It's January, and if you're ever near slow-moving water on a rainy night, you might hear a whole chorus of frog calls. You might hear the call of more common species like the Sierran Treefrog or the California Toad. If you were extremely lucky, you might hear call of sensitive Foothill Yellow-legged Frogs or California Red-Legged Frogs, whose populations are both at risk right now. During and after a good rain, frogs and toads croak to attract a mate. These rainy-season songs were much more wide-spread in the last century, before the region was as developed as it is now. If amphibians continue to face severe threats, it's possible that by the next century we won't be able to hear them at all.
Several decades ago, most streams and meadows of the northern Yosemite region were home to robust populations of amphibians, from California Red-legged Frogs and Foothill Yellow-legged Frogs to Yosemite Toads and Sierra Newts. These species are integral to their ecosystems, and are highly specialized to their environments. Unfortunately, as mentioned in our recent newt article, amphibians are the most threatened vertebrate group on Earth, with one in three species threatened by extinction. California Tiger Salamanders have lost about 75 percent of their historical breeding habitat. Red-legged Frogs, the largest native frog species west of the Mississippi River, have been driven from 70 percent of their range. There are multiple proposed causes for these population declines, but the answer is likely a combination of factors.
A major cause of population decline in California amphibians is the deadly fungal pathogen chytrid, or Batrachochytrium dendrobatidis (Bd). The fungus causes frog skin to cease its normal functions, including absorbing oxygen and nutrients, which in most cases is fatal. Having already led to the extinction of 200 frog species, scientists are striving to understand the spread of the disease and hope to someday halt it. Mountain Yellow-legged Frogs, whose numbers have already been reduced by 90 percent, are heavily impacted by Bd in the alpine lakes of the Sierra Nevada. These frogs are the subjects of an experimental treatment by UC Santa Barbara: capture wild specimens, immunize them to the disease, and release them.
Both invasive Bullfrogs and the common Pacific Tree Frog are considered carriers for this disease. Humans can help reduce the spread of the fungus by disinfecting footwear after use in water, and by discouraging amphibian pet imports and bait use.
The Sierra Nevada mountains are often downwind of agricultural land in the Central Valley, allowing for agrochemicals to travel from farmland into ground and surface water at higher elevations. Pesticide residue has been documented in the bodies of Southern Mountain Yellow-legged Frogs and Pacific Tree Frogs, and a study on California Red-legged Frogs found a significant association between their population decline and wind-borne agrochemicals. Herbicides like the ubiquitous Atrazine can affect amphibian hormones (as well as bird, rodent, and human endocrine hormones), effectively sterilizing them and sometimes causing males to become female. Other pesticides can cause delayed metamorphosis, suppression of the immune system, and fatality. Nitrate pollution from fertilizers can also cause deformity or be fatal to California Toads and Pacific Tree Frogs.
Habitat Destruction and Degradation
Like other wildlife, amphibians are heavily impacted by habitat loss and alteration. Urbanization, especially near aquatic habitats, can devastate a population by polluting the water, diverting water for human use, and fragmenting habitable areas. Habitat fragmentation can split a large population into many small ones with reduced genetic diversity and movement capabilities, which limits their ability to respond to environmental changes. Activities like clear-cutting and grazing alter plant ecosystems and numbers. This produces a cascading environmental effect that can make the land uninhabitable for some amphibian species.