3 Reasons Why a Large Deer Herd's Bubble Burst

3 Reasons Why a Large Deer Herd’s Bubble Burst

We lean on longtime DDH contributor Keith McCaffery, a retired deer research biologist, to explain the intricacies of whitetail overabundance on northern deer range. His report follows:

During fall of 2014, I began an attempt to put my own thoughts in order regarding the 20-year run-up (irruption) and decline of the deer population in northern Wisconsin from 1988 to 2007. There was public concern and loud debate about the causes of the population decline in the decade after 2000. However, I was especially interested in how it was possible for deer densities to reach the high levels that we saw. My outline grew into the accompanying article.

Three factors seem paramount in explaining the bubble in deer abundance: mild winters, supplemental feed and public distrust of deer population estimates.

The population correction after 2000 was imparted by liberal harvest management, likely degraded habitat, and unexpected help from one severe and two moderate winters. Buck harvest trends alone tell an incontrovertible story and avoid arguments about population estimates. Bottom line: starting in 2008, northern deer populations were back in the range of normal based on habitat constraints and traditionally established population goals.

There was a temporary dip below traditional goals following the late 2013 spring and very severe 2013-14 (Polar Vortex) winter. Following three consecutive mild winters (2015-17), herds recovered and began crossing a silvicultural threshold above which regenerating favored timber species may again become problematical as occurred during the bubble.

Early History

Deer population changes in northern Wisconsin during the past 100-plus years can usually be explained by harvest intensity, winter weather or habitat. The overshooting by market hunters prior to 1897 and continued subsistence hunting (poaching) by settlers caused deer numbers to plunge to very low levels by about 1910. With the imposition of bag limits, refuges and restrictive hunting seasons, deer numbers increased.

A deer herd is roaming grassy a field.
Photo courtesy of Getty Images

Exploitative logging and wildfires in the late 1800s and early 1900s prepared a habitat that was very favorable for deer. Legal protections and the improved habitat allowed deer populations in the north to reach record densities by the early 1940s. Chronic starvation and deer damage to forest reproduction became a concern. A significant transition had to be made from rebuilding the deer herd (politically popular) to herd reduction and control (unpopular). 

The transition to herd control was very controversial and spawned the Save the Deer Club in northern Wisconsin.

Aldo Leopold was among those who led the way to responsible deer management. He was threatened and criticized by the public and media. Talk of herd reduction provided the spark that started the infamous Deer Wars in the late 1930s. Supposedly, the wars continued until about 1951, but one wonders if they ever ended as herd reduction, whether by plan (harvest) or nature (severe winters), continued to cause conflict between hunters and the managing agency (Wisconsin Conservation Department (WCD)/Department of Natural Resources (DNR)) to the present time.

Midcentury

During the late 1950s, the northern forest was recovering from the extensive logging and slash fires. The super abundance of deer in the ’40s had temporarily exceeded carrying capacity, causing habitat damage. But now habitat quality for deer was also naturally declining. Trees began to mature, shade returned to the understory, and the abundance of aspen, grassy forest openings and preferred foods was declining. 

Modern deer management in Wisconsin began in about 1962. In-person checking of harvested deer had already begun in 1953, but by 1962, deer management units (DMUs) and unit-specific deer density goals were established, and a population monitoring system was in place. Limited use of the new Variable Quota System began in 1963. Antlerless harvest quotas could now be prescribed by unit with predictable results.

But, there were two severe and three very severe winters* in the eight years from 1965 through 1972. There was not a fawn crop in that period that was not affected by the winter prior to or after its birth, or both. Deer populations declined by half in the north. Quickly, accusations were voiced blaming the new Variable Quota System for shooting mother deer. Coyotes were also blamed by many citizens when the cause was clearly harsh winters. This was a difficult start for modern deer harvest management in Wisconsin.

Deer Herd Build-Up and Decline

November weather and the opening date of the firearm deer season can significantly affect antlered-buck harvest success. But, the buck harvest does provide an index to deer population size because Wisconsin has similar hunting seasons and similar hunting efforts each year. Without using population estimates, the figure below is a rough depiction of northern herd status beginning 1984 until 2014 when in-person registration of harvested deer ended.

Following the modern low in northern deer numbers in the early ’70s (only 12,000 bucks harvested in 1972), deer populations had recovered to the prescribed goal by the mid-1980s. This had followed some ups and downs depending on winter severity. The Winter Severity Index (WSI=102*) for 1985 suggested major losses of deer. However, deep snow completely disappeared in one weekend in early spring, apparently just in time to prevent major deer losses. In retrospect, herds were under-harvested in fall of 1986. This was followed by the mildest winter (WSI=14) in 114 years, based on records of Northern States Power Company and another conservative harvest. The result was a rapidly increasing northern deer herd until a record 98,000 antlerless harvest (2.0x buck kill) in 1991 and a poor fawn crop in 1992 turned the herd downward.

