Amphibian malformations
 Beginning around the mid-1990s, numerous reports of malformed amphibians generated widespread concern among scientists, health officials, and state and federal agencies. In large part, these malformations involved limb deformities in recently metamorphosed frogs. Extra limbs, partially and completely missing limbs, and a variety of other limb malformations (e.g., skin webbings, bony triangles) comprised many of the observed deformities. Although initial reports mentioned eye abnormalities, internal irregularities, and tumors, these problems were likely over-emphasized and are not discussed here. In the United States, concentrations of hotspots occur in the western US, the Midwest, and the Northeast (including parts of southern Canada). This issue is still very much alive, and new malformations sites continue to be discovered each and every summer. |
Amphibians as model hosts
 Global declines in amphibian populations have caused amphibians to become the most threatened class of vertebrates. Alongside habitat loss, infectious disease is one of the major driving forces behind such declines. Because of their biphasic life cycle, which typically involves an aquatic larval stage and a semi-terrestrial adult, amphibians are assaulted by a variety of pathogens, including fungi, flatworms (trematodes), tapeworms (cestodes), roundworms (nematodes), bacteria and various protozoa. As such, amphibians offer a valuable model system in which to explore how environmental change broadly affects infection prevalence and intensity in different pathogens. In cooperation with the US Fish and Wildlife Service, we are examining parasites of amphibians collected across National Wildlife Refuges in the United States . To date, we have assembled the largest amphibian parasite database, with well over 150,000 examined parasites from >20 amphibian species representing 20 US states. These data will be invaluable toward understanding ( i ) how community interactions among parasites affect the abundance of pathogenic species, ( ii ) exploring how parasite abundance and richness covary in response to latitudinal, longitudinal and land use gradients, and ( iii ) evaluating whether amphibian parasites can be use as indicators of environmental condition.
|
Diversity and disease
 Disease emergence and biodiversity loss are two of the most pressing environmental problems confronting human society. Each is driven predominantly by human-mediated changes in the environment. Although changes in the levels of biodiversity and infectious diseases have often been studied separately, the importance of reciprocal interactions between them has received comparatively little attention. Under what circumstances will disease epidemics drive species losses? And, reciprocally, can higher levels of biodiversity reduce parasite transmission? Through what ecological mechanisms? We have been using trematode parasites to study these questions in more detail, including Schistosoma mansoni (the causative agent of human schistosomiasis) and Ribeiroia ondatrae (the causative agent of amphibian malformations). Our results indicate that higher levels of host diversity can reduce transmission of both parasites. Candidate host species vary in their susceptibility to parasite infection and, in more diverse communities, the presence of less competent hosts act as parasite “decoys”, distracting infectious parasites away from more competent hosts and thereby reducing parasite abundance and host pathology.
|
Interactions among invasions and habitat alteration
 Identification and prediction of systems that function as “invasion hubs,” facilitating the subsequent invasion of numerous additional lakes, is a critical step toward focused and effective prevention of exotic species introductions. Together with Julian Olden (University of Washington) and Jake Vander Zanden (University of Wisconsin), we are evaluating the hypothesis that artificial impoundments (e.g., dams and reservoirs) facilitate biological invasions into freshwaters. By coupling data on waterbody physicochemistry and the distributions of five nuisance aquatic invaders (Eurasian watermilfoil, rusty crayfish, rainbow smelt, zebra mussels and spiny waterfleas), we found that invaders are between 2 and 300x more likely to occur in impoundments relative to natural lakes. Our current focus lies with ( i ) expanding this approach to other invaders and geographic regions, ( ii ) testing hypothesized mechanisms for this pattern, and ( iii ) using spatially-explicit models to evaluate the significance of reservoirs as “stepping stones” for invasion of natural systems.
|
Consequences of biological invasions in freshwater ecosystems
 Freshwater ecosystems are in the midst of a biodiversity crisis, with more threatened or extinct species than terrestrial and marine ecosystems combined. Human-mediated introductions of nonindigenous species, including invasive predators, parasites and competitors, are a major contributor to these alarming patterns. In many cases, however, the mechanisms through which invaders affect native biota, and how multiple nonindigenous species interact with one another, remain poorly understood. We are working to understand ( i ) the environmental factors that facilitate introduction and spread of invaders (e.g., see above) and ( ii ) the consequences of such invasions for native species, ecological communities, and ecosystem processes in freshwaters. We currently have projects focused on invasions by the Chinese mystery snail ( Bellamya chinensis ), the New Zealand mudsnail ( Potamopyrgus antipodarum ), the spiny water flea ( Bythotrephes longimanus ), amphibian chytridiomycosis ( Batrachochytrium dendrobatidis ), and the American bullfrog ( Rana catesbeiana ).
|
Ecological significance of disease in lake ecosystems
 Despite the ubiquity of parasites and pathogens, our understanding of their significance in mediating predator-prey interactions, competition, primary production and disturbance frequency is severely limited. To address these questions, we examine diseases of zooplankton with a focus on Daphnia hosts. Because of the keystone importance of Daphnia in lake food webs, both as a primary consumer and as a food resource for fish, any pathogen that regulates Daphnia populations will have cascading effects throughout the food web. For example, by combining experiments and histology, we found that infection by a chytrid fungus ( Polycaryum laeve ) caused reproductive castration, increased mortality, decreased growth, increased respiration, and a near cessation in migratory behavior within Daphnia hosts. Owing to increased conspicuousness of infected individuals, the parasite also caused a fourfold increase in the susceptibility of Daphnia to predation by planktivorous fish. Correspondingly, lake characteristics, especially water color and planktivore density, strongly influenced the spatial pattern and severity of epidemics. Current analyses of long-term data suggest that chytrid epidemics can regulate Daphnia populations, potentially altering both water clarity and food availability for fish.
|
|
|
