The cascading effects of top scavenger loss on bird communities : a case study on a shifting strait

The loss of species from ecosystems reduces biodiversity but can also result in shifts to the structure and composition of ecological communities. Understanding how community dynamics are altered by species loss is imperative for the conservation of species by guiding appropriate management strategies. Land-use change can impact community structure through the loss of species following the removal of vital habitat. However, declines of species at the top of food webs, such as large mammalian carnivores, can also influence community structure and trigger trophic cascades throughout an ecosystem. This impact, in turn, can cause changes to the behaviour and abundance of mesoscavengers and thus increase predation on smaller prey species that are not the direct targets of top predators, such as woodland birds. As top carnivores continue to decline globally, it is essential that we understand the cascading impacts of carnivore loss. In this thesis, I use the contemporary loss of native mammalian carnivores and post-colonisation land-use change on the Bass Strait Islands (south-eastern Australia) as a case study to investigate the relationships between woodland bird communities, habitat modification, carnivore removal and shifts in scavenger dynamics. Specifically, I aim to: (1) explore how habitat modification and the removal of top mammalian carnivores impacts scavenging dynamics and mesoscavenger behaviour (Chapters 2-4); (2) investigate any cascading impacts of top carnivore loss on woodland bird communities through shifts in mesoscavenger abundance (Chapter 5); and (3) explore the utility of a novel conservation management strategy‚ÄövÑvÆtrophic rewilding‚ÄövÑvÆto mitigate the effects of top carnivore loss on mesoscavengers and woodland birds (Chapter 6). Through analysis of sites on the large island of Tasmania, as a comparison to the Bass Strait Islands, I show that mesoscavengers have greater access to carrion where top carnivores are absent, potentially leading to increases in their abundance. In Chapter 2, I reveal that forest ravens, a native mesoscavenger, were common around road sections with higher densities of roadside carrion. However, the loss of carnivores led to year-long use of roadkill, even when other resources were available, likely due to increased access to roadside carcasses. In Chapter 3, I show how carnivore loss leads to the increased discovery and use of carcasses by both feral cats and forest ravens, likely due to the increased persistence of carcasses within environments without native mammalian carnivores. In addition, I demonstrate in Chapter 4 that carrion usage and discovery by mesoscavengers across natural and modified environments is homogenous in areas without top carnivores, likely due to increased carcass persistence and reduced competition for this high-quality resource. Increases in carrion access are likely leading to higher densities of mesoscavengers, which could have consequences for their prey species, such as small woodland birds. In Chapter 5, I found that the abundance of small woodland birds was negatively associated with modified landscapes and forest raven abundance. This highlights the need for approaches that tackle these issues from both the bottom-up (e.g., restoration of habitat) and the top-down (e.g., control of overabundant mesoscavengers). Finally, within Chapter 6, I argue that trophic rewilding of top carnivores could be an important conservation tool for woodland bird communities on the Bass Strait Islands, leading to a reduction in both predation and overgrazing of vital habitat, by controlling overabundant herbivores and mesoscavengers via the restoration of top-down processes. Overall, I demonstrate that the loss of top mammalian carnivores can result in the loss of essential ecosystem functions and have cascading impacts on the structure of ecological communities. In areas without top carnivores, mesoscavengers have greater access to carrion, potentially leading to higher densities and heightened predation on woodland-bird communities. This highlights that while bottom-up strategies, such as revegetation, are essential for conserving woodland birds, top-down approaches should also be considered. The findings from this thesis highlight the integral role played by native mammalian carnivores within an ecosystem, providing backing for novel management approaches, such as trophic rewilding, and emphasising the importance of conserving mammalian carnivores.