Assessing deer densities and impacts at the appropriate level for management: a review of methodologies for use beyond the site scale

Population density alone is unlikely to be a good predictor of the impacts of deer on their environment. The assessment of management requirements should therefore be based on assessment of deer impacts, alongside estimates of density. Both density and impacts need to be monitored at a landscape scale, and there is a need to develop appropriate methodologies that allow managers to consider the current and likely future impact of deer. The relevant scale for assessment (and management) varies both with deer species and context of impact, but should always encompass at least the estimated biological range of the population of deer present in an area. Some impacts (e.g. deer-vehicle collisions, and risks of disease transmission) may need to be assessed at a wider regional level. In this review we consider various approaches available for assessing: absolute or relative animal abundance; impacts of ungulates on agriculture, forestry, amenity woodlands and other conservation sites; impacts on public safety (e.g. through road traffic accidents) and on humans or livestock through potential spread of disease. In each case the advantages and disadvantages of a variety of methods are considered, before recommendations are made for methodologies which are sufficiently accurate, sufficiently robust and sufficiently practical to be favoured in a management context. To address impacts at the landscape scale requires management policies that integrate information on both positive and negative impacts of deer in order to ensure appropriate and holistic management. We present a decision-support framework suitable for use within the UK, using inputs from a variety of possible impact types to assist managers and forewarn of situations where current management may need to be modified.