Declining survival rates of red foxes (Vulpes vulpes) during the first outbreak of sarcoptic mange in Sweden

Rapid declines in red fox (Vulpes vulpes) populations have followed
outbreaks of epizootic mange caused by the mite Sarcoptes scabiei. In
Sweden, the first outbreak of sarcoptic mange started in 1977/78 and
affected the whole country by 1984. Here we used data on number of
harvested red foxes from Gävleborg county (18 199 km2) in Sweden between
1970 and 1994. We used data on the prevalence of sarcoptic mange in
harvested red foxes from 1974 to 1982. A rapid decline in harvested foxes
occurred two to three years after the prevalence of sarcoptic mange first
became evident. In the same period, Mark-Recovery data were used to
estimate changes in survival rates, and the best model included an effect
of age (young or adult) and period (annual) on the survival and recapture
probabilities. The analysis was based data from 701 young foxes of which
523 were recovered, and 133 adults of which 131 were recovered. Average
annual survival was 0.55 (range=0.53-0.58) for adult and 0.36
(range=0.32-0.39) for young foxes in the three years preceding the
outbreak. During the outbreak and the remaining six years of the study,
the average survival was reduced to 0.41 (range=0.30-0.48) for adult and
to 0.25 (range=0.17-0.30) for young foxes. A population model, based on
our results on survival and literature data on fecundity, was developed to
project the decline of the fox population. The rate and magnitude of the
reduction in the projected population and harvested foxes were similar,
with both reduced by almost ninety percent. Harvest statistics indicate
the fox population recovered to pre-mange densities in less than 10 years
after the first detection of mange indicating a rapid development of
resistance in the host. This study shows the importance of long-term
population monitoring in combination with large-scale field-experiments to
device alternative management options.