Demographic analyses of a fox population suffering from sarcoptic mange

Abstract

We reconstructed a red fox population through a cohort analysis based on age-specific numbers of culled individuals between 1987 and 2008, a timeframe that covers the pre-mange and mange periods, and analyzed the impact of sarcoptic mange on the fox population in Nemuro, Japan. In the process of the reconstruction, we estimated annual mortality (caused by natural factors and culling) and birth rates by referring to the annual change in an index of population abundance and the recovery of tagged individuals. The estimated total population size decreased greatly from 303.2 to 70.8 between 1996 and 2000 in the study area (136.5km 2 ); the first mange-infected individual was found in 1997. The natural mortality estimate (0.613) in 1998 was high, and it was also high in 1997 and 1999 (0.504 and 0.507, respectively), whereas mortality caused by culling did not show any particular change in these years. The birth rate (number of cubs per adult) was high in 1998 and 1999 despite the mange prevalence. Although the infection rate (number of infected individualsperinspected fox)decreasedafterthepeak in1999, foxeswerefound infected withmange in every study year between 1997 and 2013, except for 2002. Natural mortality was higher during the mange period (1997-2008) than during the pre-mange period (1987-1996), whereas we did not detect a significant difference in birth rates between the periods. A generalized linear model analysis showed significant effects of adult density and birth rate on natural mortality together with the effects of mange. We interpreted higher natural mortality in years with higher birth rates as simply a result of the population in those years including more cubs with higher mortality. The positive effect of adult density during the pre-mange period on natural mortality can be explained by competition for territory, whereas high adult density may also have contributed to the spread of mange for the mange period, causing higher mortality. This study shows that the use of cohort analysis can be extended to estimating demographic parameters by referring to external data. 2014 The Wildlife Society.