Effects of multiflora rose (Rosa multiflora) invasion on blacklegged tick (Ixodes scapularis) abundance at two spatial scales
Non-native, invasive shrubs in eastern deciduous forests may affect tick-borne pathogen transmission by increasing tick abundance. At a fine scale, the dense growth form of these shrubs can create a more favorable microclimate for tick survival, with increased humidity and protection from extreme temperature fluctuations. However, whether these shrubs also confer changes to habitat characteristics that could influence tick abundance at a broader spatial scale has remained largely unexplored. We investigated the association of multiflora rose (Rosa multiflora), an exotic invasive understory shrub, on the abundance of blacklegged ticks (Ixodes scapularis), an important vector for zoonotic pathogens, at two spatial scales: within and among 11 hardwood forest fragments in the urban landscape of Newark, Delaware. During 2013 and 2014, we used CO2-baited traps to sample nymphal tick abundance, and we conducted vegetation surveys surrounding each trap. At the forest fragment scale, we compared tick abundance between rose-invaded and rose-free forest fragments sampled at four randomly chosen points within each fragment. At a finer scale within rose-invaded forest fragments, we compared tick abundance between four pairs of points. Each sampling pair consisted of a randomly-chosen point under rose and a point 25 meters away and not under rose.
Within rose-invaded forests, we trapped about 2.5 times more blacklegged tick nymphs under rose compared to paired points away from rose (t19=2.52, p=0.019). Yet, at the broader spatial scale, we trapped more than 3 times as many ticks in rose-free forest fragments compared to rose-invaded forest fragments (t42=3.50, p=0.001). Thus, non-native understory plant invasion has scale-dependent effects. Additionally, among several habitat characteristics that we tested, only leaf litter volume was a good predictor of tick abundance across all traps (R2=0.22, p=0.0001). At the forest fragment scale, points in rose-invaded sites had less leaf litter compared to those in the uninvaded sites (F(2,60)=37.68, p<0.0001). We suggest that the availability of leaf litter at the fragment scale appears to play a greater role in constraining the abundance of ticks than the fine-scale positive association of tick abundance with multiflora rose.