Controversies arose over the deer population estimates and sparked reviews of the deer management program by the Legislative Audit Bureau and Natural Resources Board. Plus, early blizzards before and during the gun season in 1991 left the DNR without reliable population estimates and led to lower than predicted harvests in the middle-north in 1992. The 1992 harvest weakened DNR arguments. A following period of public upheaval resulted in very conservative antlerless harvests for a few years and herd growth again as eight of nine winters from 1987 to 1995 were mild (WSIs from 14 to 50). 

This mild sequence of winters had clearly offset any ongoing decline in deer habitat quality, reminding folks that two factors drive deer carrying capacity: habitat quality (food) and weather. Deer populations in 1995 achieved their highest level since 1943.

A deer herd is roaming a wintery landscape.
Photo courtesy of Mark Kayser

Back-to-back severe winters in 1996 and 1997 reduced the herd, but it remained well above the established overwinter goal, perhaps due in part to widespread recreational feeding. Hunters challenged the continued high population estimates and harvest quotas in the north and especially in east central Wisconsin, prompting yet another major citizen review of the deer program by Deer 2000 study groups. 

Baiting and feeding of deer had become increasingly popular starting in the early 1990s. Just how much this artificial energy contributed to the run up of deer numbers and to buffering the effect of the few severe winters is unclear. But, it is safe to assume that the thousands of tons of bait provided by over 100,000 hunters during both archery and gun seasons, plus unknown numbers of rural residents recreationally feeding deer, had changed the productivity, survival, distribution and behavior of deer. It became like an acorn crop every year everywhere. Baiting also changed behavior of deer and hunters (fewer deer drives) and lowered deer-sighting rates during November hunts.

In 2000, there was a record statewide harvest of 618,000 deer of which nearly 200,000 were harvested in the northern forest, including a record northern buck kill of 68,000. The following winter was severe (WSI=83), and the antlered buck harvest in the north fell to 59,000. From 2002 through 2007, the northern deer population remained above goal with an average annual buck harvest of about 47,000 despite generous antlerless harvests (average 1.4x buck kill). Baiting and feeding of deer was suspended statewide in 2003, a result of chronic wasting disease (CWD) confirmed in 2002, and feed quantities were limited to 2 gallons per site thereafter. It wasn’t until another moderate winter (WSI=70) in 2007-08 that the harvest fell to 34,000 near the expected buck harvest when the herd is at the traditionally prescribed goal.

Meanwhile, the dramatic reduction in northern deer numbers fomented much unrest among some hunters. Hunters dislike herd reduction whether imposed by plan or nature, and this reduction was mostly by plan. Unrest spread statewide, exacerbated by the continued herd reduction efforts elsewhere in much of the central farmland and in the newly defined southern CWD management zones. 

To address hunter concern, the newly elected governor in 2011 appointed a Deer Trustee from Texas to review the deer management program. Unlike previous reviews, this audit generated 82 recommendations for change. All were implemented by 2014 and, in effect, gave hunters major responsibility for deer harvest management.

From 2008 to 2012, the northern population was near the traditional goal. But, a late spring in 2013 and a record severe 2013-14 winter (WSI=149) reduced deer numbers to below goal when the 2014 northern buck kill fell to 28,000. Meanwhile, and like the early 1970s, it remained popularly believed by many hunters that coyotes, wolves and bears (CWB in the north!) were contributing to deer losses during the decade.

Research projects were designed to address the predator concern. Results dispelled allegations but changed few perceptions. Overlooked in all of this was the question of how overbrowsing may have also contributed to the herd decline and whether lasting habitat damage may affect future herd performance.

Reality Check

If we were talking about the stock market, we might call the downward change in population after 2000 a needed correction. White pine, red oak and other forest plants benefited and so did the deer. But, the 50% decline in buck harvest in fewer than 10 years was not well accepted by the hunting public. Hunters had fond memories of hunting during the bubble in deer numbers. The period, especially from 1995 to 2007, seems to have become a new hunter expectation.

How realistic is this? Historically, winters have been a major controlling force driving deer populations in the north with severe winters occurring about every three to four years. But, in the 21 years preceding 2008, there were an unprecedented 16 mild winters. 

