The role of large herbivores in Ixodes ricinus and Borrelia burgdorferi s.l. dynamics

Large herbivores are the most important reproduction hosts for Ixodes ricinus, and, as such, play a major role in maintaining tick populations. As one individual deer can already feed many females during the tick season, we propose that the relationship between deer density and tick density can best be described by a step function rather than a linear function. At high densities, herbivores may negatively affect tick numbers through their effects on vegetation structure and composition by creating and maintaining a short and open herb layer, reducing the shrub layer and decreasing the thickness of the litter layer. These effects may also have a negative effect on rodent densities. Domestic herbivores as added grazers will likely not have a major added effect on tick numbers but at high density they may have, both through their effects on the vegetation and because they may negatively affect the habitat use of the wild ungulates through competitive interactions. Large herbivores are mainly incompetent, in the sense of not-transmitting the parasite Borrelia burgdorferi s.l. to ticks, but to what extent this will affect the density of infected nymphs in a system is dependent of the host community as a whole and cannot be predicted from the density of the large herbivores alone.