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Resource use and coexistence of sympatric Tasmanian devils and spotted-tailed quolls
thesisposted on 2023-05-27, 23:13 authored by Andersen, GE
Interactions between sympatric carnivores are among the key factors influencing community structure and function, and strongly affecting the population dynamics, distribution and behaviour of the interacting species. Interspecific competition occurs in two ways: exploitation competition occurs when a resource unit is consumed by one species so it cannot be consumed by another; interference competition involves direct aggressive encounters (e.g. fighting) or the threat of aggression, thereby excluding a competitor from a resource. The strength of competition between carnivores varies with factors such as body size, diet and population density. Interspecific competition is often asymmetrical such that the smaller carnivore is most affected. Due to the competitive effects that apex predators can have on sympatric carnivores, their removal from a system can result in a 'mesopredator release', which in turn can lead to increased predation on smaller prey species. To minimise competition and facilitate coexistence, the subordinate species can either avoid spatial overlap with the dominant species or modify temporal or behavioural patterns to reduce the chance of encounters and interactions, but still allow for spatial overlap. The intensity of competition can be reduced in species that have co-existed for a long time through coevolution of divergent ecomorphological and behavioural traits that influence the size and type of prey that are eaten. Current competition can be difficult to measure, as competition may only occur when resources are severely limited such as during drought, and can be subtle where there is a long coevolutionary history among the species. As interspecific competition between carnivores can strongly influence abundance and distribution, understanding how carnivores interact and the extent that competition might influence their ecology and demography is crucial in managing and conserving them. The Tasmanian devil (Sarcophilus harrisii) is Tasmania's largest marsupial carnivore (5 - 14kg) and coexists with the smaller spotted-tailed quoll (Dasyurus maculatus) (0.9 - 5kg). The devil population is currently declining due to a fatal transmissible cancer (devil facial tumour disease, or DFTD) and it has been hypothesized that this could result in a mesopredator release of quolls. There is a paucity of evidence on the mechanisms by which these species coexist and how interactions may influence distribution, habitat use and temporal activity of the smaller carnivore. I investigated the feeding ecology, movement behaviour, habitat utilisation and interactions between devils and quolls to aid the management and conservation of both species in the wild. First, I investigated diet composition and overlap of devils and quolls by analysing scats from several sites across Tasmania. Devils and quolls prey predominately on Tasmanian pademelon (Thylogale billardierii), Bennett's wallaby (Macropus rufogriseus) and birds but also consume a wide range of prey species at lower frequencies. This suggests that they are flexible and opportunistic foragers. Diet overlap was very high (Pianka index: 0.92). Second, I investigated whether there was temporal separation or spatial separation at the home-range level between devils and quolls. I did this at a site that is still free of DFTD and where devil and quoll densities are high. Using GPS collars, I found little spatial segregation at the home-range and core-area level between devils and quolls. Devils and quolls exhibited different activity patterns during the night. Devils were active from dusk until 4am, while quolls were most active in the early and latter parts of the night. This pattern of activity could allow quolls to avoid agonistic encounters with devils, but could also reflect the different hunting modes of the two species. Third, I investigated the selection of habitat types and linear features by both devils and quolls in the same landscape. I found that both species respond to moderate anthropogenic modification of intact habitats to enhance movement and facilitate prey acquisition. They used the pasture/cover interface for foraging and roads for movement and foraging. Devils utilised fence lines, while quolls showed little preference for them. Devils and quolls used all vegetation types and did not avoid the agricultural matrix. However, living in these landscapes makes them susceptible to human persecution and collision with vehicles. Human tolerance and mitigation measures to reduce the effect of road kill combined with maintaining connectivity in the agricultural matrix should be the focus of management strategies in these habitats. While moderate landscape alteration can enhance the natural features that devils and quolls use to forage, there is likely to be a threshold of fragmentation beyond which devils and quolls may not be able to exist. Fourth, I assessed the behavioural responses of free-living devils and quolls to one other's odour to help understand their behavioural interactions and test mechanisms of competitive interaction. Behavioural responses exhibited by devils and quolls are indicative of a dominant predator-mesopredator relationship and suggest the potential for interspecific competition. This study found an extensive overlap of resource use, which suggests that current competition is not occurring at my study site. Bennett's wallaby and Tasmanian pademelons, which are the preferred prey species of devils and quolls, both reach high population densities in fragmented areas, such as my study site, and could facilitate coexistence. When resources are abundant, losing devils from an ecosystem is unlikely to result in a mesopredator release of quolls. This study also enhances our understanding of devil and quoll spatial ecology and reveals several conservation and management implications in fragmented areas.
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