The bubble developed, and surveys by forestry staff in 2005 and 2006 documented serious problems getting timber regeneration due to overbrowsing by deer. This finding prompted letters of concern from the Wisconsin Council on Forestry, County Forest Administrators Association and a chapter of the Society of American Foresters. Decades of experience (1960 to 1990) have shown that silviculture can be successfully practiced in the north with overwinter densities averaging up to 18-20 deer/2 miles of habitat (a northern buck kill of about 30,000-35,000 or fewer), but higher deer densities posed problems of overbrowsing.

Maximum deer carrying capacity (KCC) in the north was estimated during the 1980s and 1990s to average about 26-30 deer/2 miles of habitat. No one should want to maintain herds at or above average KCC as this would not allow for much if any hunting harvest, deer would tend to be in poor condition, severe habitat damage would take place, and KCC would decline. Habitat quality has also naturally declined in the decades since these estimates of KCC were made, a result of forest maturation and succession toward longer-lived, shade-tolerant trees.

Density goals had been set by DMU at about 65-70% of KCC, which is higher than optimum for maximum sustained yield. But public input suggested that seeing deer is more important to most hunters than shooting more and bigger deer. Plus, it is counterintuitive to believe that a smaller northern herd can produce a higher sustained harvest. So, compromise led to higher than optimum goals.

Future Resource Management

Climate change, baiting and feeding may be increasing carrying capacity by offsetting the effect of natural decline in habitat quality. But, the impact of high deer numbers on the forest system remains a concern. Deer population response to favorable conditions can occur within a year (1,000 deer in winter may become more than 1,400 by the next fall) whereas forest adaptation to climate change may take many decades. Maintaining responsible densities of deer during these changing circumstances will challenge scientists and elected politicians and may frustrate hunters.

The Deer Trustee Report (2012:82) acknowledged that hunters tend to have unrealistic expectations that confound deer management. Many hunters seem to always want more deer, irrespective of herd status. Deer hunting represents a billion-dollar industry. However, the forests are not only the foundation for the deer herd but also for a $24 billion wood industry that has very significant economic importance to Wisconsin.

Photo courtesy of Daniel E. Schmidt

Heretofore, state resource managers have done their best to balance all the interests in the deer herd (hunters, farmers, forestry, motorists, etc.) while keeping deer populations within ecological sideboards.

Public trust resources (air, water, wildlife, public land) should be managed based on the best science. The deer science (e.g., ecological sustainability of woodlands and the counterintuitive elements of density dependent herd responses) is not always well understood by the public despite educational efforts. Blending of all the public interests with ecological imperatives are demanding challenges. Gaining and keeping public acceptance of and trust in science-based decisions has compounded the challenge.

Conclusions 

There was an overrun of deer in the north from about 1988 to 2007. The runup in numbers was fed by an unusual combination of influences. Eleven of 14 winters leading up to the peak deer population in 2000 were mild.

Deer baiting and feeding also became increasingly popular during the 1990s, adding thousands of tons of supplemental energy to the system. And finally, controversies over deer population estimates and herd reduction efforts throughout much of the state created a sociopolitical environment that impaired timely and proper harvest responses. This combination of factors allowed northern deer numbers to overshoot average carrying capacity for deer. 

A “correctionxe2x80x9d followed in 2000 with generous antlerless harvests, a severe winter and two moderate winters. Starting in 2008, northern deer populations overall were back in the range of normal based on habitat constraints and traditional population goals. The late spring in 2013 and the very severe (Polar Vortex) winter of 2013-14 caused a temporary drop to below the traditional population goal.

The conservative northern harvests starting in 2014 and the succession of mild winters through 2017 caused deer populations in most northern counties to exceed a silvicultural threshold where forest regeneration may again become problematical.

With continued mild winters, the factors that seemed to drive the runup of deer numbers prior to 2000 are mostly still in place. But, only the unfolding future will show whether narrow self-interests, popular opinion, or science will guide deer herds and deer harvest management from now into the mid-2000s.

Meanwhile, CWD and farmland deer abundance continue to add to the challenges of managing the deer population in Wisconsin.

*Winter Severity Index (WSI) is a scoring of one point per day whenever the temperature reaches 0F or below or snow-depth reaches 18 or more inches from December 1 to April 30.

*Days when both conditions occur count as 2. An ending score of 50 or less is Mild, 50-80 is Moderate, 80-100 is Severe, and 100+ is Very Severe.

Keith McCaffery is a retired deer research biologist from northern Wisconsin.

Literature Cited

Deer Trustee Report. 2012. A 21st Century model for deer management in Wisconsin: Final report and recommendations by Wisconsin Deer Trustee and review committee (J.C. Kroll, D.C. Gwynn, Jr., and G.L. Alt.). Wis. Dep. Administration. Madison. 136pp.

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