Small mammal surveys in the Little Campbell River Watershed, British Columbia

In the autumn of 2004 A Rocha Canada began small terrestrial mammal monitoring at their study site in South Surrey. The site contains two main habitat types of grassland and mixed woodland that are bordered by a riparian zone containing the Little Campbell River. The aims of the study were to gather baseline data on small mammal populations within the area and gain insight into their long-term trends. Additionally the data will feed into A Rocha’s wider assessment of habitats within the Little Campbell River Watershed.

Townsend's Vole, captured at A Rocha Canada's British Columbia study site

The survey was set up using the ‘pitfall’ live trapping system. Five arrays of traps were set up on the site, two within the grassland of pasture and wet meadow and three amongst the mixed woodland. Each array consists of four 25 cm diameter and 30 cm deep buckets spaced 5 m apart, dug into the soil and inter-connected by polythene fences (30 cm high and embedded 2 cm into soil). Monitoring sessions were 48 hours long with each array being checked every three hours in a consistent pattern. In 2004, pilot trapping was done in October in preparation for 2005, when monthly sessions were carried out from June until August. In 2006, monthly sessions were undertaken between June and September.

Since 2004 seven different species have been recorded, from four families, these being:

Trowbridge’s Shrew Sorex trowbridgii
Common Shrew Sorex cinereus
Vagrant Shrew Sorex vagrans
Townsend’s Vole Microtus townsendii
Creeping Vole Microtus oregoni
Shrew-mole Neurotrichus gibbsii
Deer Mouse Peromyscus maniculatus

The most often caught species was the Trowbridge’s Shrew, which was recorded 43 times in 2005 and 11 times in 2006. The large variation in numbers between 2005 and 2006 could represent a fluctuation in the population or the avoidance of established trap arrays by the species. The Trowbridge's Shrew was found in all arrays. It is known to prefer woodland but these results show that it moves into adjacent grassland areas.

Trowbridge's Shrew, captured at A Rocha Canada's British Columbia field study site

Common Shrews were trapped in 2004 and 2006, but not in 2005. This species is known to have pronounced population fluctuations year to year, related to abundance of prey, which may explain these variations. Vagrant Shrew was recorded three times within July and August 2005. Two species of vole were recorded within the survey: Townsend’s Vole was trapped twice in 2005 and three times in 2006, whilst Creeping Vole was caught three times in 2006. Also present within the 2005 and 2006 surveys were Shrew-mole and Deer Mouse.

The 2005 survey recorded 56 trappings; this number fell to 31 in 2006 although the variety of species increased during this time from 5 to 6.

From 2007 A Rocha Canada will be expanding the survey to include an area further upstream on the Little Campbell River, which comprises broadleaf deciduous woodland, meadow and riparian habitat and which will provide useful comparative data to the present survey. The proximity of the site to the Little Campbell may also provide opportunities for recording the provincially red-listed, Pacific Water Shrew (Sorex bendirii).

***

Project Leader: Glen Carlson. For more information please contact canada@arocha.org