A before-after control-impact experiment reveals that culling reduces the impacts of invasive deer on endangered peatlands.
Abstract
Invasive deer have undesirable impacts on native ecosystems. Culling is commonly used to reduce those impacts, but is expensive and sometimes controversial. It is therefore important to robustly assess how culling reduces the impacts of invasive deer. We conducted a Before-After Control-Impact (BACI) experiment to determine how culling affects the impacts of invasive sambar deer Cervus unicolor in an endangered alpine peatland ecosystem in south-eastern Australia. We established two blocks, each with four 4300-ha experimental units, in which two units were randomly assigned to a treatment of deer culling and the other two were non-treatment areas. Sambar deer relative abundance (faecal pellet count) and three impacts (ground degradation, pugging intensity and water turbidity) in three peatland features (peatlands, natural pools and sambar deer wallows) were surveyed before and after culling was implemented. We used four metrics to quantify the culling intensity in each treatment unit: treatment versus non-treatment, number of deer culled on each unit, deer culled within a 500-m buffer of impact monitoring sites and local probability of use by the cullers (1-ha grid). The effects of culling on deer abundance and impacts were estimated using Bayesian generalised linear mixed models. Our analysis revealed that the effect of culling differed between the two experimental blocks. In one block, culling reduced deer abundance and three impacts. In the other block, culling did not reduce deer abundance but reduced three impacts and increased two impacts. The effects of culling were generally stronger when culling was estimated using finer-scale (500-m) than coarser-scale (experimental unit) culling metrics. Synthesis and applications. Our study provides experimental evidence that culling invasive sambar deer reduces some of their undesirable impacts on endangered peatlands. That not all impacts were reduced by culling suggests that some effects of invasive species are slow to recover or require higher culling intensities to be reversed.