Time Interval Photography Monitoring of Cinder Butte Snake Hibernaculum (2013)
Investigators and Affiliations
- Charles R. Peterson, Ph.D., Department of Biological Sciences, Idaho State University, Pocatello, ID
- Jeremy P. Shive, Environmental Surveillance, Education, and Research Program, Gonzales-Stoller Surveillance, LLC., Idaho Falls, ID
- David Bush, Department of Biological Sciences, Idaho State University, Pocatello, ID
- Idaho State University Department of Biological Sciences
- Idaho National Environmental Research Park
The T-17 wildland fire burned approximately 17,807 ha (44,000 acres) in 2011, including the area around Cinder Butte (Figure 2). The basalt outcropping near Cinder Butte supports multiple snake hibernacula, including the primary North den, which has been monitored by the ISU Herpetology Laboratory for over 15 years. Anecdotal field observations following the T-17 fire found there was a lot of soil and sand movement in the areas devoid of vegetation. The wind-blown sand was beginning to fill in the interspaces of the basalt rock and there was some concern whether access to the den would be restricted, and the individuals returning for winter hibernation would be stranded with no alternative refuge.
The primary goal of monitoring the Cinder Butte snake hibernaculum is to document the continued use of the den site, and to identify which species of snakes are currently present following the T-17 wildland fire.
Accomplishments through 2013:
Photographic monitoring is a relatively inexpensive method to monitor snake den activity. We initially attempted to use the infrared thermal trigger on the camera during fall of 2012, however, snakes are ectothermic and body temperature is closely tied to environmental temperatures. Because snake body temperatures were not substantially different than background temperatures, the thermal trigger did not capture snake movements as the animals were essentially ‘invisible’ to the thermal sensor.
We modified the camera setting in the spring of 2013 to a fixed time interval of one minute from sunrise to sunset. We collected over 52,000 images during the spring from 4/14 – 6/18, and over 31,000 images in the fall from 9/12 – 10/22. All images were reviewed and each observation event was recorded. An observation event is defined as a single snake observed for one or more consecutive images. If an individual moved out of view or retreated back into the den it concluded the observation event, even if an individual was seen back at the same spot minutes later. Because we cannot be sure it was the same individual, we treated each instance as a new observation event.
The spring dataset was further analyzed at ISU and compared to previous results from radiotelemetry surveys, and to investigate how time and temperature affect snake surface activity at the den. To test for observer error during the image review process, two independent people reviewed all of the images, and the number of observation events was compared.
Time-interval photography was found to be an effective method for monitoring snake species at the Cinder Butte hibernaculum. All four species (Great Basin Rattlesnake [Crotalus oreganus lutosus], Gopher Snake [Pituophis catenifer], Striped Whipsnake [Masticophis taeniatus], and Western Terrestrial Garter Snake [Thamnophis elegans]) previously documented at the Cinder Butte hibernaculum by ISU Herpetology Laboratory were successfully detected and present during the spring of 2013.
Observer accuracy was high with an average detection rate of 91 percent. One observer missed four of the observation events and the other observer missed six. Snake surface activity ranged from April 26 to June 12 with a median activity date of May 15. Interestingly, the May 15 median date coincides with the previous median day of surface activity calculated from more intensive radiotelemetry, trapping, and hand capture surveys conducted by the ISU Herpetology Lab from 1990-1992 at Cinder Butte. Snake surface activity ranged from 10:08 to 20:59 with median activity occurring at 13:41. The camera temperature sensor showed a range of -9.4ºC to 39.4ºC with a median temperature of 25.9ºC for snake observation events.
Plans for Continuation:
Further analysis including the fall 2013 and spring 2014 datasets is planned. Once a few seasons of data have been collected, a baseline of observation events can be established. Future time interval photographic monitoring could then be conducted and compared to the established baseline to potentially detect changes in population status.
We would like to experiment with different camera settings and systems to optimize the accuracy of snake detections while minimizing overall effort. Solar-powered battery attachments are becoming more common and less expensive. Memory card capacity is also growing rapidly, and it is now feasible to store the entire spring/fall season on a single SD card. Once we are able to power the camera system for the entire season of imaging and store all images on a single memory card without having to make multiple visits, we would like to test the omission error rate of the current time-interval setup. We plan to alter the time interval and capture an image every second (rather than every minute) from sunrise to sunset and essentially record a continuous sampling of the hibernaculum. Then that dataset would be artificially subsampled at different time intervals (e.g. 30-second, 1-minute, 2-minute, etc.), and reviewed for observation events. This analysis will provide insight about how the time interval affects detection rates. If fewer images need to be collected, and detection rates do not vary considerably, we could minimize the required processing time. We have also considered testing a different camera system, such as a GoPro, which has higher image resolution and a larger field of view.
We plan to determine actual snake operative temps using physical snake models attached to a temperature datalogger. By comparing the internal camera system thermometer with the physical models, it will allow us to understand the relationship between camera measurements and the temperatures the snakes are more realistically experiencing at the den.
Publications, Theses, Reports:
An oral presentation summarizing the spring dataset was given as at the joint meeting of the Idaho Herpetological Society and Idaho Partners in Amphibian and Reptile Conservation at the Northwest Nazarene University in November 2013. The spring data analysis was also presented as a poster at the Idaho Chapter of the Wildlife Society annual meeting held in Boise, ID February 2014.