Rodents, cowpox virus and islands: densities, numbers and thresholds
Mike Begon, School of Biological Sciences, The University of Liverpool, Liverpool L69 7ZB, U.K. Tel. 44 (0)151 7954525. Fax: 44 (0)151 7954408. E-mail: email@example.com.
- 1The population dynamics of bank voles and wood mice, and of cowpox virus infection in these two species, was studied over a 2-year period in a mainland population and in 14 nearby island populations of varying sizes.
- 2For both species, there was no intrinsic variation in the pattern of host dynamics with island size: small island populations behaved as though they were small subsamples of a larger population, displaying no more than the expected random variation from the general pattern.
- 3None the less, the relative numbers of bank voles to wood mice increased markedly with decreasing island size; and bank vole densities tended to be higher on smaller islands.
- 4Only 22 animals were discovered to have moved either between islands or between the mainland and the islands, out of 1883 captured in all. None the less, it was apparent that males were more likely to move than females.
- 5Overall patterns of cowpox virus dynamics were similar in all cases. However, on all islands there were extended periods when cowpox virus infection was apparently absent, and on the small islands the numbers of infected individuals were mostly very low and in many cases infection was never found.
- 6For both host species, there was no evidence for a threshold population size for cowpox virus (critical community size) in terms of density, but clear evidence for one in terms of the numerical size of populations. This suggests little support for density-dependent transmission, despite this having been the usual default assumption for non-sexually transmitted infections.
- 7There was also evidence for a separate invasion threshold (between ecological and epidemiological invasion) and persistence threshold (between epidemiological invasion and persistence). This is contrary to the output of the most-quoted (deterministic) models – persistence and invasion threshold one and the same – highlighting the fact that little attention has been paid in the past to the practical meaning of the theoretical concept of a threshold.
- 8In the case of the wood mice, a superficial similarity to the bank vole thresholds was potentially misleading. Wood mouse thresholds were influenced at least as much by the bank vole thresholds as they were by the dynamics within the wood mouse populations themselves.