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Collared white-tailed deer walking through a forest in winter. WisDNR photo

DNR announces primary results of Southwest Wisconsin CWD deer and predator study

MADISON, Wis. – The Wisconsin Department of Natural Resources (DNR) today presented the primary results of the Southwest Wisconsin Chronic Wasting Disease (CWD), Deer and Predator Study to the Natural Resources Board (NRB). A recording of the NRB meeting, including this presentation, is available on the DNR’s YouTube channel.

The purpose of this study was to determine how CWD impacts deer populations.  This involved estimating deer survival rates and how they were influenced by CWD.

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.

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. Annual survival estimates are listed in the table below.    

Annual Survival Probability  

 UninfectedCWD-Infected
Females83%41%
Males69%17%

These figures 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 this study indicate that when the CWD prevalence rates of females surpasses about 29%, deer populations are expected to begin declining. 

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. 

The public can learn more about these results by visiting the study’s results webpage.

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.

To stay informed about these announcements and results from additional analyses on topics like movement, habitat use and predator survival rates, subscribe to the Field Notes Newsletter.

STUDY RESULTS

How does CWD affect Wisconsin’s deer population?

Fieldwork during phase 1 of the Southwest Wisconsin CWD, Deer and Predator Study saw data collection from over 1,200 captured animals (adult deer, fawns, coyotes and bobcats) in northern Iowa, Dane and Grant counties. This region is where chronic wasting disease (CWD) was first detected in Wisconsin in 2002 and over the years, has seen an increase in CWD prevalence over the years. Phase 2 then began to analyze that data to fuel the study’s research objectives. These objectives included:  

  1. Estimate survival rates as a function of time, CWD infection status, age and sex. Document the breakdown of causes of mortality (i.e., starvation, disease, predation and hunting) and how these differ by CWD infection status and sex.
  2. Integrating survival estimates of CWD-infected and uninfected deer and recruitment (pregnancy rates and fawn survival) into an age-structured population model to determine deer population response across a gradient of CWD prevalence.  

Unprecedented Analysis  

The sheer size of the Southwest Wisconsin CWD, Deer and Predator Study has been unprecedented in size. It involved collaborations between the Wisconsin DNR’s Office of Applied Science, the USGS National Wildlife Health Center, the Wisconsin Wildlife Cooperative Research Unit, the UW-Madison Department of Forest & Wildlife Ecology and the Montana Wildlife Cooperative Research Unit. The study also could not have occurred without the hundreds of private landowners throughout southwest Wisconsin who graciously allowed their properties to be used for animal capture.

The sample size of this project is notable as well. While there have been previous research projects to estimate deer survival, relatively few have ever focused specifically on how CWD might affect it. Those that have were focused in the Western U.S. and had sample sizes ranging from 136 to 217 collared cervids. To put that into perspective, the Southwest Wisconsin CWD, Deer and Predator Study deployed 1,089 collars on white-tailed deer (766 adult and 323 fawns), not to mention additional coyote and bobcat capture.

In a dense green setting, two individuals wearing gloves are measuring a collared fawn. The fawn has a tracking collar and is surrounded by foliage. One person holds a measuring tape, while the other carefully positions the fawn.
Photo Credit: Wisconsin DNR

The model used to estimate survival is novel in that it can account for an animal’s changing disease status, such as deer that were CWD-negative at capture but later acquired the disease. It must also consider how survival differed over time and among sex and age groups. Before this study, survival models had never before incorporated all of these elements of disease, and to do so here required tremendous patience and innovation. As a direct result of this patience and innovation, that we present the results of the Southwest Wisconsin CWD, Deer and Predator Study below.

Primary Results 

Objective 1 Results 

Objective 1 saw scientists analyze various causes of death within their sample of collared white-tailed deer. The differences in survival between CWD-infected and uninfected deer over one year are listed in the table below.  

