Changes in forest-bird assemblage structure in response to multiple pressures: climate and vegetation change

Climate change is linked to negative effects on vegetation, including drought-induced vegetation dieback. Large-scale dieback leads to considerable carbon emissions and loss of ecological resources for fauna. The consequences of vegetation change for biodiversity include; reductions in breeding success, population sizes, dispersal, resistance to other pressures, and changes in species composition. Changes in species composition alter interspecific interactions, such as competition, predation, parasitism and pollination, which collectively affect assemblage dynamics and ecosystem function. Competitive species that are resistant to, or have benefited from disturbances may monopolize resources (nesting sites and food) and limit the survival and recruitment of disturbance-sensitive species. I investigated whether and how the vegetation structure, composition and carbon content changed over a period of extended drought in a much-modified forest ecosystem. I explored if landscape configuration, management practice or soil type influenced vegetation change and identified the factors that influence the spatial and temporal distribution of bird assemblages, including climate, vegetation structure, vegetation loss/fragmentation, interspecific species interactions and resource provision. Lastly, I assessed if the recruitment of native birds is influenced by vegetation loss/fragmentation, drought-driven habitat degradation, and interspecific interactions. Box and ironbark forests of Victoria in south-eastern Australia are an example of a highly modified, ecosystem in which the joint effects of fragmentation and climate change have not been systematically measured. These forests experienced drought stress from 1997 until 2010, in conjunction with the on-going loss, degradation and fragmentation of native vegetation by human encroachment. The study design incorporates vegetation fragments of a range of sizes, allowing the evaluation of fragmentation/habitat loss effects. I made direct comparisons of changes in avian and vegetation assemblage structure and condition over the last 15 years by revisiting sites measured previously (1997). These data, used in conjunction with observations of breeding success, provided an opportunity to investigate the long-term interaction between vegetation change and climate-change. Forest structure was systemically different between the two periods. Canopy cover, shrub cover and litter decreased between 1997 and 2010, while the total basal area of dead trees in all size classes increased. The effects were amplified in fragmented vegetation, probably due to greater water and heat stress. Avian assemblages changed significantly between 1997 and 2010, with many woodland bird species halving in both prevalence (proportion of sites occupied) and abundance (numbers when present). Declines were largely independent of species ecological traits (i.e. nesting, foraging, range etc.). Changes in the bird community were associated with precipitation, temperature, vegetation cover (canopy, ground and mid-storey cover) and the density of the noisy miner Manorina melanocephala. The reduction in canopy foliage, ground-litter and shrub cover probably has reduced food resources and nesting sites. Greater noisy miner abundance as an additive effect of fragmentation and degradation had a greater negative effect on species’ breeding behavior than on-transect vegetation characteristics. Nectarivores tracked spatial and temporal variation in flowering, but this relationship was disrupted in the presence of noisy miners. The box-ironbark region serves as an exemplar for other regions undergoing increases in the frequency and duration of extreme climatic events, such as North America, Europe, southern Africa and Asia. My work is among the first to consider interacting pressures that arise from major drivers of ecological change. The loss, fragmentation and degradation of habitat have caused declines in many native species but have facilitated increases in the abundance and distribution of other native species. Projected climate change (i.e. hotter and drier conditions) may exacerbate changes in the bird assemblage by accelerating vegetation degradation, especially in smaller fragments. I found that greater drought-driven vegetation degradation in smaller fragments facilitated the expansion of a highly competitive native, the noisy miner. The noisy miner by acting as an interference competitor (defending space) disproportionately excluded smaller-bodied birds from sites where the noisy miner occurs. This reduced the access of small-bodied birds to valuable resources (nesting and food) which may limit their capacity to recover from adverse climate events (e.g. long drought), with long-term consequences for the persistence of small-bodied species. As climates become more extreme, similar effects to those that I report are likely to arise in other fragmented regions and for other taxa, because interspecific interactions are not limited to avifaunas, and fragmentation has been linked to altered species interactions in many regions of the world.