Diversity loss and disease emergence

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 (i.e., the dilution effect). 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 (see Johnson et al. 2008, 2009, 2012, 2013a, b).

Coinfection and parasite community assembly

Almost all hosts are infected with multiple parasite species. And despite growing evidence that parasite interactions can affect host pathology and parasite transmission, most research focuses on single host, single parasite interactions. We are interested in studying the importance of coinfection for disease ecology and evolution. In particular, we are combining cross-sectional surveys, experiments, and immunology to understand the full range of interactions within a parasite community, which may include dozens or more species. This work explicitly explores the scales at which parasites interact, how the outcome of such interactions depends on the type of parasites involved (e.g., viruses, fungi, and helminths) or host condition, and the consequences of parasite diversity for disease risk (see Hoverman et al. 2011, Johnson and Hoverman 2012, Johnson et al. 2013).

Climate change and infectious disease risk

Climate change is projected to influence the dynamics and distributions of many parasitic diseases of both humans and wildlife. Whether such changes will serve to increase or reduce disease depends on the differential thermal physiologies of parasites and their hosts. Available forecasts suggest that climate change will involve changes in both mean temperature and temperature variability, yet relatively little is known about how shifts in temperature affect parasite-host relationships.  We are combining experimental-based approaches both in the laboratory and in mesocosms with metabolic theory to develop a more predictive framework for how and when climate shifts will affect disease risk (see Paull and Johnson 2011, Paull et al. 2012, Hoverman et al. 2013, Altizer et al. 2013)

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 year. Click here for more information.

Amphibian declines

Amphibian populations are at the forefront of the global biodiversity crisis, with more extinct or declining species than any other class of vertebrates. In Colorado, the northern leopard frog (Rana pipiens) is classified as a Tier 1 “species of most concern,” but data on the severity and extent of apparent declines are currently lacking.  Our current goal is to advance amphibian habitat restoration and conservation in Colorado by examining interactions among major drivers of declines, including land use change, biological invasions, and the emergence of infectious diseases.  To this end, we are combining resurveys of historical sites (1900-1990) known to support native amphibians in Colorado with contemporary field sampling across different land use types to evaluate the individual and combined effects of land use change (e.g., urbanization), bullfrog invasions, and infections by the chytrid Batrachochytrium dendrobatidis.  We anticipate that results of our research will be directly applicable to managing and restoring amphibian wetlands throughout the western USA.  Click here for more information or see Blaustein et al. (2011), Johnson et al. (2011), McMahon et al. (2013).

Aquatic Parasite Observatory

Ongoing emergence of infectious diseases in wildlife have prompted renewed interest in the effects of disease on conservation. Infections ranging from salmonid whirling disease to crayfish plague have had devastating effects in managed and wild populations of aquatic organisms. The purpose behind the Aquatic Parasite Observatory is to investigate infections and their consequences for freshwater taxa, with a focus on amphibians, birds, snails, and fishes. This effort was initially launched to examine the role of infectious agents for amphibians, which have become the most threatened class of vertebrates worldwide due in part to infectious disease threats. In cooperation with the US Fish and Wildlife Service, we are examining parasites of amphibians collected across National Wildlife Refuges in the United 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 aquatic parasites can be use as indicators of environmental condition (see Johnson and Paull 2011).

Interactions between 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 (see Johnson et al. 2008).

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) (see Johnson et al. 2009, 2011, Olden et al. 2011, Preston et al. 2012).

Ecological significance of disease in food webs

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 (see Johnson et al. 2006a, b, 2009, Forshay et al. 2008, Peñalva-Arana et al. 2011).

 

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