The State of Amphibians in the United States
The number of extinctions in the United States is not as extreme as in other parts of the world, but serious declines are occurring. For example, every native ranid frog species in the western United States is on a state or federal list of concern, and infectious disease remains one of the primary causes of declines. Salamander chytridiomycosis (i.e., Bsal), a recently described disease that is originally from Asia and now causing amphibian declines in Europe, could arrive in the U.S. via the pet trade. Bsal would be a particular threat to salamanders in the U.S., where we have the greatest salamander diversity in the world.
To address this serious issue, the U.S. Geological Survey (USGS) Amphibian Research and Monitoring Initiative (ARMI) conducts research to support the needs of management agencies in the U.S. ARMI estimates a 3.7 percent average annual decline in the proportion of sites occupied by amphibians within ARMI study areas (Adams et al. 2013). This rate of decline means that the average amphibian species will be gone from half of the places where it now occurs in < 20 years (Grant et al. 2016). Extinction is a real outcome, with declines threatening 30 percent of the world’s approximately 7,000 amphibian species (Scheele et al. 2019).
Why are Amphibians Declining?
Habitat Destruction and Alteration
Land-use change is a key reason for the loss of global biodiversity and is the most obvious cause of amphibian declines in the United States (e.g., Bodinof Jachowski and Hopkins 2018, Haggerty et al. 2019). Destroying habitat directly eliminates populations and increases the isolation of remaining populations in a landscape. When the space between habitat patches is increased, the ability of amphibians to move across the landscape to find food, breed, or recolonize areas decreases (e.g., Billerman et al. 2019, Hansen et al. 2019). The effects of habitat loss are dramatic but in some ways could be the easiest to manage (e.g. Cayuela et al. 2018), and habitat restoration or creation is an effective recovery tool for many species (Petranka et al. 2007, Chandler et al. 2015).
Many species of plants and animals around the world are now found in areas outside their natural range. Some of these introduced species become invasive and cause can have both direct and indirect negative effects on amphibians. For example, the introduction of sport fish to formerly fishless habitats is a major concern for amphibians in the western United States (Pilliod et al. 2010), and the American Bullfrog is a widely introduced species that, among other things, can act as a vector for disease (Yap et al. 2018, Brunner et al. 2019). Invasive American Bullfrogs are associated with declines of many native amphibians in the western United States (Jennings and Hayes 1986, Fisher et al. 1996, Hossack et al. 2017).
Emerging infectious diseases, especially ranaviruses and the amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) are contributing to amphibian declines (Scheele et al. 2019). Evidence suggests that Bd is a dominant factor in some of the more puzzling amphibian declines around the world and has affected species in the United States. The long-term effects of Bd on amphibian populations in the United States are still poorly understood. Recent ARMI syntheses indicate the pathogen has a negative effect on many species, although effects vary across species and even among populations within the same species (Russell et al. 2019).
We are faced with the potential disappearance of a significant proportion of an ancient group of vertebrates. Adding to the complexity of amphibian decline, climate change is an overarching threat with the potential to interact with other causes of decline (Rollins-Smith 2017, Miller et al. 2018, Cohen et al. 2019). Our challenge is to refine our understanding of amphibian declines by identifying causes , and then developing appropriate management responses. Meeting this challenge is critical to conserving the unique amphibian resources of the United States.
Given the limited dispersal ability and specific habitat requirements of most amphibian species, developing conservation and management strategies is paramount for ensuring viable populations. Even in protected areas, populations are at risk from past management practices, habitat changes occurring along boundaries, and global threats such as disease that affect entire landscapes. ARMI is working with natural resource managers at local and national levels to develop strategies for effective management. Effective strategies combine our understanding of population trajectories with detailed research into factors that are related to these trends.
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