Impacts of feral deer on threatened ecological communities in south-eastern New South Wales
Creators: name: Burns, Heather
Publisher: name: The Australian National University
Year: 2019

description: Feral deer populations and their associated ecological impacts are variously increasing across Australia. High density deer populations can cause extensive damage through intensive browsing of native plant species, rubbing or ringbarking trees, thrashing saplings, exacerbating soil erosion and reducing water quality. Despite the ecological risks that deer theoretically pose, relatively little quantitative research has so far assessed realised impacts, particularly in Australia. Additionally, the existing limited research has generally focused on high-density populations, while areas with emerging low-density deer populations are often overlooked. As a result, many land managers primarily rely on anecdotal evidence to make decisions about management and whether to apply control measures. The South Coast of New South Wales is a prime example: anecdotal evidence suggests deer populations are expanding, but land managers lack key information needed to create effective, targeted management plans. In this thesis I aimed to (1) predict the current distribution of deer in the South Coast region to be used as a baseline; and (2) establish the extent to which deer are a current and likely future threat to the conservation of threatened ecological communities within the region. My study examined the area from the coastal city of Nowra, south to the Victorian border, and extending inland to approximately the coastal escarpment. Fieldwork underpinning my studies was conducted between April and July 2019. To address the first aim, I compiled over 500 presence-only records of feral deer (including 76 records of fallow deer and 60 records of sambar deer) from online databases, knowledgeable National Parks and Wildlife Service staff, and field-based surveys. Maxent species distribution software was used to combine these data and a selection of environmental variables and model the probability of occurrence of deer across my study area. Using these methods I produced three models: one for all species of deer, and two species-specific models for fallow and sambar deer. Two bioclimatic variables, mean temperature of the coldest quarter and precipitation of the driest quarter, had a high contribution to all three models. The predicted probability of occurrence of deer decreased with increasing distance from non-woody vegetation across all models. The geographic representations of all three models agreed with anecdotal reports of deer distribution across this area. However, these models were derived from opportunistic observations only, and they should therefore be interpreted with due caution. To address my second aim, I focused on the threat that browsing by deer currently poses to plant species within threatened ecological communities in my study area. Three hundred fifty-six transects were surveyed across 89 sites representing eight threatened ecological communities, and browse data was collected for both woody species and non-woody life forms. Data on the presence or absence of deer sign and the abundance of macropod pellets was collected at the same time. Generalised linear mixed models (GLMMs) were developed to predict: (1) the average browse intensity and (2) proportion of individuals browsed for woody species and non-woody life forms. Sites where deer were present had a higher average intensity and proportion of non-woody life forms browsed, and a higher proportion of woody species browsed. Where deer were present, there was increased browsing pressure on rushes, cycads, sedges, and grass life forms. In contrast, average browse intensity of woody species was not affected by the presence of deer. I also used GLMMs to predict whether average browse intensity and proportion of individuals browsed for woody species and non-woody life forms varies in different threatened ecological communities. Where deer were present, there was increased average browse intensity of non-woody plants in Coastal Saltmarsh, Freshwater Wetland, and Littoral Rainforest threatened ecological communities. The comparison of my models indicates that plant life form (or species) is a better predictor of deer browsing pressure than threatened ecological community. Although the deer population across this region currently appears to be at a low density, their presence is nevertheless resulting in changes to the browsing impacts on a range of plant species and life forms. My results can be used by land managers to prioritise where deer control should occur, and as a baseline for ongoing monitoring of deer impacts in threatened ecological communities across the South Coast of New South Wales. My results also contribute to a growing body of research about the impacts of feral deer in Australia.