The significance and future prospects of floodplains for birds in a drying climate

Abstract

Climate change is becoming an increasingly significant driver of biodiversity loss. Changes to climatic extremes, such as those associated with drought, may pose a more severe threat to ecosystem functioning than gradual changes in mean conditions. The duration and frequency of extreme droughts are projected to increase in many parts of the world including Australia, especially in the south of the continent. Identifying refugia that increase the capacity of biota to withstand climate extremes such as drought (‘resistance’) and to recover if or when adverse conditions abate (‘resilience’) will be critical for mitigating the effects of climate change. Locations with reliable availability of water and mesic microclimates, such as floodplains, are likely to be particularly important for supporting species persistence during drought. In this thesis, I conducted a global meta-analysis to compare the relative effects of climate change and land-use change on demographic rates in plant and animal populations. I measured the resistance and resilience of floodplain bird assemblages to a thirteen-year drought (the ‘Big Dry’) in southeastern Australia. I assessed the potential of floodplains as drought refugia by comparing the resistance and resilience of birds in floodplains to those in non-floodplain zones, and assessed whether floodplains moderate the effects of aridity on bird occurrence. I investigated the importance of landscape context, the physical landscape, vegetation structure and ecological productivity in promoting the resistance of bird assemblages to prolonged drought. In the meta-analysis, I found, on average, that climate variables had equally strong effects on demographic rates of plants and animals as did land-use change. This is significant, given that the pressures of climate change will continue to intensify in coming decades. In southeastern Australia, where climate change is causing increased drought frequency and severity, I found that bird assemblages were severely affected by the Big Dry: a large proportion of species declined during the Big Dry, and few recovered after the drought broke. Floodplains showed potential as drought refugia because fewer species declined in floodplain zones than in non- floodplain zones, and declines in floodplains were less severe for many species. I found that floodplains moderate the effects of aridity on bird occurrence, extending the distribution of many species into more arid regions than they would otherwise be expected to occur. Vegetation productivity was the most important factor for increasing the resistance of bird assemblages to drought across the study region. Climate change is an important driver of population viability. Drying climate conditions, such as increased drought frequency and severity, are likely to have significant effects on bird assemblages. However, the effects of drought and aridity on birds may be reduced in highly productive ecosystems such as floodplains.