Annual Survival Probability

 UninfectedCWD-Infected
Females83%41%
Males69%17%

These figures indicate that CWD substantially reduces the annual survival probability of both male and female white-tailed deer. Note how the above survival probabilities are averaged across the years of the study and ages (1 year of age and older). Once deer reached one year of age, annual survival did not differ much. However, there were some differences in survival between years. In particular, survival rates during 2018 and 2020 were somewhat lower than survival rates during 2017, 2019 and 2021. This lines up with the deer harvest patterns we observed during those years, which were higher in 2018 and 2020.

While the table above highlights key takeaways from this objective, previously completed additional analyses that were previously completed, examine fawn and adult deer survival further.

  • Fawn survival: Fawn survival in the study area was high enough to help sustain the population. Coyote predation was the highest source of mortality, followed by disease (pneumonia or enterocolitis), hunter harvest and human-related causes (car collisions, haying equipment, domestic dogs).
  • Adult deer survival: Uninfected deer have high survival rates. CWD substantially reduces that survival rate and is closely associated with severe starvation and loss of bone marrow fat. Additionally, 51% of necropsied deer in the study had pneumonia, with bronchopneumonia more commonly found in CWD-positive deer. 

Objective 2 Results

The survival estimates referenced above were combined with data on deer recruitment into a population model, which allowed researchers to determine how deer population growth changes across a range of CWD prevalence levels. This includes an estimate of the CWD prevalence threshold, which indicates the percentage at which CWD infection within a population will begin to cause population declines.  

The graph below depicts the relationship between CWD prevalence and deer population growth rate. The y-axis shows the population growth rate. Specifically, this tells us how much a population increases or decreases over a given period; a growth rate greater than 1 means the population is growing, whereas a value less than 1 indicates a decline. The x-axis here is denoted as prevalence within adult deer. This refers to the percentage of adult deer in the population infected with CWD (i.e., 0.0 = none infected and 0.6 = 60% infected). Within the graph itself, the dashed line provides a reference for population equilibrium, so when the straight line is above the dashed line, the population is growing, and when it is below that dashed line, the population is declining. Results from this study indicate that when the CWD prevalence among females surpasses about 29% (where the two lines intersect), deer populations are expected to decline.    

The line graph illustrates the relationship between Chronic Wasting Disease (CWD) prevalence in adult deer and population growth rates. As CWD prevalence increases, the population growth rate declines, with a notable decline expected when prevalence surpasses approximately 29%. A dashed horizontal line at a growth rate of 1.0 indicates population stability.

What does this mean for Wisconsin Deer

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.  
  • If CWD continues to spread, it will eventually impact deer populations elsewhere.

While our research shows the general relationship between CWD prevalence and deer population growth, varying local conditions, such as harvest and recruitment rates, will play a role in the deer population trend that is ultimately observed. It is important to note that researchers do not expect CWD-affected deer herds to become extirpated (locally extinct). Deer populations have a strong ability to increase reproduction when deer abundance is lower, due to less competition for food and space.

What’s Next

The results presented here are the primary findings of the Southwest Wisconsin CWD, Deer and Predator Study. 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. At the same time, this was a novel study, and there will still be more results, including cause-specific mortality rates, to come moving forward. As this analysis continues, information will continue to be released to the public. 

These results will inform population management in currently infected areas and the agency’s future CWD management decisions. For areas currently seeing low or undetected CWD, this research may provide a glimpse of the future should CWD continue to spread as it has over the past 20+ years. The DNR’s current approach to managing CWD in Wisconsin can be found via the CWD Response Plan.

Furthermore, the following additional analyses are still underway:  

  • Bobcat and Coyote Movement and Habitat Use  – This analysis will inform scientists which areas each species favors or avoids. It will provide a better understanding of how predators fit into the bigger picture of deer survival.  
  • Seasonal Deer Movement and Habitat Use  – This will tell researchers how deer respond to seasonal changes in the environment and where there are likely to be higher rates of CWD transmission. 
  • The Impact of CWD on Deer Movement  – This will shed light on how deer movement changes once they are infected with CWD. The analysis will also look for sex, age and seasonal patterns. 
  • Deer Contact Rates – This analysis will look for contact networks among deer and how CWD is likely being spread between them.

The results of these additional analyses will also continue to be reported upon in the Field Notes newsletter as they are completed. 

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