COMBATING CHYTRID HOW ONE DISEASE HAS CHANGED AMPHIBIAN POPULATIONS WORLDWIDE

Jaclyn Reifeiss

Batrachochytrim dendrobatidis, or chytrid fungus, is one of the most destructive forces to have influenced amphibian populations in the last halfcentury. Sweeping across Central America in the 1980’s, the fungus caused the mass death of amphibians and total eradication of at least 90 species. While the populations remaining have mechanisms to resist the fungus and may even be increasing in numbers, there are few studies in place which monitor the long-term changes in amphibian abundance and distribution. It is therefore difficult to fairly assess the health of these populations. To address this issue, Becca Burks and I spent two months establishing a long-term amphibian monitoring project in Costa Rica’s largest private reserve.

When designing a comprehensive study aimed at improving conservation efforts, understanding the history of chytrid fungus is crucial. Chytrid is one of many diseases that has been transferred globally throughout a specific species due to human influence. First identified in southern Africa in 1938, chytrid was prevalent in 2.7% of all frog species in the region (Weldon et al. 2004). The global spread of chytrid fungus is attributed to the exportation of one species – the African clawed frog – which was found to function well as a pregnancy test. When female frogs were injected with the urine of a pregnant woman, the human chorionic gonadotropin in the urine would cause the frog to ovulate. The African clawed frog proved to be an invaluable medicinal tool, and thousands of individuals were exported each year beginning in 1940. With the development of chemical pregnancy tests the trade for medicinal purposes declined – however, the species remains in high demand for their use in scientific studies as well as the pet trade (Measey 2017).

Chytrid was first detected outside of Africa in 1989 in Australia, and throughout the 1990’s the remainder of the world began to find chytrid within its amphibian populations. Later evidence suggested, however, that chytrid fungus began its spread through Central America in the 1980’s undetected by scientists.

This fungus is particularly lethal because it targets the skin of amphibians, which is an integral part of maintaining homeostasis. As chytrid attaches to and degrades the keratinized portions of the skin of amphibians, gas exchange and fluid transport is disrupted. The resulting low sodium and potassium levels causes heart failure and death (CISR 2022). Today, chytrid fungus is recognized as the cause of sudden, catastrophic amphibian declines in Central America in the 1980’s. A 2019 study reported that the decline of at least 501 amphibian species across the planet is attributable to the spread of the fungus, and declines have continued through today (Scheele et al. 2019).

Reports of an amphibian comeback is largely based on the population trends of a few species. For example, Holdridge’s toad was believed to be extinct due to chytrid but was rediscovered in 2009 (Abarca 2010). Many regions of central America are lacking monitoring programs which would allow for a comprehensive understanding of amphibian population changes. This impacts our ability to select how and where to best spend conservation funds.

It is important that organizations overseeing large sections of land put long-term monitoring systems in place. The Children’s Eternal Rainforest (Bosque Eternode los Niños), located on the Pacific slope of Costa Rica, is the largest private reserve in the country. The reserve spans 55,800 acres and is home to a disproportionate percentage of the world’s biodiversity (ACMCR 2022). In June and July, Becca Burks and I served as interns at the reserve, working to establish the first long-term monitoring program of amphibians in the BEN.

Our work included the establishment of both stream and land survey transects at all four of the reserve’s stations. Between five and nine sites were established at each station. Every site was either a 100 meter land transect, a stream transect, or a listening point. Each type of site was designed to target a specific group of frogs. Leaf litter and arboreal species are abundant at land transects, while stream species such as the red-eyed stream frog and the emerald glass frog are likely to only be found at stream transects. Some species are rarely found below the canopy, so we created points where we counted the number of frog calls within five minutes. This can give us an idea of their relative abundance across years. We worked with local scientists to determine where best to establish these transects to observe the highest species diversity. We then recorded each site location with a GPS unit so these specific locations can be revisited each year.

We surveyed every site two times throughout the summer. This allowed us to both establish a repeatable methods system and collect preliminary data on the species present at each station. Our results yielded 20 species total and two species at locations outside of their known range. Hendrix’s professor emeritus, Dr. Matthew Moran, will continue to perform these surveys every rainy season for the foreseeable future. Our data will allow the Monteverde Conservation League, which oversees the Children’s Eternal Rainforest, to implement well-informed conservation strategies that better serve some of its most threatened occupants.

Costa Rica has an incredible diversity of habitat, which results in a plethora of species endemic to small regions of the country. Much of this biodiversity is dependent upon amphibians as part of their food chain – keeping insect populations in check while providing a food source for a number of groups such as bats, snakes, and raptors. Because of their permeable skin and narrow environmental range, amphibians are used as indicator species which allow scientists to assess the health of an ecosystem. Many species, such as the red-eyed tree frog, are used as conservation flagships and continue to draw in funding for the conservation of their habitats. Amphibians are currently the most threatened vertebrate group, but are key species in maintaining habitat for the immense biodiversity of Central America. Our implementation of a long-term monitoring program designed to track amphibian populations will better our ability to protect amphibians and maintain the ecosystems within protected areas.

References

Abarca, J., Chaves, G.,García-Rodríguez, A., & Vargas, R. (2010). Reconsidering extinction: rediscovery of Incilius holdridgei (Anura: Bufonidae) in Costa Rica after 25 years. Herpetological Review, 41(2), 150.

Bosque Eterno de los Niños, La Reserva Privada más grande de Costa Rica. ACMCR. (2022, March 10). Retrieved September 13, 2022, from https://acmcr.org/contenido/

Chytrid fungus. Center for Invasive Species Research. (2022). Retrieved September 13, 2022, from https://cisr.ucr.edu/invasive-species/chytridfungus

Measey, J. (2017). Where do African clawed frogs come from? An analysis of trade in live Xenopus laevis imported into the USA. Salamandra, 53(3), 398-404.

Scheele, B. C., Pasmans, F., Skerratt, L. F., Berger, L., Martel, A. N., Beukema, W., ... & Canessa, S. (2019). Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity. Science, 363(6434), 1459-1463.

Weldon, C., du Preez, L. H., Hyatt, A. D., Muller, R., & Speare, R. (2004). Origin of the Amphibian Chytrid Fungus. Emerging Infectious Diseases, 10(12), 2100-2105.