The worldwide trade in frog legs is massive, and is undoubtedly a significant contributor to the decline and extinction of amphibian populations worldwide. For example, Europeans alone consumed roughly 120 million frogs per year in the 1990's. The Californian gold-miners nearly ate the California red-legged frog (Rana draytonii) to extinction in the 19th century, and the species has never fully recovered.
The frog legs trade is problematic whether the frogs are wild-caught or farm-raised. Specifically, the harvest of wild frogs leads to depletion of wild populations, and trade in farm-raised frogs leads to the spread of harmful infectious diseases and invasive species.
The depletion of wild populations
The trade in wild-caught frogs has been driving frog populations to near extinction since the 19th century, when the Californian gold miners decimated populations of California Red-Legged Frogs (Rana draytonii) in the 1800's. The harvesting of amphibians for the food trade is often unregulated, and in many developing countries (like Indonesia and Thailand) is likely a primary contributor to amphibian declines. India actually banned the export of frog legs in the 1980's because mosquito populations were increasing as the frog populations declined. The French ate so many of their native frogs that the government had to ban the eating of native frogs, and now France relies on imports from Indonesia and other distant countries. The Goliath Frog (Conraua goliath), the world's largest frog, is being hunted to near extinction in western Africa.
Frog legs and the spread of infectious diseases
Even in the most developed countries, there are virtually no protocols in place to ensure that diseased amphibians do not get imported or exported. American Bullfrogs (Rana catesbeiana) are commonly farmed and transported worldwide. They are known carriers of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) and thus are likely to be primary contributors to the global spread of chytridiomycosis, a disease that has decimated amphibian populations worldwide. In a recent study, 62% of the captive-raised bullfrogs sampled in shops in New York, Los Angeles and San Francisco were infected with the chytrid fungus. These three cities alone have been importing over five million amphibians per year.
Frog legs and the spread of invasive species
American Bullfrogs are the frog species most commonly farmed worldwide. They are also quite adept at establishing populations in areas to which they have been introduced, and they have become invasive species in at least 15 countries worldwide. Bullfrogs compete with and eat native amphibians. Actually, bullfrogs eat all types of native wildlife: frogs, bats, snakes and more. If you eat frog legs in an American or European restaurant, there is a high chance it is this fungus-prone invasive species, which is native to the eastern United States.