Large-Scale Model-Based Assessment of Deer-Vehicle Collision Risk

Citation: Hothorn T, Brandl R, Mu ller J ( Large-Scale Model-Based Assessment of Deer-Vehicle Collision Risk Torsten Hothorn 0 Roland Brandl 0 Jo rg Mu ller 0 Adina Maya Merenlender, University of California, Berkeley, United States of America 0 1 Institut fu r Statistik, Ludwig-Maximilians-Universita t M u nchen , Mu nchen, Germany , 2 Fachbereich Biologie, Philipps-Universita t Marburg , Marburg, Germany, 3 Sachgebiet Forschung und Dokumentation, Nationalparkverwaltung Bayerischer Wald, Grafenau, Germany, 4 Lehrstuhl f u r Terrestrische O kologie, Technische Universita t M u nchen, M u nchen , Germany Ungulates, in particular the Central European roe deer Capreolus capreolus and the North American white-tailed deer Odocoileus virginianus, are economically and ecologically important. The two species are risk factors for deer-vehicle collisions and as browsers of palatable trees have implications for forest regeneration. However, no large-scale management systems for ungulates have been implemented, mainly because of the high efforts and costs associated with attempts to estimate population sizes of free-living ungulates living in a complex landscape. Attempts to directly estimate population sizes of deer are problematic owing to poor data quality and lack of spatial representation on larger scales. We used data on w74,000 deer-vehicle collisions observed in 2006 and 2009 in Bavaria, Germany, to model the local risk of deer-vehicle collisions and to investigate the relationship between deer-vehicle collisions and both environmental conditions and browsing intensities. An innovative modelling approach for the number of deer-vehicle collisions, which allows nonlinear environment-deer relationships and assessment of spatial heterogeneity, was the basis for estimating the local risk of collisions for specific road types on the scale of Bavarian municipalities. Based on this risk model, we propose a new ''deervehicle collision index'' for deer management. We show that the risk of deer-vehicle collisions is positively correlated to browsing intensity and to harvest numbers. Overall, our results demonstrate that the number of deer-vehicle collisions can be predicted with high precision on the scale of municipalities. In the densely populated and intensively used landscapes of Central Europe and North America, a model-based risk assessment for deer-vehicle collisions provides a cost-efficient instrument for deer management on the landscape scale. The measures derived from our model provide valuable information for planning road protection and defining hunting quota. Open-source software implementing the model can be used to transfer our modelling approach to wildlife-vehicle collisions elsewhere. - During the last century, ungulates have gained substantial public relevance in several temperate ecosystems of Europe and North America. Especially the roe deer Capreolus capreolus in Europe and the white-tailed deer Odocoileus virginianus in North America dominate more and more the fauna of ungulates [1,2]. Ungulate densities are relevant on larger scales because of browsing damage, which renders deer densities a major issue in farming, forestry, and conservation of biodiversity [3,4,5,6]. Selective browsing by roe deer decreases the diversity of tree species during forest regeneration, which interferes with the conversion of conifer plantations to diverse forests. Browsing can even lead to the local extinction of rare tree species, such as the white fir Abies alba in Central Europe [4,7,8]. Furthermore, collisions of vehicles with either deer species are a serious threat to human health and animal welfare [for whitetailed deer, see 9]. Each year in Germany, approximately 200,000 roe deer collide with vehicles, which is almost 20% of the roe deer harvest by hunters (online at http://www.jagdschutz.de). These collisions lead to approximately 3,000 injured people, with 50 fatalities and costs of about 490 Mio. J (online at http://www. gdv.de). The number of deervehicle collisions (DVCs) is even expected to increase in many countries with the increase in traffic [10,11,9]. Most attempts at decreasing the number of DVCs through various road construction techniques (e.g., repellents, wildlife crossings, reflectors) have had little success, except for fencing [10]. Management programs that aim at decreasing DVCs and browsing damage require information on the spatial distribution of deer. However, all methods currently available for a direct estimation of deer densities, e.g., using visual methods such as thermal imaging or spotlight surveys [12,13] or indirect measurements such as fecal counts or measuring jaw length [14], are only applicable to small areas. On larger scales, as required for statewide management actions, these methods are rendered difficult or even impossible due to the high costs associated with these survey methods. However, Morellet et al. [2] argue that for management purposes it is not necessary to obtain absolute measures of population size. Instead, cost-effective relative indices of deer density are of considerable value for data-driven management decisions. One relative index for deer abundance is the intensity of browsing of palatable tree species. Several authors noted a close relationship of deer density and browsing intensity [15,16,6]. This method is attractive because one directly measures the risk of browsing on saplings, i.e., the event forest managers are primarily interested in. However, the costs associated with large-scale browsing surveys are also rather high, and the relationship between deer densities and browsing is nonlinear [16]. Given the high costs and low precision of many survey methods for estimating absolute or relative deer abundances, it is not surprising that the idea of monitoring deer densities indirectly by means of the number of DVCs was published as early as 1959 [17]. Even though the number of these accidents was quite low at that time, a positive correlation between deer density and the number of DVCs was found. More recent studies also found a positive correlation between the number of DVCs and absolute density estimates of roe deer and white-tailed deer [18,11,19]. The relationship between deer density and DVCs cannot, however, be assumed to be linear. In addition, the strong effects of the environment and traffic, which confound the situation, have to be taken into account [10]. DVCs have received considerable interest also in the emerging field of road ecology, e.g., for measuring the impact of traffic on species [20]. Our research does not target the interaction between roads and deer per se; instead, we consider DVCs as a simple sampling method for indirectly obtaining information on deer densities across a complex landscape. The number of DVCs is an attractive and cost-efficient relative index for deer management. For example, in Germany, most accidents are registered because a police report is required for (...truncated)