Last week, the Wisconsin Department of Natural Resources (DNR) presented the primary results of the Southwest Wisconsin Chronic Wasting Disease (CWD), Deer and Predator Study to the Natural Resources Board (NRB).
The figures from the study indicate that CWD is substantially reducing the annual survival probability of both male and female white-tailed deer. Reduced female survival lowers the growth rate of the population, and when sufficiently suppressed, may result in population decline.
Specifically, results from the study indicate that when the CWD prevalence rates of females surpasses about 29%, deer populations are expected to begin declining.
Counties already edging up in percentages of positives of deer tested include:
• Richland County – 12%
• Sauk County – 10%
• Iowa County – 8%
• Columbia – 7%
• Lafayette – 7%
• Grant – 6%
• Green – 6%
• Dane – 5%
Crawford County is now at 4%, Vernon at 3%, and Monroe at 1%.
Results presented
Daniel Storm, WDNR Deer Research Scientist, presented the results of the study to the NRB. Storm said that key research contributors to the study included:
• Dr. Daniel Walsh, U.S. Geological Survey Montana Wildlife Research Unit
• Dr. Wendy Turner, U.S. Geological Survey Wisconsin Wildlife Research Unit, UW-Madison
• Dr. Marie Gilbertson, UW-Madison.
The purpose of the study was to determine how CWD impacts deer populations. This involved estimating deer survival rates, and how they were influenced by CWD. The disease has now been detected in wild deer in 48 of the state’s 72 counties.
Fieldwork for this project took place in northern Iowa, Dane, and Grant counties. This area was selected for study as it is the region where CWD was first detected in Wisconsin in 2002, and has maintained a high CWD prevalence in the years since.
As part of this study, over 1,200 animals (adult deer, fawns, coyotes and bobcats) were captured, 766 GPS collars were deployed on adult deer, and 323 radio tracking collars were placed on deer fawns. There were 400 cooperating landowners in Iowa County, and 1,000s of fawn capture volunteers. The study employed 26 seasonal DNR field staff, and five full-time staff annually.
“From 2017 to 2020, 799 deer were captured and collared with GPS Trackers,” Storm explained. “In addition, 323 newborn deer were collared.”
Scientists working on the study monitored a large sample of deer for size, and tested them both at capture for collaring, and at the time of death, for CWD. The fates of the animals in the study were monitored for how, when and where they die.
Based on the results, DNR scientists are able to model deer dynamics as a function of:
• annual survival rate of CWD-negative females
• annual survival rate of CWD-positive females
• proportion of CWD positive and CWD negative females (prevalence)
• recruitment (the process by which fawns survive to become part of the fall deer population. It involves the number of fawns born and the number that survive to around six months old).
Study results
Scientists analyzed data from the sample of collared adult white-tailed deer in order to estimate the differences in annual survival (the probability of surviving from one year to the next) between CWD-infected and uninfected deer.
CWD infection rates in the highest prevalence areas show 44% prevalence in adult females and 55% prevalence in adult males.
“The primary issue is the survival of adult female deer,” Storm told the NRB. “CWD in adult females chips away at population growth, but there is still growth potential in the deer herd – it crosses the threshold at 29% prevalence in adult female deer. When it reaches 30%, then we’ll see the populations decline.”
Storm said study results revealed survival rates of uninfected deer at 83% of females, and 69% of males. This contrasts with the survival rate of infected deer at 41% of females and 17% of males.
Study implications
The key takeaways from these results are:
• CWD substantially reduces deer survival rates and suppresses population growth
• where CWD prevalence is high, deer populations are likely declining
• CWD will eventually impact deer populations elsewhere if it continues to spread and increase in prevalence.
If CWD continues to spread and its prevalence continues to increase, populations will likely face further declines. The exact degree of these declines, however, will depend on local harvest and recruitment rates. It is important to note here that researchers do not expect CWD-affected deer herds to become extirpated (completely eliminated in a given area), as deer populations have a strong ability to increase reproduction when deer abundance is lower, due to less competition for food, space and other resources.
“Will there be more research for live testing?” NRB member Marcy West asked.
“Yes,” Storm responded. “There will be a further study to determine if life is ending early, and we will look at Richland County where prevalence is high. Now, 20 years after our first detection, we see that we aren’t losing all of the deer herd, but we are increasingly seeing CWD infection occurring at a younger age.”
Storm said this will require ongoing monitoring for the herd, and by the public in general.
“The importance of the study is seeing how our understanding of how the impacts of the disease are changing,” Storm explained. “It used to be that the disease was impacting the big bucks, and now we’re seeing the disease even in juveniles. You should expect fewer deer in high prevalence areas. Support for our studies is crucial, not the least to protect human health.”
“We need to see more messages to the public about managing the population of does,” West responded.
Looking forward
It should be noted that the results presented here are the primary findings of the Southwest Wisconsin CWD Deer And Predator Study. Although the scope of this study provides us with a rich dataset from which we can continue to learn about our deer herd and CWD’s impact, there are sure to be more results to come as analysis continues. Additional findings will continue to be released to the public as completed.
