Functional connectivity and landscape genetics of Box–Ironbark birds

2017-02-23T04:31:08Z (GMT) by Amos, John Nevil
Habitat loss, fragmentation and degradation are drivers of major declines in biodiversity and species extinctions. The actual causes of species population declines following habitat change are more difficult to discern. In this thesis I attempt to identify, based on published data on mobility and their ecological responses to fragmentation, some of the processes that may have led to patterns of decline for 10 woodland-dependent birds in fragmented woodlands of central Victoria, Australia. Eight species had been identified as ‘decliners’ (species that disappear from suitable patches when landscape-level tree cover falls below species-specific thresholds) and two as ‘tolerant’ species (whose occurrence in suitable habitat patches is independent of landscape tree cover). I investigated the contribution of decreased structural connectivity on functional connectivity as a cause of the observed declines. A set of landscape connectivity models was constructed of each species covering a range of plausible values. The two dominant algorithms for summarising effective distances used in modelling complex fragmentation patterns in landscape genetics—least-cost path and circuit distance—were used to construct these models The results of the two methods were compared and circuit distance was determined to be the more appropriate approach for use in my study system. Choice of algorithm and null model were important influences on inferences in landscape genetics. I predicted (1) fragmentation would impede dispersal and gene flow of ‘decliners’ but not of ‘tolerant’ species; and that fragmentation effects would be stronger (2) in the least mobile species, (3) in the more philopatric sex and (4) in the more fragmented region. These predictions were then tested with a large empirical genetic dataset (2198 individuals from 63 sites across a 170 x 50-km study area). I fitted models specific to sex and geographic zone in order to account for sex-biased dispersal and potential scale- and configuration-specific effects. As expected, four of the least mobile decliners showed reduced genetic connectivity. Responses were sex specific in the two least mobile species. The tolerant species and (unexpectedly) four of the more mobile decliners showed no reduction in gene flow. Weaker genetic effects were observed in the geographic zone with more aggregated vegetation, consistent with gene flow being unimpeded by landscape structure. These results indicate that, excepting the most sedentary species in our system, the movement of the more dispersive sex maintains overall genetic connectivity across fragmented landscapes in the study area. I examined relationships among configuration, extent and status of native vegetation and three commonly used indicators of individual body condition and chronic stress in 13 species, two measures of changes to population processes (sex ratio and individual homozygosity) in 10 species and allelic richness in five species. Little support for relationships between site or landscape characteristics and individual or population response variables was found. These findings, along with related work to which I contributed, but that does not form part of the thesis, highlighted the need for management to increase both connectivity, for the least mobile species, and critical resource availability for other species to conserve these declining species.