The effect of climate on host-parasite interactions

Ecological systems are responding to changes in climate, and due to their ubiquitous nature, parasites will not be exempt from such changes. Understanding the effect that global climate change will have on parasites is of utmost importance, because there will be climate-driven implications for them and their hosts. This thesis addresses fundamental questions about the effect that a changing global climate will have on host-parasite interactions. First, a meta-analysis investigating general trends of parasites exposed to climatic perturbations found that warmer temperatures increased parasite abundance, did not affect fecundity and accelerated development time (Chapter 2). Field experiments were conducted in the Italian Alps, focussing on the yellow-necked mouse (Apodemus flavicollis) and its parasites as a model system. Low altitudinal sites (500 m) were significantly warmer and less humid than high altitudinal sites (1400 m), and as such, the change in altitude was used as a proxy for climate change. The effect of climate, host factors and parasite infection on one parasite life-history trait, in utero fecundity, was examined, and female hosts at low altitude harboured the least fecund parasites, which was posed to be driven by male-biased sex ratios in this cohort (Chapter 3). The effect of coinfecting parasites on the egg shedding-infection load relationship was further investigated, and this relationship was altered in coinfected hosts (Chapter 4). Interactions between coinfecting ectoparasites and helminths were quantified, and were found to be driven by host biology and climatic conditions (Chapter 5). Finally, infection with tick-borne endosymbionts and/or pathogens on tick behaviour was examined, and ticks infected with endosymbionts emigrated from dead hosts faster than uninfected individuals (Chapter 6). This thesis highlights the importance of considering the effect of host biology and coinfecting parasites in combination with climate when investigating the effect that future climatic changes may have on host-parasite interactions