Redpath Museum, McGill University Montreal, Quebec, Canada
The geographic range of blacklegged tick (Ixodes scapularis) populations in Canada has expanded due to climate warming and associated northward range expansions of its vertebrate hosts. Abiotic factors, such as temperature, precipitation, and land cover, are known to affect I. scapularis abundance. Yet, biotic factors, especially the abundance of white-footed mice (Peromyscus leucopus; a highly competent host for several tick-borne pathogens) and the diversity of other vertebrate hosts, could alter blacklegged tick abundances and consequent tick-borne disease risks. Here, we evaluate the effect of abiotic and biotic factors on tick abundance in Québec (where tick populations recently established) and in Ontario (where endemic tick populations were first detected in Canada in 1990). We expect that tick abundance will be driven differently by abiotic and biotic factors due to the distinct historical establishments in each province. We conducted field-based methods using tick dragging, small mammal trapping, and trail cameras to determine the biotic factors and I. scapularis abundances across our sites. Environmental factors and land cover were extracted from remote sensing and weather tower data. Generalized additive models were constructed to determine the relative importance of various environmental and ecological factors on tick abundance.
Blacklegged tick abundance was significantly different between provinces, with a greater amount of ticks found in Quebec compared to Ontario. The number of mammal species and abundance of P. leucopus directly affected tick abundance, but the nature and direction of these relations differed based on the degree of tick population establishment (endemic versus emergent). In Ontario, the abundance of white-footed mice and tick abundance are decoupled. However, tick populations responded positively to the number of mammal species with additional hosts providing more feeding opportunities. In Quebec, the abundance of white-footed mice showed a peaked relationship with tick abundance, which corresponds to areas expected to have the highest Lyme disease risk. The number of mammal species also showed a peaked relationship with tick abundance, which was driven locally by hosts of varying competencies to tick-borne diseases. Disentangling the influence of abiotic and biotic factors is critical to understanding how the local environment and host community affect blacklegged tick populations. Furthermore, our results highlight the importance of the temporal dimension for the nature of the interactions between the mammalian hosts and tick vectors, where disease system dynamics differ between endemic and emergent regions.