The State of Amphibians in the United States
Serious declines are occurring in the United States but the number of extinctions is not as extreme as in other parts of the world. No amphibians have been declared extinct in the United States but concern is growing. For example, every native ranid frog species in the western United States is on a state or federal list of concern. Habitat loss is still the top cause of declines, but infectious disease remains another important cause. Batrachochytrium dendrobatidis (the amphibian chytrid fungus) was identified in the late 1990s and continues to affect amphibian populations. Another pathogen, Batrachochytrium salamandrivorans causes a similar disease in salamanders. It has seriously affected salamander populations in Europe, but has not yet been identified in the U.S. (Waddle et al. 2020). Its arrival (if it arrives) will have a devastating effect on U.S. salamanders because the U.S. is the global hot spot for salamander biodiversity. There are over 50 species of salamanders in the southern Appalachian Mountains in the eastern U.S. - 10% of the overall global diversity of salamanders! In the Pacific Northwest, there are more than 10 different species).
To address the major issue of amphibian decline, 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). In the U.S., there are 10 endangered anurans (i.e., frogs and toads) and 15 endangered salamanders. Over 40 frogs, toads and salamanders are listed by the U.S. Fish and Wildlife Service as threatened or endangered (ECOS: Home (fws.gov)).
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., Etard et al. 2022, Cordier et al. 2021, Seaborn et al. 2021, 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., Wright et al. 2020, 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 (Moor et al. 2022, Chandler et al. 2015).
Introduced Species
Many species of plants and animals have been moved to habitat outside of their native ranges. Sometimes movement has been deliberate, and sometimes by accident, but in many cases these non-native species are having 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 in the western U.S. that, among other things, can act as a vector for disease (Hossack et al. 2023, Yap et al. 2018, Brunner et al. 2019). Invasive American Bullfrogs are associated with declines of many native amphibians in the western United States (Fisher et al. 1996, Hossack et al. 2017).
Disease
The amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) is an emerging infectious pathogen that causes the disease chytridiomycosis. This disease causes excessive skin sloughing, loss of righting reflex and finally an imbalance in sodium in the body leading to heart attack and death (Voyles et al. 2007). This disease is contributing to amphibian declines worldwide (Scheele et al. 2019) and evidence indicates that Bd is a dominant factor in some of the more puzzling amphibian declines in the U.S. (e.g., Muths et al. 2003) and clearly has a detrimental effect on many species (e.g., Russell et al. 2019, Bradley et al. 2019, Muths et al. 2020). The long-term effects of Bd on amphibian populations is poorly understood, but even in populations that persist with Bd, marked declines are observed over time (e.g. Pilliod et al. 2010). Other amphibian diseases such as ranaviruses can have significant effects on populations but have not been linked to specific species declines (e.g., Muths and Hossack 2022). A pathogen related to Bd is Batrachochytrium salamandrivorans (Bsal). This fungal pathogen has extirpated populations of salamanders in Europe (e.g., Martel et al. 2013) and produces a disease with similar features to Bd induced chytridiomycosis. Bsal is not known to occur in the U.S. (Waddle et al. 2020), but there is concern that it could arrive via the pet trade (Waddle et al. 2020). If it does arrive, the consequences could be dire for salamanders. In the U.S., the Appalachian Region of the eastern seaboard and the Pacific Northwest are hotspots of salamander diversity specifically, these areas support the largest biodiversity of salamanders in the world. ARMI initiated and hosted an international workshop aimed at developing a North American surveillance monitoring strategy and identifying responses for the potential invasion of Bsal into the U.S. (Grant et al. 2015). ARMI has participated in tabletop scenarios to plan for responses to Bsal (Bernard and Grant 2021) and to disease in general (Canessa et al. 2020) and continues to evaluate regulatory action and monitoring efforts (Grear et al. 2021).
Conservation Challenges
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, Muths et al. 2020, 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 and requires collaboration among researchers, managers, and policy makers (Grant et al. 2019).
Given the limited dispersal ability and specific habitat requirements of most amphibian species, the likely changes in habitat (e.g., Walls et al. 2013) and behavior (Walls and Gabor 2019) due to climate change are of heightening concern. The potential for climate refugia (e.g., Mosher et al. 2018), and how the impact of disease on species may vary with changes in climate (e.g., Price et al. 2019) are topics that require more attention.
While protected areas such as National Parks provide some respite, even in these presumed “pristine” landscapes there are threats (e.g., Halstead et al. 2022). Populations are at risk from past management practices, habitat changes occurring along boundaries, and global threats such as disease that affect entire landscapes. Developing conservation and management strategies for all public lands is paramount for ensuring viable amphibian populations and the persistence of amphibians across the landscape of the U.S. To this end, ARMI is working with natural resource managers at local and national levels to provide actionable information and assist in developing strategies to mitigate stressors, and to protect and restore amphibians (e.g. Oyler-McCance et al. in press, Grant et al. 2023, Hossack et al. 2022).
Photograph by A. Cressler, U.S. Geological Survey.
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