Local adaptation of the ectoparasite Ixodes uriae to its seabird host

In addition to reproductive rates and generation times, local parasite adaptation is predicted to be associated with relative host/parasite migration rates, parasite virulence and local resource levels. We tested for local parasite adaptation in a spatially structured natural host–parasite system with fluctuating host resources. Using a cross-fostering design replicated over 2 years, we exchanged chicks of the Black-legged kittiwake (Rissa tridactyla) between subpopulations of its ectoparasite, the tick Ixodes uriae. We found evidence that ticks were adapted to their local hosts; ticks had higher success (capacity to continue to the next life stage) and shorter engorgement times on sympatric birds than on allopatric birds. However, infestation levels were similar between the resident and non-resident chicks in a nest, implying that ticks are unable to distinguish between good and bad hosts and that selection acts during tick engorgement. Hosts appeared to be locally maladapted to their parasites; growth rates tended to be lower for sympatric birds in the presence of parasites. However, we found no effect of host group on the T-cell immune response of chicks. Overall, the results seemed to depend on the environmental quality. When resources were low, local maladaptation was expressed in the host, but adaptation was not shown by the parasite. In the higher quality year, evidence for local parasite adaptation was found, but the host seemed to be able to compensate for the pathogenic effects of ticks. This suggests that virulence (pathogenic effect on host) and the reciprocal effects of the interaction can fluctuate depending on host environmental conditions.