Assessing the long-term impact of Ranavirus infection in wild common frog populations

Amphibians are declining worldwide, and one cause of this is infectious disease emergence. Mass mortalities caused by a virus or a group of viruses belonging to the genus Ranavirus have occurred in wild common frogs Rana temporaria in England since the 1980s, and ranaviral disease is widespread in amphibians in North America and Canada, where it can also cause mass die-offs. Although there have been numerous reports of Ranavirus-associated mass mortality events, no study has yet evaluated the long-term impacts of this disease. This study follows up archived records of English common frog mortalities likely caused by Ranavirus. There is a preliminary indication that common frog populations can respond differently to the emergence of disease: emergence may be transient, catastrophic, or persistent with recurrent mortality events. We subsequently focused on populations that had recurring mortality events (n=18), and we report median declines of 81% in the number of adult frogs in these populations from 1996 to 2008. Comparable uninfected populations (n=16) showed no change in population size over the same time period. Regressions show that larger frog populations may be more likely to experience larger declines than smaller populations, and linear models show that percentage population size change is significantly correlated with disease status, but that habitat age (a possible proxy for environmental quality) has no significant effect on population size change. Our results provide the first evidence of long-term localized population declines of an amphibian species which appear to be best explained by the presence of Ranavirus infection.