Habitat occupancy assessment to help monitor status of population
By Bob Berwyn
SUMMIT COUNTY — Colorado Parks and Wildlife biologists say they’re close to finalizing a plan to monitor the state’s lynx population by assessing habitat occupancy. If successful, the strategy would enable researchers to determine whether the population of endangered wild cats is sustaining itself over time.
The habitat occupancy model was tested in a pilot program in core lynx habitat in the San Juans a couple of years ago, showing that about 50 to 60 percent of the available lynx habitat is occupied. Now the biologists are trying to figure out if they can use the same method to keep tabs on lynx across the entire state.
Lynx have been listed as a threatened species since 2000, with a population in New Mexico currently under consideration for listing as a candidate species. Colorado launched a restoration program in 1999, transplanting more than 200 lynx from Canada and Alaska to the San Juans.
The reintroduction effort was declared a success about two years ago, after 10 years of intensive monitoring, with on-the ground visits to lynx dens, as well as tracking via airplanes and satellites. The tracking shows that the population has spread northward, with resident, breeding lynx up through the Collegiate Range and even into Summit County, with pockets of populations north of I-70.
In a draft report on the pilot study, biologists said it’s not feasible to accurately estimate population numbers, but assessing habitat use and occupancy can help determine whether the population is stable, growing or declining — and might also show trends in habitat use, for example in response to changing forest conditions. Colorado Parks and Wildlife lynx research is online here.
“It has some potential to monitor lynx on a large scale, across the state and through time,” said Dr. Jake Ivan, the Colorado Parks and Wildlife biologist who is leading the effort.
Ivan said the pilot study involved setting up a grid of habitat units scaled to match the approximate size of a typical lynx home range — anywhere from 12 to 83 square miles, according to the U.S. Fish and Wildlife Service.
During the winter, biologists visited a sample of those units multiple times, looking for lynx tracks. More remote units were monitored with a network of 120 automated cameras that yielded more than 400 pictures of lynx.
The repeat visits enabled the biologists to estimate a detection probability — a correction factor — that will be important for up-scaling the assessment effort on a statewide basis.
Ivan said the best way to detect habitat use is on-the-ground snow tracking.
“If you get on a snow machine and use Forest Service roads and trails, you stand a good chance of cutting across their tracks,” Ivan said.
In other areas, including roadless tracts and wilderness, the researchers placed cameras, along with attractants and lures to try and capture pictures of the elusive carnivores.
The monitoring is focused on winter months because that’s when lynx stick relatively close to their home ranges, with one or two drainages. During summer, individual animals often make long-range treks as they seek out mates or new foraging habitat.
To effectively monitor lynx across Colorado, Ivan has to calculate how many units would need to be sampled — and whether that effort is even feasible from a financial and logistical standpoint.
“It was a heck of an investment to get those animals on the landscape in the first place,” he said, adding that logistics are not out of the question, given some cooperation with other agencies, especially the U.S. Forest Service.
“I think we could get support for that,” he said, adding that the statewide assessment would be a pioneering program in terms of landscape-level monitoring of a restored carnivore population.