The Consequences of Myxomatosis

The arrival of myxomatosis in Britain in 1953-1955 constituted a massive, far-reaching, unplanned field experiment. Myxomatosis is a lethal disease of European rabbits, and within a couple of years the national rabbit population crashed to less than 1% of its former size (Chapter 10). This sudden removal of the formerly abundant rabbits had broad environmental effects.

For example, the lifting of the enormous grazing pressure once exerted by rabbits permitted unprecedented, spectacular flowering of the countryside in the springs of 1955 and 1956 (Sumption & Flowerdew 1985). The consequences of these events for common weasels and stoats were not observed directly, but were strong enough to make unmistakable ripples in the vermin books of many game estates (see Figures 10.2 and 10.6). The numbers of each species caught can be compared directly, since both are collected in the same traps by identical methods. The records clearly show that, for at least 15 or 20 years after the epizootic, common weasels flourished, while stoats virtually disappeared.

King and Moors (1979a) suggested that one possible reason why myxoma-tosis had such different consequences for the two small mustelids was that it changed the balance of their relative advantages. When rabbits disappeared, the broad range of sizes of prey animals available to stoats suddenly narrowed. Stoats lost their main advantage over common weasels, the freedom of choice between large and small prey, and instead found themselves in fierce competition with common weasels for small rodents and with larger predators, such as foxes, feral cats, and raptors, for the remaining rabbits. Stoats lost against both the common weasels, which can reach rodents in their burrows, and the larger predators, which would not hesitate to attack a stoat. The balance of advantages was temporarily tipped in favor of common weasels.

The crash in the rabbit population resulted in a general shortage of prey at first, because all surviving predators had to concentrate on small rodents. This shortage did not affect common weasels nearly as much as it affected the larger predators, which can hunt only those rodents that show themselves out of their nests and burrows. Then, in 1956, the populations of small rodents soared, and the common weasel population followed them.

For the next few years over most of the country, the average ratio of common weasels to stoats killed was reversed, from about 1:2 before myxomatosis to about 2:1 after, depending on the habitat (Craster 1970; Hewson 1972). On one estate in eastern England, common weasels never exceeded stoats in the annual bag, but the average number killed more than doubled, from 15 per year in the 7 years 1947 to 1953, when stoats averaged 650 a year (1:40), to 38 per year in the 7 years 1957 to 1963, when stoats were at their lowest ebb, 69 a year

(1:2) (King 1980c). The main reason for the extraordinary increase in numbers of common weasels was certainly the sudden glut of small rodents, although the removal of interference from stoats (very scarce by then) probably helped.

Was this article helpful?

0 0

Post a comment