The Bahamas

Seaweed Subsidies

The flow of material from one ecosystem to another represents an important component of global biogeochemical cycles. Increasingly, these flows are influenced by anthropogenic factors such as fertilizer application, species introductions and extinctions, and climate change. Since 2005, I have been studying the effects of seaweed deposition on terrestrial ecosystems in the Bahamas. These seaweed subsidies support an abundance of detritivores, which attract terrestrial predators. Stable isotope analyses reveal that these predators consume more marine-derived prey in the presence of seaweed, increasing herbivore pressure on terrestrial plants. In addition, plant growth rates increase, as does the nutritional content of terrestrial plants, suggesting that seaweed also has a fertilization effect. In a subsequent study combining seaweed manipulations with predator inclusion/exclusion treatments on small islands, we found that the interactive effects of predator species on lower trophic levels was influenced by the presence of resource subsidies. Specifically, ants and lizards had synergistic effects on herbivory in the absence of seaweed which disappeared in the presence of seaweed. These findings suggest that resource subsidies can have profound effects on ecological communities that are propagated through multiple levels of the food web. We are currently working on evaluating the effect of variation in the timing and magnitude of seaweed subsidies community responses and comparing the effects of above-ground and below-ground pathways of resource transfer. This new study will provide insight into how spatial and temporal variation in resource availability affects ecological communities and contribute to our understanding of the effects of increased hurricane frequency on terrestrial ecosystems.

Ant-Plant Mutualisms

Positive interactions between species are ubiquitous but have received much less attention from biologists than antagonistic interactions. Ant-plant mutualisms, in which ants protect plants from herbivory and in return receive a reward from the plant, are common features in many ecosystems. There is often a great deal of variation in the strength of these interactions, which can have important consequences for the ecology and evolution of the species involved. I have been studying the causes of variation in the effect of mutualistic ants on a common mangrove in the Bahamas using a combination of field experiments and observational studies. I found that plants with a high density of leaf hairs do not receive as much of a benefit from ants as plants with a low density of leaf hairs and do not offer as much of a reward for ants, suggesting that these two morphs are on distinct evolutionary trajectories with regard to anti-herbivore defense. I also found that simulated hurricane disturbance increased the amount of reward produced by plants for ants and the strength of competition between ants and other arthropod predators and that seaweed and the presence of lizard predators influence the effect of ants on plants (as mentioned above). I am currently working with collaborators to study of the effects of multiple vertebrate predator species on the strength of the ant-plant mutualism. These studies enhance our understanding of the ecology and evolution of positive interactions between species by showing how ecological context influences the beneficial effect of mutualistic partners.

The Mountains of Northern California

Amphibian Disease

Invasive species can have profound impacts on communities and ecosystems. In particular, the introduction of novel pathogens often has disastrous consequences for naïve hosts. For example Batrachochytrium dendrobatidis (Bd), an emerging fungal pathogen of amphibians, has been implicated in die-offs and extinctions worldwide. Along with collaborators at the University of California, Davis, the US Forest Service, and the US Fish and Wildlife Service, I am examining the effects of Bd on native amphibians in the mountains of northern California. Bd infects at least four of the seven amphibians that are common in lakes and ponds in this region. We have conducted extensive sampling for Bd throughout this region in order to investigate how environmental factors influence the relationship between Bd and its amphibian hosts. Using generalized linear models (GLMMs) and multi-model inference, we found differences between amphibian species and life stages in the rate of Bd infection and pronounced seasonal patterns of infection that differ between life stages and between lakes at different elevations, suggesting that both biotic and abiotic factors affect the relationship between host and pathogen.

We are also conducting intensive studies of the interaction between Bd and the Cascades frog (Rana cascadae), which has experienced severe declines in the mountains of northern California. We have been monitoring Cascades frog populations and sampling for Bd at 15 sites. Preliminary data suggests that Cascade frog populations with higher rates of Bd infection have lower survival. This year, we have reared frogs from 7 of these populations in the laboratory and will test for population-level differences in Bd susceptibility (using Bd strains from multiple populations) under laboratory conditions. This experiment will provide insight into whether genetic differences between host and pathogen populations contribute to differences in disease susceptibility.

Symbiotic skin bacteria may help explain variation in the effects of Bd on amphibians. Naturally-occurring skin bacteria can limit the effect of Bd on amphibians. We are currently cataloguing the diversity of microbes that occur on the skin of Cascades frogs and are conducting laboratory experiments to determine if any of these bacteria have anti-fungal activity and can influence the outcome of interactions between Bd and Cascades frogs. These experiments will help determine whether symbiotic microbes contribute to disease resistance.

Introduced Trout

Introduced trout have dramatic effects on ecological communities in mountain lakes. These active predators can also sever linkages between lakes and surrounding terrestrial habitats by consuming aquatic organisms before they metamorphose into terrestrial adults. I have been working in collaboration with scientists at UC Davis and the US Forest Service to examine the effects of trout on connections between the aquatic and terrestrial ecosystems using whole-lake trout manipulations. We found that trout reduce the biomass and composition of emerging aquatic insects and reduce the proportion of aquatic prey in frog diets. We are currently assessing the long-term effects of trout removal and stocking cessation on amphibian populations.

The Eel River

River Food Webs and River-to-Ocean Nutrient Transport

Rivers represent important conduits for the flow of energy and resources from land to sea. However, there is considerable debate about the importance of in-stream biological processes in governing the magnitude and timing of resource export from rivers. I recently began working with a team of researchers to examine how species interactions in northern California’s Eel River influence resource transport through the river to the sea. In the Eel, seasonal blooms of macroalgae represent significant concentrations of resources, in part due to the presence of nitrogen-fixing cyanobacteria. The growth of these algal blooms is largely determined by hydrological factors, such as the timing and intensity of seasonal flooding, which influence the composition of the grazer community. I will combine observational studies of resource fluxes and models of watershed processes (such as streamflow and sediment transport) to investigate how biological and hydrological processes govern the downstream transport of algal resources through the river to the sea. The results of these studies will be coupled with biophysical models of the coastal ocean and regional climate models in order to explore how nutrient export from the Eel influences oceanic and atmospheric systems.