Amphibian Population Declines in Colorado
Amphibian populations are at the forefront of the global biodiversity crisis, with more extinct or declining species than any other class of vertebrates. Putative causes of observed declines are numerous and frequently controversial. Habitat loss and alteration are considered the most significant drivers of declines, but additional causes include infectious disease, introduced species, and changes in climate patterns. Ultimately, however, drivers of amphibian population declines interact through complex mechanisms, leading to the destabilization of long-term population viability. Efforts aimed at amphibian conservation and habitat restoration must recognize the significance of such interactions if they are to be successful, yet most studies continue to assume the perspective of single-factor driven declines.
Interactions between Invasions and Disease
Biological invasions and the emergence of infectious diseases represent two of the greatest threats confronting native amphibians. The introduction and spread of American bullfrogs (Rana catesbeiana), for example, has contributed to large-scale declines of native amphibians in western North America, South America, Europe, and Australia. The fast-growing tadpoles often out-compete native species, while the large adults are generalist predators that consume a broad diversity of native species. Similarly, emergence of the chytridiomycete pathogen Batrachochytrium dendrobatidis (“Bd”) has led to devastating amphibian population losses and is considered one of the greatest threats to global amphibian diversity. For example, boreal toads (Bufo boreas) in Colorado have suffered severe declines from chytridiomycosisand are now endangered in the State.
Despite advances in the scientific community’s understanding of these causes individually, we know comparatively little about how drivers of amphibian population declines interact with one another. A major goal of the current study is to adopt a multi-systems approach by integrating interactions among different forms of environmental change. Growing evidence suggests that bullfrog invasions and Bd infection may not always be independent. Although often infected with Bd, bullfrogs are relatively insensitive to its pathology. Bullfrogs may therefore act as reservoir hosts, helping to transport Bd to new environments in which it infects sensitive native amphibian species. In this manner, the introduction and subsequent spread of non-native bullfrogs could accelerate the spread of Bd and exacerbate the severity of declines. While Bd is capable of causing declines in the absence of bullfrogs (as are bullfrogs in the absence of Bd), the joint effects of both stressors may compound losses in native species. A clear understanding of the relative roles of bullfrogs, Bd and their interaction is essential for management, as any efforts directed toward amphibian conservation are unlikely to succeed unless the threats are correctly identified and contained.
In Colorado, 9 out of 16 native anuran species are classified as endangered, threatened or of special concern according to Colorado’s Comprehensive Wildlife Conservation Strategy (2006). For example, the Northern leopard frog (Rana pipiens) is classified as a Tier 1 “species of most concern” and is currently a candidate for federal protection in the western USA. Over the last two decades, herpetologists and local agencies have reported that leopard frog numbers are but a fraction of their historical abundance along Colorado’s Front Range. In high elevation sites, for example, leopard frogs appear to have been largely extirpated. In other regions, however, the status of this species is less certain. In Colorado, the apparent decline of the Northern leopard frog has coincided with increases in: 1) habitat alteration, 2) invasion by bullfrogs, and 3) emerging infectious disease. While each factor has likely contributed to decreases in native amphibians, the relative importance of each as well as their potential interactions remains poorly understood. A more complete understanding of the interactions between disease emergence and biological invasions is critical toward enabling successful efforts aimed at promoting amphibian health, and will help improve amphibian habitat restoration and conservation.
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. Specifically, 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 Batrachochytrium infections (see Johnson et al. 2011; Johnson et al. 2013; Peterson et al. 2013). While each of these factors can cause harm in native species, they may also interact to drive declines; for example, bullfrogs are immune to chytridiomycosis and can therefore spread the infection to native species (reservoir hosts). This project is also helping to build collaborations among the University of Colorado, the Colorado Division of Wildlife (DOW), the US Geological Survey and local conservation agencies. We anticipate that results of our research will be directly applicable to managing and restoring amphibian wetlands throughout the western USA.