Land-bridge Islands (land-bridge + island)

Distribution by Scientific Domains

Selected Abstracts

Ecological correlates of vulnerability to fragmentation in Neotropical bats

Christoph F. J. Meyer
Summary 1In the face of widespread human-induced habitat fragmentation, identification of those ecological characteristics that render some species more vulnerable to fragmentation than others is vital for understanding, predicting and mitigating the effects of habitat alteration on biodiversity. We compare hypotheses on the causes of interspecific differences in fragmentation sensitivity using distribution and abundance data collected on 23 species of Neotropical bats. 2Bats were captured over a 2-year period on 11 land-bridge islands in Gatún Lake, Panama, and on the adjacent mainland. We derived a series of explanatory variables from our capture data and from the literature: (1) natural abundance in continuous forest, (2) body mass, (3) trophic level, (4) dietary specialization, (5) vertical stratification, (6) edge-sensitivity, (7) mobility, (8) wing morphology (aspect ratio and relative wing loading) and (9) ecologically scaled landscape indices (ESLIs). After phylogenetic correction, these variables were used separately and in combination to assess their association with two indices of fragmentation sensitivity, species prevalence (proportion of islands occupied) as well as an index of change in abundance. 3Model selection based on Akaike's information criterion identified edge-sensitivity as the best correlate of vulnerability to fragmentation. Natural abundance and mobility or traits linked to mobility (relative wing loading and ESLI) received limited support as predictors. Vulnerability of gleaning animalivorous bats is probably caused by a combination of these traits. 4Synthesis and applications. Our findings emphasize the importance of a local-scale approach in developing predictive models of species fragmentation sensitivity and indicate that risk assessments of Neotropical bats could be based on species tolerance to habitat edges and mobility-related traits. We suggest that, in order to be effective, management efforts should aim to minimize the amount of edge-habitat and reduce the degree of fragment-matrix contrast. Moreover, if high bat diversity is to be preserved in fragmented Neotropical landscapes, conservation measures regarding reserve design should assure spatial proximity to source populations in larger tracts of continuous forest and a low degree of remnant isolation. [source]

Vegetation dynamics of predator-free land-bridge islands

Summary 1We tested the ,green world' hypothesis of Hairston, Smith and Slobodkin by monitoring vegetation change on recently created predator-free land-bridge islands in a huge hydroelectric impoundment, Lago Guri, in the State of Bolivar, Venezuela. 2Our results affirm the green world hypothesis and expose the operation of a strong top-down trophic cascade that negatively impacted nearly every plant species present, implying that community stability is maintained through the action of predators. 3To test the hypothesis, we monitored vegetation on nine predator-free islands and compared demographic parameters to those observed at control sites supporting complete or nearly complete suites of predators. 4Herbivore abundance was high on ,small' (, 0.5, < 2 ha) islands, moderate on ,medium' islands (> 3, < 15 ha) and low on the ,large' landmasses that served for reference. 5Small sapling densities on small islands were only 37% of controls in 1997 (after 11 years of isolation), and when recensused in 2002, had fallen to 25% of controls. High mortality and, especially, low recruitment contributed to the decline in sapling cohorts. 6Sapling decline occurred earlier on small islands, although recruitment failure had become equally pronounced on medium islands by the end of the monitoring period. 7Several mechanisms could potentially account for suppressed sapling recruitment, but the weight of evidence points to herbivory on seedlings and small saplings by leaf-cutter ants (Atta spp. and Acromyrmex sp.). Exposure to prevailing trade winds (windward vs. leeward slopes of islands) had no detectable effect on the density or diversity of seedlings or saplings. [source]

Life-history traits associated with fragmentation vulnerability of lizards in the Thousand Island Lake, China

Y. Wang
Abstract Following habitat fragmentation, the remnant faunal community will undergo a period of species loss or ,relaxation.' Theory predicts that species with particular life-history traits, such as a small population size, small geographical range, low fecundity and large body size, should be more vulnerable to fragmentation. In this study, we investigated the relationships between the above life-history traits and the fragmentation vulnerability index (the number of islands occupied) of five lizard species inhabiting recently isolated land-bridge islands in the Thousand Island Lake, China. Data on life-history traits were collected from field surveys (population density) and from the literature (body size, clutch size and geographical range size). The species,area relationships for lizards sampled from the mainland versus on the islands differed significantly (i.e. the number of species inhabiting islands was decreased relative to similar-sized areas on the mainland), indicating that species extinction has occurred on all of the study islands following isolation. For the fragmentation vulnerability index, model selection based on Akaike's information criterion identified natural density at mainland sites as the best correlate of vulnerability to fragmentation, supporting the hypothesis that rare species are most vulnerable to local extinction and will be lost first from fragmented landscapes. In contrast, there was little evidence for an effect of lizards' snout,vent length, clutch size or geographical range size on fragmentation vulnerability. Identification of species traits that render some species more vulnerable to fragmentation than others has important implications for conservation and can be used to aid direct management efforts. [source]