Powerhouse Restoration Site Small Mammal Trapping

Due to site damage caused by the construction of the BC Hydro Seton Dam and Cayoosh Creek Canal complex the Lillooet Naturalist Society together with other parties are working on a Powerhouse Foreshore Restoration project to create a more diverse and healthy habitat for wildlife. Even though the project is aimed at restoring the site for all wildlife the identification of the Western Screech Owl, by the Conservation Data Center of red-listed species, creates an opportunity to evaluate both the availability of prey for the owl as well as to identify various species of small mammals, fish, birds, insects and other invertebrates. Small mammals are important as prey for land and flight predators and distributors of fungi, seeds and spores as well as in many other groups.

The project’s objective was to assess the small mammal’s population and diversity within three areas of the Powerhouse Restoration Area, the restoration area where the Western Screech Owl was located and a referenced area of Ponderosa pine. This was done to provide a baseline for trapping sessions and for assessing the availability of prey for the owl as well as to attain an understanding of how the restoration site compares to other adjacent sites.

The project occurred in three different areas: The Restoration grid, where a gravel filled landscape was restored to one with soil and native plants; The Riparian grid is a dense area with shrubs and trees, located between Fraser River and an upland Ponderosa pine stand; and The third and last grid was the Reference grid, located within an area filled with Ponderosa pine and native shrubs, grasses and forbs. In the Restoration and Reference grids the trapping grids were 8×8. In the Riparian grid it was 16×4, but was later changed to 3×21. The traps used were Longworth-style live-traps. The traps were prebaited and left open for two weeks after that the mammals were live-trapped for three nights. Sex, weight and species were determined as well as if the animal was a juvenile or adult. After the animals were marked they were released again. The density estimates were calculated using the program CAPTURE. Then the estimates were transformed to animals/ha.

The deer mouse was the only species captured with a total of 91 marked animals in 2010 and 99 in 2011. The area with most captures was the Riparian site and the one with the fewest was the Reference site.

The cause for the cyclic population fluctuations with small mammals is still unknown. This fluctuations create a probability for other small mammals other than the deer mouse to occur in these areas. Surprising, the moderately healthy Ponderosa pine stand Reference grid showed the lowest number of trappings. This could not be explained. Samples taken across seasons might confirm the trends that were identified in the past trappings.

The Reference grid also showed an imbalance in sex ratio. Some areas in this grid are currently being restored and that could be the reason. The later trappings in 2011 showed an increase in both male and female. The high density in the Riparian grid was not surprising due to it’s diversity of this habitat. All the trapping grids appeared more productive in 2011. In the future more evaluations may help to track a change in population and in general gain more understanding of how the restoration affects the habitats for the small mammals.

Adding of coarse woody debris to the restoration area should be continued in the future as the increase of debris piles also shows an increase of small mammal populations. Further trappings should be conducted to identify a more diverse appearance of small mammal species and to confirm the trends shown in the past trappings. The Western Screech Owls should be monitored frequently by non-invasive surveys.

In conclusion, the trappings in both 2010 and 2011 were already a good start however, neither could confirm nor deny the trends shown in the assessments. Therefore, trappings should be continued. There was an increase in reproduction and juveniles in 2011, so restoration should be continued, especially with the use of coarse woody debris as it doesn’t just increase the population of the deer mouse but also other small mammal species.

  • Source Dustin Oaten.Powerhouse Restoration Site Small Mammal Trapping – Fall 2010 and Summer 2011. For Lillooet Naturalists Society. Available at : http://www.env.gov.bc.ca/wildlife/wsi/reports/4998_WSI_4998_RPT_MAMMAL.PDF [Accessed 03/2021].