Effects of Fragmentation and Sea-Level Changes upon Frog Communities of Land-Bridge Islands off the Southeastern Coast of Brazil

We investigate the composition of anuran communities of land-bridge islands off the southeastern coast of Brazil. These islands provide natural long-term experiments on the effects of fragmentation in the Brazilian Atlantic Forest (BAF). We hypothesize that Pleistocene sea-level changes, in combination with other abiotic variables such as area and habitat diversity, has affected anuran species richness and community composition on these islands. Data from the literature and collections databases were used to produce species lists for eight land-bridge islands and for the mainland adjacent to the islands. We assess the effects of area, number of breeding habitats and distance to the mainland upon anuran species richness on land-bridge islands. Additionally we use nestedness analysis to quantify the extent to which the species on smaller and less habitat-diverse islands correspond to subsets of those on larger and more diverse ones. We found that area has both direct and indirect effects on anuran species richness on land-bridge islands, irrespective of distance to the mainland. However, on islands with comparable sizes, differences in species richness can be attributed to the number and quality of breeding habitats. Anuran communities on these islands display a nested pattern, possibly caused by selective extinction related to habitat loss. Common lowland pond-breeders were conspicuous by their absence. In the BAF, the conservation of fragments with a high diversity of breeding habitats could compensate for the generally negative effect of small area upon species richness. We suggest that sea-level changes have an important role in shaping composition of anuran species on coastal communities.

本研究调查巴西东南沿海陆桥岛（land-bridge islands）的无尾目（Anura）群落组成。这些岛屿为探究巴西大西洋森林（Brazilian Atlantic Forest, BAF）的生境破碎化效应提供了天然长期实验平台。本研究提出假说：更新世（Pleistocene）海平面变化，结合岛屿面积、生境多样性等非生物变量，共同影响了这些陆桥岛上的无尾物种种丰富度与群落组成。本研究通过整合文献与馆藏数据库数据，为8座陆桥岛及其邻近大陆编制了物种种名录。我们评估了岛屿面积、繁殖生境数量以及与大陆的距离对陆桥岛无尾物种种丰富度的影响。此外，本研究采用嵌套性分析（nestedness analysis），量化生境较少、多样性较低的小型岛屿上的物种，在多大程度上可作为面积更大、生境更多样岛屿的物种种群子集。研究结果显示，无论与大陆的距离如何，岛屿面积对陆桥岛的无尾物种种丰富度均存在直接与间接影响。但在面积相近的岛屿中，物种种丰富度的差异可归因于繁殖生境的数量与质量。此类岛屿上的无尾目群落呈现嵌套格局，这一现象可能由生境丧失引发的选择性灭绝所导致。常见的低地型池塘繁殖物种在此类岛屿上显著缺失。在巴西大西洋森林中，保护繁殖生境多样性丰富的生境片段，可抵消小面积生境对物种种丰富度的普遍负面影响。本研究认为，海平面变化在塑造沿海无尾物种种群组成中发挥了重要作用。