By Renata Vidovic, Evolution and Ecology ’15
To some, the phrase climate change evokes images of dry lakes, melting icebergs, and rising oceans. However, the effects of global warming are not simply cataclysmic geological changes. There are links between all biotic and abiotic features of an ecosystem. Unsurprisingly, climate change has an immense impact on frog populations around the world. Home range, abundance, breeding cycles, pathogen epidemics, and physical degradation in frogs are all affected by the changing climate.
Agalychnis annae, the blue-sided tree frog, is a poster child for the effects of climate change on frogs. The Agalychnis genus, which contains five species (including the red-eyed tree frog, pictured above), lives in the canopies of the moist lowlands, mountains, and forests in Central and South America (Doha). The International Union for Conservation of Nature (IUCN) Red List of Endangered Species lists A. annae as endangered with very rare sightings in Parque Nacional Tapantí and the Reserva Biológica Monteverde, where it was once common (IUCN). As with most frogs, it prefers moist, shady habitats that are relatively unpolluted. Frogs are sensitive to environmental change due to their highly permeable skin as well as their combined terrestrial and aquatic behaviors.(Blaustein).
As with most frogs, those of the Agalychnis genus are carnivorous, feeding mostly on insects such as moths, beetles, crickets, and flies, as well as smaller frogs on occasion. Since frogs have three stages of life–egg, tadpole, and adult–and each has their own slew of predators. However, predatory hazards are not the only threats that impact A. annae populations. Deforestation, trade, pesticide use, destruction of breeding sites, disease, and climate changes all threaten the Agalychnis species (Wilson). The impact of these issues can be seen in the populations of A. annae, which have declined by 50% in the last 10 years and are still receding. This frog has also virtually disappeared from almost all of its historical range, and now is only found near San José, Costa Rica (Pounds).
When discussing climate change in relation to amphibians, one might think the greatest threat would come from drying water systems, which are thought to provide these frogs with a place to live and breed. However, for the A. annae especially, the increased spread and breeding of pathogens, such as the chytridiomycosis fungus. Melting glaciers that create new lakes and warmer, more humid climates have expanded the territory of this once-rare disease. A recent survey in El Salvador has shown that 98% of the A. moreletti and A. annae tadpoles have malformations caused by the chytrid fungus (MARN). The warming has also meant that the fungus can now be found in higher elevations, between 1,500 and 1,950 meters.
Unfortunately, it is difficult to gauge exactly how climate change will affect these frogs in the future. Though this species has been observed to be highly adaptable, it seems the changes are happening too fast for it to keep up (Pounds). In frogs, the two processes most affected by temperature and climate change are reproduction and development. For example, A. annae relies on the rainy season as a cue for a safe reproduction period (Rome).Increased temperature, increased length of dry season, decreased soil moisture, and increased inter-annual rainfall variability will affect not only individuals, but populations and even communities of frogs. If the rainy season shortens, then the temporal partitioning of spatial resources that occurs naturally between frogs of different species will decrease in efficiency as more species are forced to breed at the same time (Donnelly). A. annae are also known for their practice of depositing their eggs on leaves 350mm to 3m above water, where they gestate and then hatch, dropping the tadpoles into the pools below (Duellman). If temperatures increase and the dry season lengthens, humidity is expected to drop, which would provide harsh conditions for the A. annae eggs (Donnelly).
As mentioned earlier, amphibians are bioindicators of environmental health and climate change due to their highly permeable skin and aqua-terrestrial habits (Blaustein). Frogs are a huge part of the ecosystem, as both predators and prey. Therefore, it is extremely important that we pay attention to their reactions to our environment. Amphibians exist in the fossil record about 150 million years before dinosaurs. They have survived the most extreme upheavals of this planet, from extinction episodes to ice ages. Now they show signs of drastic decline due to environmental factors, so it is prudent to pay attention to their plight. There already exists legislation in Costa Rica protecting the Agalychnis genus of frogs, including bands on illegal trade and increased protection of habitats in ecosystems (Doha). In the end, perhaps the greatest indicator that our world is changing is the resounding sound heard in affected wetlands around the world–the sound of silence, as the last frog croaks.
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