Last year I posted quite a few blurbs on Craig Fiehler's and my carnivore survey at the California Department of Fish & Game's Chimineas Ranch.
The purpose of the survey is to determine the distribution and relative abundance of mesocarnivores on the Chimineas Unit.
To do a proper survey we needed more cameras than I own, so we started modestly and conducted a pilot study.
The ranch is probably home to 12 species of carnivores, but only 10 of them have been confirmed.
The cast of characters includes the mountain lion, bobcat, black bear, raccoon, coyote, gray fox, kit fox, badger, striped skunk and long-tailed weasel.
Spotted skunk and ringtail should also be there, but no one has seen them (dead or alive), or collected voucher specimens.
A lot of data piles up in 8 months of camera trapping, but before reporting the results you should know what we did.
So let's start with methodology, which will be of interest mainly to field workers.
Cameras: We used a maximum of 16 homemade camera traps and one Cuddaback Trail Camera. The components of these “homebrew” camera traps were Sony s600 6 MP digital cameras and controller circuits made by Pixcontroller, Snapshot Sniper, White-tail Supply (XLP), or YetiCam. The controllers activate and trigger the cameras with passive infra-red (PIR) sensors. Controllers require 2- 3 seconds to power the camera and take the first photo, and the trigger successive photos at minimum intervals of 3 seconds.
Cameras were wired to supplemental batteries that normally insured operation for at least a month, and all components were housed in Pelican 1060 or 1040 camouflaged cases.
PIR sensors are highly responsive to air-borne temperature differentials, and on warm days moving vegetation and breezes rapidly fill a camera’s memory with false (blank) exposures. Consequently we set the cameras for both 24 hr and nighttime operation depending on lighting conditions.
Attractants: Cameras were mounted about 2 feet above ground on metal posts driven into the soil. Scents and/or bait were placed within each camera’s view to attract wildlife. Canned mackerel was the bait most often used. The tin was punctured on both ends and secured with rocks, wire or reinforcing bar. Scents were dabbed on vegetation, twigs, or rocks, usually at an elevated position to increase scent dispersal. We also set two cameras at a cow carcass for 2 nights.
Camera locations: We used Geographic Information Systems (GIS) to develop a property-wide grid system of 100-ha sample units. The 100-ha sample unit size was chosen because it encompasses the minimum home range size of two target species: ringtail (Bassariscus astutus) and Western spotted skunk (Spilogale gracilis) (Zielinski and Kucera 1995). We set single remote camera traps in each of 4-12 100-ha plots during each sampling period. We assigned a number to each camera trap location and refer to the location and associated information as the camera trap set.
Schedule: We attempted to service and move cameras to new locations monthly, but occasionally weather conditions and other factors delayed the schedule. The mean duration of a camera set was 29 days (range: 1-68), and the mean operational duration was 21 days (range: 0.15-68).
Data analysis: GPS location, altitude, habitat and topographical features were recorded at each camera trap set (Appendix 4), together with other details such as bait and camera settings (24 hr or night time). Animal species, date, time, and individual jpeg number were tabulated for each set from photos downloaded to computer (Appendix 6).
Additional variables for each camera set included:
- Camera trap days: Camera trap day was a convenient measure of effort defined as 24 hrs of camera operation. Nighttime sets were calculated as half days, so that 30 days of camera operation yielded 15 camera trap days. When the camera’s batteries were dead upon collection, the duration was calculated from the day of the last photo. The percentage of all photos that were of animal subjects. Low success rates indicate that many images were false triggers due to weather conditions during daylight hours or rodent traffic at night.
- Success rate: The percentage of all photos that were of animal subjects. Low success rates indicate that many images were false triggers due to weather conditions during daylight hours or rodent traffic at night.
- Number of species: We were able to identify most vertebrates except for certain rodents of the genus Peromyscus and bats of the genus Myotis.
- Number of visits: Multiple photos of a species were usually clustered in time and considered a single visit. Photos of any species that were separated by more than one hour were considered separate visits. No effort was made to identify individuals. We did not calculate the number of visits by rodents because of the large number of photos obtained, uncertain identification and the range of interval durations.
That's enough for now.
In a few days I'll start posting the findings.