The Decline and Increase of Feral Sheep (Ovis aries) on Campbell-Island
- Ecology Division, DSIR, Private Bag, Lower Hutt, New Zealand
Sheep introduced to Campbell Island in 1895 for farming reached numbers of over 8000 in 1916 then declined to 1000 by 1961 (exponential growth rate r = -0.05 p.a.). Numbers increased to around 3000 from 1961 to 1969 (r = 0.14 p.a.). The island was divided into halves by a fence in 1970, and all sheep north of it were killed. The southern population continued to grow from 1970 to 1984 (r = 0.053 p.a.). The southern half of the island was cleared of sheep in 1984 except for about 800 fenced off on a peninsula.
Censuses in 1961 and 1969, and the sample shot in 1970, showed that population growth rate, lambing rate, sex ratio, and time of breeding varied in different places on the island, and from year to year. The average number of lambs present per 100 females ranged from 30 to 40 in the farming years up to 1931, and from 60 to 80 by 1970. The age structure in 1970 was consistent with a stable age-distribution in an expanding population.
During the farming years the southern hemisphere was in a cold climatic phase. From the mid 1930s there was a general warming, especially during the 1950s. These better conditions, together with falling grazing pressure and absence of burning, allowed an increase in the island's woody and herbaceous vegetation. The sheep developed traits for self shedding of the unshorn fleece and for breeding at one year old. These changes are discussed in relation to the decline and subsequent increase of the population.