Non-random lizard extinctions in land-bridge Amazonian forest islands after 28 years of isolation

Abstract

Major hydropower infrastructure has become a leading driver of biodiversity loss in the lowland tropics. Terrestrial species typically become stranded in post-isolation land-bridge islands within hydroelectric reservoirs. Understanding the resulting extinction dynamics of insular communities is critical to inform, if not to avert, the ongoing blitzkrieg of dam development. Here we assess the effects of forest patch and landscape metrics on diurnal lizard species richness and composition within the Balbina Hydroelectric Dam and surrounding areas in the Central Brazilian Amazon. This 28-yr-old dam created a reservoir of ~4438km2, comprising 3546 islands. We sampled 25 of these islands (0.83–1466ha) and five mainland continuous forest sites, one of which placed along stream banks. We further related morpho-ecological traits and the geographic distribution of lizard species to the spatial metrics of islands where they occurred. Using 100L-pitfall traps operated over 5447 trap-days, we recorded 1123 lizards from 17 taxa, two of which exclusively found along stream banks within continuous forest. Island area was the best predictor of species richness and composition. Small islands (≤2ha) harboured fewer than a third of all species typically observed in larger islands and continuous forest (≥8 species), and only islands ≥100ha retained nearly complete lizard faunas. Lizard assemblages inhabiting small, isolated islands consisted almost exclusively of an oversimplified set of widely distributed, large-bodied, habitat generalist, heliophile species associated with open areas and forest edges, and that feed on a wide spectrum of prey sizes. These wholesale changes in lizard community structure were characterized by severe losses in functional traits, and may profoundly affect ecosystem functioning.