Nuttall’s cottontail (nuttallii subspecies) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

Most of the occurrence records for this species are either historical museum specimens or incidental observations from naturalists and biologists. Only two surveys have been done on this subspecies in Canada. Sullivan et al. (1989) sampled animals through spring and summer in 1984, 1985, and 1986 on a 25.6 ha live-trap grid at the Federal Agriculture Research Station at Summerland. According to Sullivan (1985), 19.7 ha of the grid was natural steppe habitat; 5.9 ha were cultivated orchards, old field or vineyards. Using recapture data, they estimated population density and survival rates. Because few animals were captured in the cultivated habitats, the density estimates apply to natural undisturbed habitat. No estimates exist for Canadian populations in cultivated habitats or in natural steppe habitats disturbed from intense livestock grazing.

In 1990, Carter et al. (1993) surveyed most of the known range in the southern Okanagan and Similkameen valleys using spotlight surveys along transects. Surveys were done on foot and from a vehicle. Not all habitat types were sampled and sampling effort was unequal for the habitats sampled. Because their report simply lists locations where S. n. nuttallii was observed and does not identify sites where the species was not detected, it provides no data on sampling effort, relative abundance, or presence-absence.

Abundance

Sullivan et al. (1989) estimated the population in their Summerland study grid at 0.23 to 0.43 animals per ha. Their data are generally consistent with estimates for a population in Oregon, although that population reached a peak of 2.5 animals per ha in a year of high abundance (McKay and Verts 1978). The total number of individuals and the number of mature animals in Canada is unknown.

Fluctuations and trends

There is no evidence that S. nuttallii has regular cyclic fluctuations in population density. However, McKay and Verts (1978) demonstrated marked population fluctuations over a three-year period in Oregon. For example, late winter-spring densities differed by 50% in two consecutive years. McKay and Verts (1978) concluded that reproduction and survival of the young were linked to precipitation during the breeding season. Population densities in autumn and early winter were negatively associated with low temperatures. Sullivan et al. (1989), who monitored populations from May 1984-August 1986 in British Columbia, found that the average densities in 1985 and 1986 had declined to about one-third of the densities recorded in 1984. They attributed this decline to variation in summer precipitation and winter temperatures. No data are available on population trends ofS. n. nuttallii in British Columbia.

Rescue effect

S. n. nuttallii inhabits Okanogan County in Washington adjacent to the Canadian border. Washington has designated this species as S5 (secure) and it is hunted as a small game animal. The state’s 2002 game harvest summary lists 168 cottontails shot in Okanogan County. Nuttall’s Cottontailis the only species of cottontail rabbit found in Okanogan County (Johnson and Cassidy 1997). The general habitat model in Johnson and Cassidy (1997) shows potential habitat for this species widespread in the Okanagan and Similkameen valleys adjacent to the Canadian border. There are no physiographic barriers that would impede dispersal from Washington and suitable habitat for immigrants would be available particularly in the South Okanagan Grasslands Provincial Park adjacent to the international border.

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