For thousands of years, no amphibians inhabited the Galapagos Islands due to their isolation, 1,000 km away from the mainland. The salty ocean acts as a natural barrier, preventing frogs and toads from colonizing the islands. However, this changed in the late 1990s when a small tree frog, Scinax quinquefasciatus, arrived probably as a stowaway on cargo ships and established populations on Isabela and Santa Cruz.
This 3–4 cm small frog, native to the coastal provinces of Ecuador, is highly resistant to salinity, drought, and human-modified environments. In Galapagos, it has rapidly spread across urban and agricultural areas on both islands, but also into natural pools within Galapagos National Park (GNP). How do we know they're everywhere? Because of their loud calls!
Frogs typically call during the breeding season (January–May), when males gather near water bodies to attract females that then lay the eggs. The eggs hatch quickly, releasing hundreds of tadpoles that remain in the water until they have metamorphosed into frogs.
In January 2023, we deployed automatic recorders to capture their nighttime calls in agricultural and remote areas. To our surprise, we detected loud choruses in isolated locations within GNP, likely produced by thousands of frogs. How do we know? Because we have also counted them!
Joshua Vela/CDF
During the 2023 and 2024 breeding seasons, we visited three natural ponds and three reservoirs in Santa Cruz for five consecutive nights. Each night, we captured, marked, and released frogs to estimate population size through recaptures. Our findings? Hundreds to thousands of frogs per site! Larger natural ponds could even host tens of thousands.
This invasion is escalating. Local farmers report that frogs only appeared a few years ago, suggesting they were initially unnoticed—possibly because their numbers were low. However, water reservoirs, essential for agriculture, have provided ideal breeding sites for these frogs. Residents near these reservoirs describe the frogs' deafening calls, making sleeping difficult.
Beyond the noise, frogs threaten native wildlife. They are insectivores and studies carried out since 2017 have shown that they primarily feed on moths, spiders and over 11 different insect orders —the same insects that Galapagos finches and other birds rely on for food. With thousands of frogs at a single site, they consume countless insects daily, potentially competing with native birds for food.
Joshua Vela/CDF
Joshua Vela/CDF
Joshua Vela/CDF
What Can Be Done?
The first step is understanding the scale of the problem—where the frogs are, how many exist, and their ecological impact. Next, we need to further study their ecology, for example:
- How do they survive the dry season, especially within GNP, where pools dry out?
- How far do they move, and how do they spread?
- How do they interact with giant tortoises and other species in their habitat?
This knowledge is critical for developing effective control strategies that do not harm other species in these pools. Previous control efforts have not been successful, so actions based on a deep understanding of the frogs’ ecology in Galapagos will be key to managing this noisy invader.
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Miriam San José
Investigator
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