Deer Management: Population Assessment
Impacts Indicators of Deer Populations
While deer population estimates and goals are helpful when considering deer management strategies and when providing information to the public, reliable estimates of free-ranging wildlife require a large amount of data, are not always accurate, and do not necessarily provide information that is critical to management. It is generally more beneficial to know whether there are more, the same, or less deer than before, and what impacts those deer have on themselves, their environment, and on people, than it is to know precisely the number of deer.
Current Population Status and Range in Michigan
Regional deer densities in Michigan have changed a great deal since the 1970s. Historically, deer hunting opportunities in the UP and NLP attracted hunters from southern Michigan to hunt the relatively abundant deer populations of the north woods. Statewide deer population estimates indicate that the Michigan deer population grew steadily through the ‘70s, ‘80s, and early ‘90s, but has shown a gradual long-term declining trend since 1995. Population trends are not consistent across the State, as this statewide decline has been driven by declines in both the UP and NLP even as the SLP population continued to grow.
Today, deer densities in Michigan generally increase from north to south. Deer populations are low along the northern edge of the UP as lake-effect snow associated with Lake Superior makes winter conditions tough on deer. Deer in this area are forced to seek out lowland conifer swamps or migrate south in early winter to areas that typically receive less snow. Snow depth data show a link to current estimated population densities across the entire State as deep snow and cold winter temperatures frequently result in significant winter mortality and low fawn recruitment. In the southern one-third of the state, where winter conditions are less severe and agricultural crops are more common, deer densities are mostly above goal.
In the NLP, current deer populations are at or near DNR goals in many of the current deer management units (DMUs). However, when harsh winter conditions occur, herd size can be noticeably reduced by high winter mortality rates and low fawn recruitment. Even with efforts to reduce the deer density in the bovine tuberculosis (TB) area of the northeast part of the NLP, which were initially successful, deer populations have increased and are currently over the DNR population goal for this area.
In the SLP, deer populations are over DNR population goals in nearly every DMU. The abundance of food in the form of available agricultural crops combined with the more than adequate cover of scattered woodlots and idle fields provide near perfect white-tailed deer habitat. In addition, relatively mild winter conditions, the near elimination of natural predators, and limited hunting access on private land (including numerous parcels where no deer hunting occurs at all) contribute to the growth of these populations.
Carrying Capacity is a term that refers to the maximum sustainable size of a population. Carrying Capacity of a population is limited by any number of constraints, both biological (Biological Carrying Capacity) and social (Social Carrying Capacity). The effective and appropriate management of deer populations must consider both biological and social carrying capacities.
Biological Carrying Capacity (BCC)
The Biological Carrying Capcity is based on the point where the habitat can support a species sustainably. BCC is determined by the capability of the area to provide the habitat components of a wildlife species – food, water, cover, and space. As deer populations grow, individual animals compete for the resources their habitat provides, with less of the four requisites being available per deer.
In Michigan, healthy, well-fed does are capable of producing triplet fawns and routinely produce twins. Under ideal conditions, even fawns are able to breed and produce their first young when they are about one-year-old. However, as populations near BCC, adult does raise fewer fawns, fawn survival decreases and fewer fawns are capable of breeding. Another impact when a deer population approaches BCC is antler development in yearling bucks may be retarded. In addition, more deer die from malnutrition. When BCC is reached, the number of deaths equals the number of births.
Social Carrying Capacity (SCC)
The Social Carrying Capacity of a species is determined by the degree humans tolerate or accept a species within their span of possible interactions. When creating management strategies, it is essential to take into account the views of stakeholders in order to create a strategy that is most acceptable. By seeking the social attitudes, a management plan can better manage within the socially accepted constraints, balancing between the desires to see more or less of the species.
There are three main targets of the SCC model: (1) deer abundance; (2) deer–human interactions; and, (3) human attitudes and tolerances regarding deer. The model measures success by whether issues associated with the presence of deer are manageable, or whether they are disrupting attempts at management.