It takes more than climate change to cause amphibian decline
This post contributed by Monica Kanojia, Administrative Assistant/Governance for ESA.
Amphibians have been around for hundreds of millions of years. They have survived numerous extinction events and yet somehow, in the past two decades, their numbers have been in severe decline. The population changes have been linked to many factors, including climate change and disease, habitat destruction and water pollution. Studies indicate that amphibians are sensitive to all of the proposed variables—not just one root cause.
A unique quality of amphibian biology is their transdermal water uptake ability. Transdermal uptake allows for nutrients to be delivered across the skin. For example, the skin of a frog allows for the direct exchange of oxygen, carbon dioxide and water from the environment. While in ideal situations this would be beneficial, it currently poses a threat to amphibian populations. Overexposure to any nutrient can be lethal to an organism. With increased rates of carbon dioxide in the atmosphere, heavily polluted water and loss of water, amphibians’ ability to survive is diminishing.
A majority of amphibian species go through reproductive and developmental stages that require a body of water. The eggs of amphibians are not as resilient as reptile or bird eggs because they are jelly coated and unsuitable for development on land; therefore, amphibians must return to water to reproduce. Increased agricultural and industrial run off and poor waste management has led to a decline in the quality of water available for amphibians. The main types of chemical contaminants affecting amphibian environments are pesticides and herbicides, heavy metals, increasingly acidic water and nitrogen pollution.
According to a study published in the journal Environmental Health Perspectives by Tyrone Hayes from the University of California, Berkeley and colleagues, pesticides commonly used in cornfields in the western United States have adverse affects on amphibian larval growth and development, immune system and the size prior to and after metamorphosis. High levels of pesticides enter streams and groundwater as water runs off of farms, ranches, golf courses and suburban areas. While organic and low-risk pesticide use is encouraged by the U.S. Environmental Protection Agency, it remains predominantly unregulated… That is, the EPA lists guidelines for how to safely use pesticides for commercial and agricultural needs, but it does not strictly regulate what can and cannot be used.
Herbicides, on the other hand, are made to disrupt photosynthesis capabilities of plants and were thought to have little to no effect on fish and wildlife. But, as Science Daily reported in 2008, studies have revealed otherwise. For example, atrazine—one of the most commonly used herbicides on golf courses, home lawns and soybean and corn crops—is responsible for lethal changes in early developmental stages in amphibians. Tadpoles can develop compromised immune systems, digestive problems and heart defects, the studies showed.
In one study by Michael Boone of the U.S. Geological Survey Columbia Environmental Research Center in Missouri and Stacy James of the University of Missouri, published in the journal Ecological Applications, multiple environmental characteristics played a role in the impact of contaminants in the water. The proximity of the amphibian’s habitat to an agricultural site, presence of competitive invasive species and the added chemical stressors were all variables that contributed to the multifaceted picture of population decline. Another study conducted by Hayes and colleagues explored hermaphroditism. They found that exposure to atrazine caused male frogs to turn into females, drastically decreasing their testosterone levels and making it virtually impossible for them to reproduce. The males that became females were in fact able to reproduce when mating with genetic males; however, all of the resulting offspring were males. The skewed male-to-female ratio, according to the researchers, would lead to fewer offspring with each generation greatly decreasing in population size.
Nutrient enrichment, resulting from excess nitrogen and phosphorous being dumped into the environment from various run off sources, has also caused an excess of snails that carry infectious parasites. The snail population increases from the algae growth brought about by nutrient enrichment. The snails play the role of initial hosts for the parasites, and when the parasites have developed further, they move on to infect amphibians. These parasites cause deformities in amphibians by forming cysts in their limbs, hindering proper development. This sort of deformity has the potential to affect reproductive success or the ability to avoid predators.
Climate change can also have a major effect on amphibians due to their delicate transdermal uptake system, which makes them sensitive to small changes in temperature and moisture. Amphibians also tend to be highly dependent on weather patterns for their breeding behavior. Weather that is too hot or too cold promotes hibernation periods in which individuals avoid exposure to harsh conditions. When temperatures are optimal, amphibians migrate to small, fresh bodies of water to breed. Climate change, according to a study published in Proceedings of the National Academy of Sciences, has caused warmer than average winters, hotter summers and earlier changes in seasons with shortened transitions. The problem amphibians face with these altered patterns is that their breeding is triggered earlier on in the season and their offspring are faced with harsher conditions to survive later, like late melting snow, or decreases in water temperature. Since amphibians are cold-blooded, they are dependent on climatic conditions to either warm them up or cool them down, Because seasons have become much more dynamic, the chances of survival of the offspring decreases greatly, which further adds to already decreasing populations.
Another problem caused by a changing climate is the inability of amphibians to mount a defense against opportunistic parasites. Scientific American looked into whether climate change is to blame for one of the deadliest fungi to impact amphibians: the chytrid fungus. According to the article, the direct effects of temperature are negligible when it comes to chytrid growth. However, it is likely that fluctuating temperatures are preventing the amphibian immune system to mount a strong defense against opportunistic infections. In other words, environmental stressors, such as variations in normal body temperatures, may be leading to a weakened immune system in amphibians.
So what does all of this mean for amphibians? In sum, it appears populations are on the decline. Could this be a sign that we are experiencing the beginning of a mass extinction event? Amphibians have outlived dinosaurs, survived various climate and landscape changes and yet now are being placed on the International Union for Conservation of Nature’s list of threatened species. And some species, like the Kihansi spray toad, are even becoming extinct.
Ecosystems are complex and every organism within them is interlinked. Many species form mutually symbiotic relationships – if one species is removed, like amphibians, any dependent species suffer, like the many species of birds for which amphibians are food sources. Biodiversity should not be protected for the simple fact of saving a single species, but for the protection of entire ecosystems with intertwined food and life cycles.
Monica Kanojia is a George Mason University graduate with a B.S. in Biology. She has interned at the National Science Foundation and contributed articles to LiveScience.com. She has also interned with EarthShot Foundation, an NGO focused on clean energy.
Photo Credit: Marlin Harms