A Hitchhiker’s Guide to Invasion Biology: Describing the Ecological Mechanisms Underlying the Range Expansions of Two Ixodid Tick Species

Increasing incidence of many tick-borne diseases have been linked to recent expansions of tick species distributions. Many tick species are expanding their ranges because of anthropogenic changes in the landscape, shifting climatic variables, and increasing populations of suitable host species and tick habitat. Few empirical studies have been performed, however, investigating the ecological mechanisms underlying these range expansions. Ticks are parasitic organisms that disperse across landscape by hitchhiking on hosts, but must then survive in the environment for long periods of time between bloodmeals. Two species of ixodid tick, Ixodes affinis and Amblyomma maculatum, are simultaneously expanding their ranges throughout the Mid-Atlantic region of the United States, and provide a case study from which to examine the relative importance of host choice and apparent habitat preference in the resulting patterns of range expansion. The first objective of this research was to use field studies to describe the life history of each of these species in Virginia, and determine important host and habitat characteristics for the survival and expansion of these species in the Mid-Atlantic. Tick surveillance data were collected over five years in a variety of habitats throughout southeastern Virginia. These data were used to answer basic natural history questions about these ticks in novel habitats, including determining tick abundance and phenology, as well as habitat and host preferences. Although both tick species parasitized many of the same host species, differences in the habitat conditions necessary for tick population establishment resulted in different patterns of invasion. Understanding how and where these ticks establish is useful in understanding the public health risks associated with areas being invaded. The second objective of this research was to determine the ancestry of the northern populations of these ticks, using genetic connectivity among populations to determine the mostly likely pathways for dispersal from ancestral populations to the Mid-Atlantic. Despite overlapping host preferences throughout ontogeny, each species exhibited very different genetic and geographic patterns of population establishment and connectivity. Genetic evidence suggests that these species may rely on different key life stages to disperse successfully into novel environments, and that host vagility, habitat stability and habitat connectivity all play critical roles in the establishment of new tick populations. The third objective of this research was to use metrics derived from field and genetic studies to parameterize agent-based models, simulating tick range expansions under different habitat and host conditions. Incorporating parameter values specific to I. affinis and A. maculatum life history, host ranges, habitat preferences, and genetic diversity allowed for hypothesis testing on whether habitats or hosts have greater influences on the invasions of these species in the Mid-Atlantic. This research describes the first comparative case study of two tick species with unique host and habitat preferences dispersing simultaneously across a landscape, and increases our understanding of the relative importance of hosts and habitat in hitchhiker invasions