Landscape distribution and habitat use of sika deer using indirect signs of deer activity: a simple tool for forest managers

The populations of many deer species across Europe are widely reported to be increasing, leading to potentially greater impacts of deer on commercial and natural forest ecosystems, yet there is a paucity of information on the regional distribution (eg 1 km2 resolution) of deer populations, or the factors that influence these distributions. Deer distributions can be modelled at a national and regional scale based on presence-only, presence/absence data or hunter returns, but a more granular assessment of distributions, using empirical observations at multiple sites, will allow identification of local density hotspots and coldspots, and more precise information on landscape factors driving habitat use. In this study, we assessed the distribution of alien invasive Sika deer (Cervus nippon) across its putative range in the south west of Ireland, using a semi-quantitative activity index, based on indirect signs of deer activity (browse, trackways, faecal pellet and slot indices), at 249 individual forest sites, within an area of approx. 10,000 km2. We then related recorded levels of Sika activity to landscape geomorphological and land use factors. Sika deer were associated with landscapes characterised by unenclosed, higher-gradient heathland, peaty, acidic soils and low agricultural intensity. Sika deer were largely absent from landscapes dominated by improved grasslands on more fertile, better-drained mineral soils, with higher agricultural intensity. The areal extent and nature of conifer plantations and broadleaf woodlands was similar in both landscape types. The future range expansion of Sika deer within the region may be less than previously reported, if deer remain restricted to these heathland landscapes, despite increases to plantation forest cover. A wider use of the activity index method, by allowing more accurate assessment of deer distributions at landscape scales, will facilitate the risk assessment and management of deer impacts to trees, crops and vehicular traffic.