Fine-scale effects of habitat loss and fragmentation despite large-scale gene flow for some regionally declining woodland bird species
Habitat loss and associated fragmentation effects are well-recognised threats to biodiversity. Loss of functional connectivity (mobility, gene flow and demographic continuity) could result in population decline in altered habitat, because smaller, isolated populations are more vulnerable to extinction. We tested whether substantial habitat reduction plus fragmentation is associated with reduced gene flow in three ‘decliner’ woodland-dependent bird species (eastern yellow robin, weebill and spotted pardalote) identified in earlier work to have declined disproportionately in heavily fragmented landscapes in the Box-Ironbark forest region in north-central Victoria, Australia. For these three decliners, and one ‘tolerant’ species (striated pardalote), we compared patterns of genetic diversity, relatedness, effective population size, sex-ratios and genic (allele frequency) differentiation among landscapes of different total tree cover, identified population subdivision at the regional scale, and explored fine-scale genotypic (individual-based genetic signature) structure. Unexpectedly high genetic connectivity across the study region was detected for ‘decliner’ and ‘tolerant’ species. Power analysis simulations suggest that moderate reductions in gene flow should have been detectable. However, there was evidence of local negative effects of reduced habitat extent and structural connectivity: slightly lower effective population sizes, lower genetic diversity, higher within-site relatedness and altered sex-ratios (for weebill and eastern yellow robin) in 10 × 10 km ‘landscapes’ with low vegetation cover. We conclude that reduced structural connectivity in the Box-Ironbark ecosystem may still allow sufficient gene flow to avoid the harmful effects of inbreeding in our study species. Although there may still be negative consequences of fragmentation for demographic connectivity, the high genetic connectivity of mobile bird species in this system suggests that reconnecting isolated habitat patches may be less important than increasing habitat extent and/or quality if these need to be traded off.
KeywordsDispersal Demographic connectivity Landscape connectivity Genetic structure
Funding was provided by the Australian Research Council Linkage Grant (LP0776322), the Victorian Department of Sustainability and Environment (DSE), Museum of Victoria, Victorian Department of Primary Industries, Parks Victoria, North Central Catchment Management Authority, and Goulburn Broken Catchment Management Authority. Birds Australia contributed towards NA’s PhD stipend, and Monash University Science Faculty funded a Dean’s Scholarship. We thank Holsworth Wildlife Research Endowment for valuable support to NA. Samples were collected under DSE permit number 10004294 under the Wildlife Act 1975 and the National Parks Act 1975, DSE permit number NWF10455 under section 52 of the forest Act 1958 and the Australian Bird and Bat Banding Scheme permit under approval and monitoring of Monash University ethics processes (BSCI/2007/07). We thank all the volunteers for the Birds Linkage project for assistance with fieldwork, and other Birds Linkage team members for diverse inputs. Jian Yen and three anonymous referees provided very helpful comments on drafts. Computationally intensive analyses (Structure and TESS) were performed on Monash Sun Grid.
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