Quantifying variations in browsing pressure caused by feral deer for a range of threatened ecological communities and plant growth forms

Abstract

AbstractGlobally, high‐density populations of feral deer have been associated with a range of negative ecological consequences. Within Australia, limited quantitative research has assessed the impacts of deer, particularly in threatened ecological communities (TECs). Our study aimed to quantify the impact of feral deer on the herbivory of a range of woody plant species, non‐woody growth forms and TECs; and assess whether feral deer increase herbivory in TECs above background rates caused by native herbivores. We surveyed 356 transects across 89 sites representing eight TECs and collected herbivory data for woody and non‐woody plants. At each site, we recorded the presence or absence of deer sign and abundance of macropod pellets to account for variations in herbivory between groups of sympatric species. Generalised linear mixed models were developed to predict: (i) proportion of individual plants browsed (or grazed); and (ii) average browsing (or grazing) intensity for woody plant species and non‐woody plants. Controlling for macropod abundance, we found the average grazing intensity and proportion of non‐woody plants grazed was higher when deer were present compared to absent. Specifically, grazing pressure on rushes, cycads, sedges and grasses was significantly higher where deer were present. In contrast, we did not observe a significant impact of deer on woody plant species. The presence of deer resulted in higher grazing intensity in coastal saltmarsh, freshwater wetland, littoral rainforest and bangalay sand forest TECs. Although the deer population across our study region generally appears to be at low density compared to neighbouring regions, their presence is resulting in grazing pressure above that caused by native herbivores for a range of non‐woody plants. Our results contribute to a growing body of research about the ecological impacts of feral deer in Australia and can be used as a baseline for ongoing regional monitoring of deer impacts in TECs.