierra Nevada

In this project we are investigating the factors that allow for persistence of populations of mountain yellow-legged frogs, Rana muscosa, in the presence of a potentially highly-lethal fungus, Batrachochytrium dendrobatidis.

Mountain yellow-legged frogs were once extremely abundant in the high elevation regions of the California Sierra Nevada. Now they are almost on the endangered species list. Although a number of factors (including introduced trout and chemical contaminants) have contributed to the decline of this species over the past century, the disease cytridiomycosis, caused by the chytrid fungus, B. dendrobatidis has resulted in population crashes and local extinction of a large number of populations over the last decade. However, in some regions of the Sierra, small infected populations of R. muscosa are apparently persisting with B. dendrobatidis. We are using a combination of field surveys, experiments, genetic and molecular techniques, and mathematical modeling to try to understand how some frog populations are able to persist with this pathogen while other populations are driven rapidly to extinction.

The Effects of Sudden Oak Death on Lyme Disease in California

In this project we are quantifying how Sudden Oak Death, caused by the introduced pathogen, Phytophthora ramorum, is potentially affecting the risk of Lyme Disease to humans.

Lyme disease is a vector-borne disease caused by the spirochetal bacteria, Borrelia burgdorferi (Bb). It is maintained in the wild in vertebrate hosts, and transferred from host to host through feeding by ticks. Lyme disease tends to be more of a problem to humans in the eastern US than in California. Differences in the tick species (Ixodes pacificus in California, versus Ixodes scapularis in the eastern US) and the timing of the tick life cycle contribute to this difference. Probably the largest factor leading to the low prevalence of Lyme disease in California, however, is the difference in the vertebrate host community in the different areas. In California, one of the best reservoir species for Bb is dusky-footed woodrats (Neotoma fuscipes), but a large fraction of the larval and nymphal ticks are fed instead by western fence lizards, Sceloporus occidentalis. Rather than acting as a host for Bb, these lizards have the ability to cleanse ticks of Bb infection. That is, any tick that is infected when it starts to feed on Sceloporus is uninfected when it detaches. We are developing and parameterizing models to quantify how the relative abundance of lizards, woodrats, and other vertebrate species in an area influences the prevalence of Lyme disease. The next step is to test the models through perturbing the system to determine if the system responds in the way predicted by models. That is where Sudden Oak Death comes in: we are using the changes to the forest community caused by Sudden Oak Death as a large-scale perturbation.

Sudden Sudden Oak Death (SOD), an introduced disease that is sweeping through California's oak woodlands and destroying dominant tree species such as Coast live oak (Quercus agrifolia) and tanoak (Lithocarpus densiflorans). SOD is caused by an introduced pathogen, Phytophthora ramorum and has the potential to impact local acorn production, leaf litter production and change the abiotic environment for the associated animal community.

The Effects of Multiple Parasitoid Species on the Gall-forming Midge, Rhopalomyia californica

In this project we are combining mathematical modeling with large-scale field experiments to understand the mechanisms of coexistence of multiple parasitoid species on a single host species.

The cecidomyid midge, Rhopalomyia californica, forms galls only on the shrub, Baccharis pilularis in coastal regions of California. The midge is attacked by a diverse assemblage of parasitoid species. I have found that the midge populations are suppressed to a small fraction of their potential density by the action of the community of parasitoids. I am interested in how the large number of parasitoid species coexist, and what effects the interactions between the parasitoid species have on the abundance and population dynamics of the midge.

Cherie Briggs | Research | Publications | Curriculum Vitae

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