Extinction debt on reservoir land-bridge islands

Large dams cause extensive inundation of habitats, with remaining terrestrial habitat confined to highly fragmented archipelagos of land-bridge islands comprised of former hilltops. Isolation of biological communities on reservoir islands induces local extinctions and degradation of remnant communities. “Good practice” dam development guidelines propose using reservoir islands for species conservation, mitigating some of the detrimental impacts associated with flooding terrestrial habitats. The degree of species retention on islands in the long-term, and hence, whether they are effective for conservation is currently unknown. Here, we quantitatively review species' responses to isolation on reservoir islands. We specifically investigate island species richness in comparison with neighbouring continuous habitat, and relationships between island species richness and island area, isolation time, and distance to mainland and to other islands. Species' responses to isolation on reservoir islands have been investigated in only 15 of the > 58,000 large-dam reservoirs (dam height > 15m) operating globally. Research predominantly originates from wet tropical forest habitats and focuses on mammals, with species richness being the most widely-reported ecological metric. Terrestrial taxa are, overall, negatively impacted by isolation on reservoir islands. Reservoir island species richness declines with isolation time, and although the rate of loss is slower on larger islands, all islands exhibit depauperate species richness < 100 years after isolation, compared to continuous mainland habitats. Such a pattern of sustained and delayed species loss following large-scale habitat disturbance is indicative of an extinction debt existing for reservoir island species: this pattern is evident across all taxonomic groups and dams studied. Thus, reservoir islands cannot reliably be used for species conservation as part of impact mitigation measures, and should instead be included in area calculations for land impacted by dam creation. Environmental licensing assessments as a precondition for future dam development should explicitly consider the long-term fate of island communities when assessing biodiversity loss vs energy output.