The effect of deer management on the abundance of Ixodes ricinus in Scotland

Abstract

The management of wildlife hosts for controlling parasites and disease has a history of mixed success. Deer can be important hosts for ticks, such as Ixodes ricinus, which is the primary vector of disease-causing zoonotic pathogens in Europe. Deer are generally managed by culling and fencing for forestry protection, habitat conservation, and commercial hunting, and in this study we test whether these deer management methods can be useful for controlling ticks, with implications for tick-borne pathogens. At different spatial scales and habitats we tested the hypotheses that tick abundance is reduced by (1) culling deer and (2) deer exclusion using fencing. We compared abundance indices of hosts and questing I. ricinus nymphs using a combination of small-scale fencing experiments on moorland, a large-scale natural experiment of fenced and unfenced pairs of forests, and cross-sectional surveys of forest and moorland areas with varying deer densities. As predicted, areas with fewer deer had fewer ticks, and fenced exclosures had dramatically fewer ticks in both large-scale forest and small-scale moorland plots. Fencing and reducing deer density were also associated with higher ground vegetation. The implications of these results on other hosts, pathogen prevalence, and disease risk are discussed. This study provides evidence of how traditional management methods of a keystone species can reduce a generalist parasite, with implications for disease risk mitigation.

The wild life of tick-borne pathogens

“…Our findings agree with Gilbert et al 38 , who found a strong decrease in the density of I. ricinus nymphs in both large and small (<1 ha) exclosures. However, they disagree with a meta-analysis by Perkins et al 194 who found that exclosures >2.5 ha were needed to reduce densities of I. scapularis and Amblyomma americanum.…”

“…Van Buskirk and Ostfeld 27 , for example, modelled how nymph densities of I. scapularis responded to differences in densities of hosts for larvae and adults, and found that the density of hosts for adults was limiting nymph density only at very low host densities, where the availability of hosts for larvae then became limiting. Thus, nymph and adult density appear related to deer density according to a nonlinear threshold relationship, rather than the linear relationship used in most studies (e.g., Gilbert et al 38 and Sprong et al 39 ). Furthermore, Van Buskirk and Ostfeld 27 suggested that the threshold adult-host density is close to zero.…”

“…Ixodes ricinus is sensitive to desiccation and is mostly found in forested areas throughout Europe 36 . Within forests, it is dependent on the availability of hosts for all stages of the life cycle, where the presence and abundance of deer, as hosts for the adult stage, is theorized to be an important determinant of I. ricinus density 38,39 . Ixodes ricinus encounters a host by sitting on the vegetation and waiting, called 'questing' 8 .…”

“…It is widely assumed that I. ricinus density is determined by deer abundance, but empirical evidence is scarce 38,55 . Therefore, I performed a deer exclusion experiment using a Before -After Control -Impact design with four pairs of 0.75-ha forest plots in a forest near Apeldoorn, the Netherlands.…”

“…It is widely assumed that deer are essential hosts for ticks in the I. ricinus complex. Indeed, some studies found a strong correlation of tick density with deer presence and density 38,191,192 . Several other studies, however, found that deer exclusion (by fencing) and deer culling did not always reduce tick densities 193,194 .…”