Occurrence of Batrachochytrium dendrobatidis in Sweden: higher infection prevalence in southern species

Abstract

The chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused worldwide declines in amphibian populations. While Bd is widespread in southern and central Europe, its occurrence and distribution in northernmost Europe is mostly unknown. We surveyed for Bd in breeding anurans in Sweden by sampling 1917 amphibians from 101 localities and 3 regions in Sweden (southern, northern and central). We found that Bd was widespread in southern and central Sweden, occurring in all 9 investigated species and in 45.5% of the 101 localities with an overall prevalence of 13.8%. No infected individuals were found in the 4 northern sites sampled. The records from central Sweden represent the northernmost records of Bd in Europe. While the proportion of sites positive for Bd was similar between the southern and central regions, prevalence was much higher in the southern region. This was because southern species with a distribution mainly restricted to southernmost Sweden had a higher prevalence than widespread generalist species. The nationally red-listed green toad Bufotes variabilis and the fire-bellied toad Bombina bombina had the highest prevalence (61.4 and 48.9%, respectively). Across species, Bd prevalence was strongly positively, correlated with water temperature at the start of egg laying. However, no individuals showing visual signs of chytridiomycosis were found in the field. These results indicate that Bd is widespread and common in southern and central Sweden with southern species, breeding in higher temperatures and with longer breeding periods, having higher prevalence. However, the impact of Bd on amphibian populations in northernmost Europe remains unknown.

Body size mediates latitudinal population differences in response to Bd infection in two amphibian species

“…Since Ra has higher infection prevalence in in the wild (Meurling et al 2020) and higher infection tolerance (this study), we suggest that Ra may act as a reservoir species and a possible vector for Bd-transmission to more sensitive species such as Bb. Indeed, Kärvemo et al (2019) showed that Bb populations coexisting with Ra had higher Bd-prevalence than populations breeding in ponds without Ra.…”

“…Bd is widespread in the southern parts of Sweden (Kärvemo et al 2018, 2019, Meurling et al 2020). However, in a pattern similar to much of Europe (Lips 2016, Scheele et al 2019, no cases of chytridiomycosis or unusual die-offs have been found in Sweden.…”

“…Two important conclusions can be drawn. Firstly, very few surveys have been conducted in northern Scandinavia (Meurling et al 2020). As populations at higher latitudes can be more vulnerable to infection, it is important to investigate the occurrence of Bd in these areas and, if still possible, prevent or limit the northward spread of the fungus.…”

“…Both species are explosive breeders and mate in early spring. In southern Sweden, Bd prevalence in breeding adults is 15.3% (n = 288) and 3.4% (n = 941) in Ra and Bb, respectively (Meurling et al 2020).…”

Environmental DNA metabarcoding elucidates patterns of fish colonisation and co‐occurrences with amphibians in temperate wetlands created for biodiversity

“…Fish are reported to have negative impacts on the growth and prevalence of amphibian populations (Pope, 2008;Semlitsch et al, 2015;Shulse et al, 2012) as well as their behaviour (Winandy et al, 2015). Amphibian population declines are also related to habitat loss or deterioration, over-exploitation, climate change and diseases (Cohen et al, 2019;Meurling et al, 2020;Stuart et al, 2004) and therefore are of conservation interest when creating new aquatic habitats. Negative effects of fish on amphibians, especially focusing on prey-predator occurrences, are well documented in natural wetlands (Hartel et al, 2007) and created ponds (Semlitsch, 1987).…”