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Coral reef systems can undergo rapid transitions from coral-dominated to macroalgae-dominated states following disturbances, and models indicate that these may sometimes represent shifts between alternative stable states. While several mechanisms may lead to alternate stable states on coral reefs, only a few have been investigated theoretically. We explore a model that illustrates that reduced vulnerability of macroalgae to herbivory as macroalgae grow and mature could be an important mechanism: when macroalgae are palatable to herbivores as juveniles, but resistant as adults, coral-dominated and algae-dominated states are bistable across a wide range of parameter space. We compare two approaches to global sensitivity analysis to rank the relative importance of the model parameters in determining the presence and magnitude of alternative stable states, and find that the two most influential parameters are the death rate of coral and the rate of maturation of algae out of the vulnerable stage. The Random Forest approach for global sensitivity analysis, recently adopted by ecologists, provides a more efficient method for ranking the relative importance of parameters than a variance-based approach that has been used frequently by computer scientists and engineers. Our results suggest that managing reefs to reduce chronic stressors that cause coral mortality and/or enhance the growth rates of algae can help prevent reefs from becoming locked in a macroalgae-dominated state.

Coral reef systems can undergo rapid transitions from coral-dominated to macroalgae-dominated states following disturbances, and models indicate that these may sometimes represent shifts between alternative stable states. While several mechanisms may lead to alternate stable states on coral reefs, only a few have been investigated theoretically. We explore a model that illustrates that reduced vulnerability of macroalgae to herbivory as macroalgae grow and mature could be an important mechanism: when macroalgae are palatable to herbivores as juveniles, but resistant as adults, coral-dominated and algae-dominated states are bistable across a wide range of parameter space. We compare two approaches to global sensitivity analysis to rank the relative importance of the model parameters in determining the presence and magnitude of alternative stable states, and find that the two most influential parameters are the death rate of coral and the rate of maturation of algae out of the vulnerable stage. The Random Forest approach for global sensitivity analysis, recently adopted by ecologists, provides a more efficient method for ranking the relative importance of parameters than a variance-based approach that has been used frequently by computer scientists and engineers. Our results suggest that managing reefs to reduce chronic stressors that cause coral mortality and/or enhance the growth rates of algae can help prevent reefs from becoming locked in a macroalgae-dominated state.

Declines in amphibian populations have been reported throughout the world in recent years. Chytridiomycosis, a disease of amphibians caused by the chytrid fungus, Batrachochytrium dendrobatidis, is one of a number of factors that have been shown to contribute to these population declines. B. dendrobatidis is associated with rapid population declines and local extinctions of populations of mountain yellow-legged frog, Rana muscosa, in some areas of the California Sierra Nevada mountains, however in other areas R. muscosa populations are persisting in the presence of this fungal pathogen. We conducted field surveys and laboratory experiments to investigate several factors that may explain these different population-level outcomes of B. dendrobatidis. Our field surveys revealed that R. muscosa at persistent sites are carrying very low fungal loads, which lead to sub-clinical infections. At these sites, B. dendrobatidis is having no detectable effect on adult R. muscosa survival (although subadults may be experiencing high disease-induced mortality), and some adults are apparently losing the infection. At sites experiencing frog population crashes, a large fraction of R. muscosa adult are carrying extremely high fungal loads, and very few infected adults are surviving over the course of a summer, or between years. We found no consistent differences in temperature profiles between the two types of sites. Our laboratory experiments detected no difference in transmission rate or virulence of fungal strains from persistent versus die-off sites. We are investigating differences in susceptibility of frogs from the two types of sites in an experiment currently underway.

Declines in amphibian populations have been reported throughout the world in recent years. Chytridiomycosis, a disease of amphibians caused by the chytrid fungus, Batrachochytrium dendrobatidis, is one of a number of factors that have been shown to contribute to these population declines. B. dendrobatidis is associated with rapid population declines and local extinctions of populations of mountain yellow-legged frog, Rana muscosa, in some areas of the California Sierra Nevada mountains, however in other areas R. muscosa populations are persisting in the presence of this fungal pathogen. We conducted field surveys and laboratory experiments to investigate several factors that may explain these different population-level outcomes of B. dendrobatidis. Our field surveys revealed that R. muscosa at persistent sites are carrying very low fungal loads, which lead to sub-clinical infections. At these sites, B. dendrobatidis is having no detectable effect on adult R. muscosa survival (although subadults may be experiencing high disease-induced mortality), and some adults are apparently losing the infection. At sites experiencing frog population crashes, a large fraction of R. muscosa adult are carrying extremely high fungal loads, and very few infected adults are surviving over the course of a summer, or between years. We found no consistent differences in temperature profiles between the two types of sites. Our laboratory experiments detected no difference in transmission rate or virulence of fungal strains from persistent versus die-off sites. We are investigating differences in susceptibility of frogs from the two types of sites in an experiment currently underway.