March 22, 2019 at 4:57 p.m.
NRB hears update on deer/predator study
NRB hears update on deer/predator study
The study, part of former Governor Scott Walker's 2016 chronic wasting disease (CWD) initiative, takes a comprehensive look at deer populations and what effect predators, as well as CWD, have on populations in the endemic area of the state in the southwest.
Roberts, a DNR carnivore and furbearer research scientist, explained to the board that while there are many carnivores in Wisconsin, coyotes and bobcats are the most common in the southwest portion of the state. He did not discount bear and wolves as factors, but noted the only looks at the most common carnivores in the equation.
The study looks to see what influence these two predators have on deer in the endemic zone. The hope is to answer several questions including: What are carnivore densities in this area? What factors drive carnivore population (and what are survivorship rates)? And what habitat do carnivores use, meaning are some areas "riskier" than others for prey?
Roberts told the board part of the difficulty in completing the survey was that carnivores, by their nature, can be difficult to capture. While modern trapping equipment makes it easier, help from landowners was really key in attaining the sample size, he explained. Trapper and landowner buy-in in the 2017-18 season brought the number of trapped coyotes from seven in the previous season to 32 and bobcats from seven to 12. In the 2018-19 season, 30 coyotes and 13 bobcats were captured and collared.
Carnivores were captured throughout the study area, Roberts said, noting that this is important to ensure proper results.
Of those carnivores collared, researchers documented 25 coyote mortalities and seven bobcat. Twenty-one of the coyotes were taken by legal hunting or trapping and five of the coyotes were also harvested through those means. He reiterated it is completely legal to hunt or trap a collared animal via legal means.
Researchers found a 50 percent annual survivorship in coyotes and a 74 percent annual survivorship in bobcats over the first two years of the study thus far, Roberts added. This means there is a 50 percent chance a coyote alive today will be alive at this time next year, and a 74 percent chance of a bobcat being alive this same time next year. All of the mortalities seen in the study were human-caused. Those not taken by legal hunting and trapping were all road kill animals.
Roberts also talked about home ranges. For a bobcat, the average home range is about 30 square miles, he said. For a coyote, the range is a bit smaller, at 13 square miles. There is a lot of variability in home ranges, with some being bigger than others, and ranges, especially for bobcats, tend to be considerably bigger in males, he added.
Roberts also said there is a surprising amount of home range overlap. Bobcats, he said, do not exclude other bobcats, and bobcats and coyotes can occupy the same home range.
Next steps on the carnivore side of the project will be to estimate habitat use. What types of habitats are these animals using the most? The study also looks to estimate densities. From there, habitat use and abundance can be used to gauge risk to deer populations, Roberts explained.
More to learn
At that point, DNR deer and elk research scientist Dan Storm took over the presentation to discuss the finding on the deer side of the study. Last year, very early in the study, Storm presented very early estimates to the NRB, however it was already clear the CWD had an impact on deer populations. This year, he said, quantifying that data would be a bit clearer, as now there are two years of data in the study. There is much more to learn as researchers are only halfway into the project, he noted.
In 2017, researchers were able to capture and collar 138 deer and test 122 of them. There were 12 deer found to be CWD positive at capture. In 2018, 194 deer were captured. Of those, 185 were able to be tested and 15 came back positive for the disease.
According to Storm, all of the CWD-positive deer from 2017 were dead by 2018 from some cause.
One of those deer, however, was a collar that went dark mysteriously near the end of 2017, he explained.
Ten of the CWD-negative deer in 2017 tested positive upon death in 2018. From the deer captured, collared and tested in 2018, seven of those testing negative at capture died CWD-positive in the same year. The test used on live deer does have a false-negative possibility, Storm noted. It is not a high possibility, but it is a possibility, he explained when questioned by the board.
He rehashed the preliminary results from Year One, showing CWD positive deer have a much higher mortality rate than those that tested negative. He then showed the updated Year Two graph. It showed CWD-positive deer continue to die at a much higher rate than negative deer. While the magnitude of difference is smaller than in Year One, the confidence level was higher. Although the difference did not look as pronounced on this graph, Storm cautioned against putting too much emphasis on that, but rather look at it as illustrating the reasons for more confidence in the mortality rates among positive deer versus negative-testing deer.
Eventually, Storm explained, researchers will want to break this down into different categories such as bucks versus does and young versus old deer.
He showed some preliminary results broken down by gender. Those results showed bucks have a higher mortality rate than does. This was expected, but the study will look to refine what is already known, he explained.
The difference between bucks and does, with all bucks having a lower survival rate, Storm said, is much less than the difference between CWD-positive and CWD-negative deer.
The ability to quantify the numbers is key as researchers want to understand how does the deer population is changing as a result of the prevalence of CWD?
From the carnivore standpoint, Roberts told the board, the hypothesis is carnivores have the ability to impact the deer population and that effect may be heightened in areas where CWD is present.
He mentioned the board's ability to control and influence carnivore populations based on those effects, whether they are having a detrimental effect on prey or they are taking the sicker animals out of the herd in a beneficial way.
As more results become available in the remaining years of the study, the hope is more questions will be answered and a clearer picture of the intricate relationship between hunter, hunted and disease will come to into view.
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