DNR study shows factors influencing deer numbers
By Houghton Daily Mining Gazette staff
HOUGHTON — In the Upper Peninsula, where wilderness and wildlife are a crucial part of daily life and commerce, the white-tailed deer is a revered species. Management decisions affecting the deer herd are closely followed and frequently debated by UP residents.
In June, the Michigan Department of Natural Resources published “Factors Limiting Deer Abundance in the Upper Peninsula,” a study that examines the impact of various factors – including weather conditions, habitat, and predation – on white-tailed deer abundance. This research may help to inform future wildlife management decisions.
The study uses buck harvest data as an indicator of trends in deer population. The DNR’s report explains that there are some limitations to using buck harvest data. For one, buck harvest has declined over the past 35 years. This decline is in part due to declining participation in deer hunting, which has fallen by almost 40% over the last 20 years.
Additionally, changes in deer hunting regulations and the weather conditions present during firearm season can impact buck harvest. Despite these shortcomings, buck harvest is still the best data source available on deer numbers in Michigan.
The study’s results demonstrate that winter weather conditions have a significant impact on deer abundance. During the winter, food sources are less abundant while cold temperatures and snow accumulation put increased energy demands on deer.
“Winter conditions, particularly consecutive severe winters, appear to have the greatest impact on buck harvest,” the report states.
A winter is considered “severe” when over a foot of snow accumulates and remains for more than 90 days. From the late 1980’s to the early 1990’s, severe winters were uncommon and deer were abundant with buck harvests reaching an all-time high in the UP.
But since this time period, severe winters have become increasingly frequent and have occurred on back-to-back years. Six of the past 11 winters have been severe, and buck harvests have declined.
In a phone conversation with the Gazette earlier this month, DNR Biologist John DePue elaborated on the significance of winter weather.
“Our winters have the biggest impact on the deer population in the western UP,” he said. “Deer are not designed for 100 inches of snow on the ground for a long period of time. It’s extremely cold and they can’t move around very much to get food resources, and they are burning a lot of energy.”
During winter, the prevalence of habitat types that provide shelter can also influence deer mortality. The report explains that hemlock and cedar trees, considered “critical shelter components,” are particularly important. A sharp decline in hemlock and a more measured decline in cedar has had a negative effect on deer numbers.
In addition to providing shelter, the habitat types that increase summer forage opportunities for deer can improve their overwinter survival rate. The study points to improvements in aspen management during the late 1980s and early 1990s as a factor contributing to plentiful food sources and increased deer abundance.
Researchers also examined the effect of predation on deer abundance. The study explains that fawn “recruitment,” or the number of fawns that survive until the fall, is the most significant factor driving deer population growth in most years.
“In the Upper Peninsula, coyotes kill more fawns than any other predator, followed by black bears, bobcats, and wolves,” the report states. “Other non-predatory types of mortality, including malnutrition, disease, abandonment, vehicle-collisions, etc. have a greater impact than predation from any specific predator in the Upper Peninsula”.
The report pays special attention to the effect of wolf predation on deer abundance, a subject that UP hunters and community members have expressed concerns over. Although wolves have little impact on fawn recruitment, they are the predator species with the greatest predation on adult does.
Wolves are responsible for 8.6% of adult doe mortality. However, adult deer survival rates are high and the majority of does killed by wolves are nutritionally stressed and unhealthy. Almost half of all does killed by wolves are malnourished and at high risk of starvation.
While buck harvest has varied widely over the last 12 years, the wolf population has been stable with a population estimate of 557-695 animals in the UP. The report explains that wolves are one species within a complicated predator-prey system, and they do not have an outsized impact on deer abundance.
“In the last 11 years there have been six severe winters that have impacted buck harvest. During this time the wolf population has remained stable, emphasizing that winter weather has a much greater impact on the deer numbers than wolves,” the report states.
The DNR reports that due to a mild winter, rapid spring green-up, and an ample fall food supply in 2020, the UP deer herd is in good shape with population trends on the upswing in 2021.
The full report and additional information on deer population dynamics and DNR research is available at www.michigan.gov/dnr.