Ireland’s Changing DEER POPULATIONS

By KILIAN MURPHY

Deer are the largest wild terrestrial mammal species in Ireland. Red deer Cervus elephus are native to Ireland and have been present here since the glacial maximum (c. 25,000 years ago) though reintroductions of foreign stock have likely bolstered their population. Ireland also has two established nonnative deer species: Fallow deer Dama dama and Sika deer Cervus nippon. Finally, there are scattered records of Muntjac deer Muntiacus reevesi but they are not considered to be widespread established populations.

Deer are an important part of our natural and cultural heritage in Ireland and thus, there is a broad suite of interested stakeholders who have different visions for the conservation and management of deer in Ireland. For instance, a portion of this group find deer to be pests that incur economic costs and pose a threat to public safety due to their perceived negative effect on forestry, crops, domestic animals, and road users. Conversely, deer are seen as a prized resource to conservationists and hunters. The goal of deer management for all stakeholders is in essence a shared one, to maintain healthy and sustainable deer population. To achieve this goal, it is essential that fundamental baseline information regarding each of the deer species populations are known, perhaps most importantly, their relative population density and their geographic distribution. New research from the Lab of Wildlife Ecology and Behaviour in University College Dublin has utilised recent advancements in statistical analysis to demonstrate how changing land-use in Ireland from 2000-2018 has played a role in shaping each deer populations’ distribution and density.

This study found that during the period 2000-2018 each of the three established deer populations in Ireland increased their spatial range across the country and increased their population density. It is likely that human introduction of deer is the strongest driver of where deer are located in Ireland and for the presence of high-density ‘hot-spots’. We found high densities of each species close to where historical introductions occurred: Donegal, Meath and Kerry for red deer, Wicklow and West Cork/ Kerry for sika deer and midland counties for fallow deer. Over the course of the study period – each species expanded their range outwards from these “hot-spots” at lower population density leading to a much-increased distribution and density of deer in Ireland. It is possible that there have been many local population introductions of deer by humans in the 20th and 21st centuries that have not been documented but may have contributed to the range expansion and/or population growth we document here.

Across the study period, there was also an observed switch in the type of habitat deer were utilising, oftentimes favouring more human-dominated environments over more natural landscapes. For instance, red deer reduced their use of natural grassland and increased the use of pasture – which may have implications for encountering domestic animals and thus disease spread e.g., bovine tuberculosis. Sika deer reduced their use of peat bogs and increased their use of conifer forestry which potentially could place this species in conflict with foresters due to incurring forest damage through browsing, fraying and bark stripping behaviours. Finally, fallow deer saw an increased reliance on artificial surfaces, areas such as roads and towns, potentially bringing this species into direct encounters with humans.

While further research will be essential to disentangle the patterns, it is important to note that Ireland is not unique in the population growth and range expansion of our deer species, in fact, it is representative of many countries where deer populations reach unsustainable levels resulting in human-wildlife coexistence issues. Research from Scandinavia, North America and continental Europe has found that shorter and milder winters due to climate change, the eradication of historically extant large carnivore species that predate on deer (e.g., brown bear, Eurasian lynx and, as recently as the 18th century, wolves in Ireland) and finally, land-use change such as increased forest cover and increased growth of agricultural crops have all facilitated deer population growth in other countries.

The result in these countries is similar to what we are experiencing in Ireland – a wildlife management conflict involving a wide variety of interested stakeholders with differing views on how to effectively manage these species to ensure sustainable populations.

This research is an important first step to understand the dynamics of deer species populations in Ireland, to inform a more granular programme of research that aims to understand fine-scale ecology and to inform management accordingly. The evidencebased management of deer is a complex but important task that underpins national debates and policy directions across sectors such as our national Forest Strategy, zoonotic disease surveillance and control, large predator reintroduction and meeting the needs of the many deer stakeholder groups who all wish for sustainable healthy populations that co-exist with humans in our highly modified landscape.

Kilian Murphy is a quantitative ecologist working out of the Lab of Wildlife Ecology and Behaviour. His research focuses on using data driven solutions to inform wildlife management when high resolution wildlife monitoring data is not available.

By GORDON D'ARCY