Invasive generalist ectoparasites provide a tool to study factors affecting expansion rates. An increase in the number of host species may facilitate geographic range expansion by increasing the number of suitable habitats and by affecting local extinction and colonization rates. A geographic perspective on parasite host specificity and its implications on range expansion are, however, insufficiently understood. We conducted a field study to explore if divergent host specificity could explain the observed variation in expansion rates between Fennoscandian populations of the deer ked (Lipoptena cervi), which is a blood-feeding ectoparasitic fly of cervids. We found that the rapidly expanding eastern population in Finland appears to specialize on moose, whereas the slowly expanding western population in Norway breeds successfully on both moose and roe deer. The eastern population was also found to utilize the wild forest reindeer as an auxiliary host, but this species is apparently of low value for L. cervi in terms of adult maintenance, reproductive output and offspring quality. Abundant numbers of roe deer and white-tailed deer were observed to be apparently uninfected in Finland, suggesting that host use is not a plastic response to host availability, but rather a consequence of population-level evolutionary changes. Locally compatible hosts were found to be the ones sharing a long history with the deer ked in the area. Cervids that sustained adult deer keds also allowed successful reproduction. Thus, host use is probably determined by the ability of the adult to exploit particular host species. We conclude that a wide host range alone does not account for the high expansion rate or wide geographic distribution of the deer ked, although loose ecological requirements would increase habitat availability.
We examined morphological and genetic differences among Fennoscandian deer ked (Lipoptena cervi L, Hippoboscidae) populations with varying expansion history: the eastern population (Finland) has expanded rapidly, whereas the western population is divided into an old and relatively stationary sub-population in Sweden and a newly established and more expansive sub-population in Norway. The genetic analysis suggests that the distinct populations represent a single species. Individuals from expansive populations were characterized by a large body size, relatively large and robust thorax shape, and wing shape with an exaggerated basal posterior margin. Yet, there was no among population variation in relative wing size or its elongated shape after variation in overall size was controlled for. Although certain size and shape variables showed thermal sensitivity, the degree of plasticity did not differ between the populations. In general, we observed that shape is more sensitive to external thermal conditions at the pupal stage than size per se, with the thermal sensitivity of the latter depending on the trait under examination. We conclude that the possible adaptive value of morphological differences relies on variation in survival during the off-host life stages or short-distance flight to reach a susceptible host instead of long-distance dispersal ability.
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