The response in native wildlife to an invading pathogen: Swedish amphibians and Batrachochytrium dendrobatidis

Abstract: Emerging infectious diseases are causing mortality and declines in wildlife populations globally. My thesis aims to get as clear a picture as possible of the effect the invasive chytrid fungus Batrachochytrium dendrobatidis has on the Swedish amphibian community.In Paper I I performed a large-scale survey testing for the presence of Bd in three regions in Sweden (Southern, Central and Northern). I sampled 1917 amphibians from 101 localities and found that Bd was widespread in southern and central Sweden, occurring in all nine investigated species and in 45.5 % of the sampled sites with an overall prevalence of 13.8%. I found a positive correlation between the temperature at spawning for each species and species prevalence. Species that require higher temperatures for egg-laying are distributed in the southern parts of the country, which led to a higher prevalence in the southern region.In Paper II, I investigated which local environmental factors in breeding habitats, landscape structure and amphibian community affect the occurrence and prevalence of Bd among breeding sites in southern Sweden. Bd prevalence in the four species with the highest prevalence (Bombina bombina, Bufotes variabilis, Epidalea calamita and Rana arvalis) was higher in ponds surrounded by less mature forest, few wetlands, and higher pH.In Papers III and IV, I looked at species and population differences in responses to Bd infection. I performed an infection experiment described in Paper III, where I exposed individuals from two common Swedish species (moor frog R. arvalis and common toad Bufo bufo) originating from two regions (north and south) with two different strains of Bd (from Sweden and the UK). I found that infection led to lower survival and growth in both species, more so in B. bufo than in R. arvalis. Small size proved to be a strong determinant of survival. As individuals from the northern population were significantly smaller than the southern ones, this may have led to the northern populations being more affected by Bd infection. In Paper IV, I studied variation in MHC Class IIB loci in B. bufo along a latitudinal gradient across Sweden. Variation in MCH genes decreased from south to north. Also, differences in survival from the experiment in Paper III could be explained by MHC haplotypes. I found that survival in the southern region was dependent on both Bd-strain and MHC haplotype.