On islands across the world, frogs and toads follow patterns that contradict a fundamental law of island biology—yet a Brazilian research team has now shown that two seemingly opposed theories actually work in harmony to explain where these salt-intolerant creatures thrive.
The puzzle lies in a simple fact: amphibians cannot survive saltwater. Unlike birds and mammals that can occasionally island-hop across oceans, frogs and toads are marooned the moment they reach the sea. This makes every marine island equally distant from the mainland in ecological terms, rendering one of biology's most cherished principles seemingly irrelevant. But Raoni Rebouças and his colleagues at the Institute of Biology of the State University of Campinas (IB-UNICAMP) have now tested whether island biogeography—the 60-year-old theory developed by Robert MacArthur and Edward O. Wilson—could still apply.
The researchers compiled data from over 5,000 marine islands worldwide, documenting the ecological characteristics of 1,924 anuran amphibian species. They measured not just the raw count of species on each island, but also functional diversity (whether frogs were terrestrial, aquatic, arboreal, or fossorial) and phylogenetic diversity (how many evolutionary lineages were represented). The results, published in Ecography, revealed something elegant: both MacArthur and Wilson's equilibrium theory and David Wright's competing species-energy theory were essential to explaining the patterns observed.
MacArthur and Wilson's model predicts that larger islands closer to mainland sources of species will harbor more biodiversity. Wright's 1983 framework argues that energy availability—the productivity of organic matter per unit area—matters more than geography. The research confirmed that both factors shape amphibian communities, but their importance shifts depending on climate and the type of diversity being measured.
"Their relevance differs depending on the climate and the diversity being considered—whether it's species richness, functional diversity, or phylogenetic diversity," explains Matheus Moroti, a co-author of the study. Consider Greenland and Borneo, the world's largest and second-largest islands. Greenland, locked in ice for much of the year, harbors no frog species despite its vast size. Borneo, smaller but tropical and productive, supports over 400. Size alone cannot explain the difference; energy and climate must be factored in.
The Brazilian white-edged tree frog (Boana albomarginata) exemplifies the nuance of these patterns. It inhabits both mainland and island populations, but island populations grow substantially larger than their mainland counterparts—a phenomenon that speaks to how islands, despite their isolation, can offer ecological advantages once colonized.
This research breaks new ground because previous tests of biogeographic theory had focused on salt-tolerant or flying organisms. Anurans, locked out of the ocean highway, offer a rare window into how foundational ecological principles apply to truly isolated communities. The findings suggest that neither historical theory need be abandoned; instead, ecologists must consider how geography, energy, and climate interact in complex ways to shape the diversity of life on islands. For Madagascar, with over 300 frog species, or the thousands of smaller islands that harbor endemic amphibians found nowhere else on Earth, understanding these dynamics becomes crucial as conservation efforts intensify.