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Rodents often act as important hosts for ticks and as pathogen reservoirs. At northern latitudes, rodents often undergo multi-annual population cycles, and the periodic absence of certain hosts may inhibit the survival and recruitment of ticks. We investigated the potential role of common shrews ( Sorex araneus ) to serve as a supplementary host source to immature life stages (larvae and nymphs) of a generalist tick Ixodes ricinus and a small mammal specialist tick I .  trianguliceps , during decreasing abundances of bank voles ( Myodes glareolus ). We used generalized mixed models to test whether ticks would have a propensity to parasitize a certain host species dependent on host population size and host population composition across two high-latitude gradients in southern Norway, by comparing tick burdens on trapped animals. Host population size was defined as the total number of captured animals and host population composition as the proportion of voles to shrews. We found that a larger proportion of voles in the host population favored the parasitism of voles by I .  ricinus larvae (estimate = -1.923, p  = .039) but not by nymphs (estimate = -0.307, p  = .772). I .  trianguliceps larvae did not show a lower propensity to parasitize voles, regardless of host population composition (estimate = 0.875, p  = .180), while nymphs parasitized shrews significantly more as vole abundance increased (estimate = 2.106, p  = .002). These results indicate that common shrews may have the potential to act as a replacement host during periods of low rodent availability, but long-term observations encompassing complete rodent cycles may determine whether shrews are able to maintain tick range expansion despite low rodent availability., (© 2022 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.)

The island biogeography theory is one of the major theories in ecology, and its applicability to natural systems is well documented. The core model of the theory, the equilibrium model of island biogeography, predicts that species diversity on an island is positively related to the size of the island, but negatively related by the island's distance to the mainland. In recent years, ecologists have begun to apply this model when investigating genetic diversity, arguing that genetic and species diversity might be influenced by similar ecological processes. However, most studies have focused on oceanic islands, but knowledge on how the theory applies to islands located on the mainland (e.g., mountain islands, forest islands) is scarce. In this study, we examined how the size and degree of isolation of mountain islands would affect the genetic diversity of an alpine bird, the rock ptarmigan ( Lagopus muta ). Within our study area, we defined the largest contiguous mountain area as the mainland, while smaller mountains surrounding the mainland were defined as islands. We found that the observed heterozygosity ( H o ) was significantly higher, and the inbreeding coefficient ( F is ) significantly lower, on the mainland compared to islands. There was a positive significant relationship between the unbiased expected heterozygosity ( H n.b. ) and island size (log km 2 ), but a negative significant relationship between H o and the cost distance to the mainland. Our results are consistent with the equilibrium model of island biogeography and show that the model is well suited for investigating genetic diversity among islands, but also on islands located on the mainland., Competing Interests: None declared.