Deer and the Numbers Explosion
In the most recent article about Ann Arbor’s deer problem, the Ann Arbor News presented a number of photographs from an anonymous contributor of deer near her residence, apparently in a public natural area. As Tanya Hilgendorf of the Humane Society of Huron Valley (who passed along the photos) said of the person who took the photos, “She has watched many families of deer grow over the years, has come to know their individual personalities and witness their bonds with each other.” The photos show quite a number of fawns, two in some pictures.
Now, the little fawn with his spotted coat is probably one of the most beloved images in our culture. Is it because we all saw the movie Bambi when we were between the ages of 6 and 10? Perhaps it is the same sentimental rush that comes with seeing the juveniles of any mammalian species, whether it is kittens, puppies, or fawns. But actually, if you care about the forests of the North American continent, or about our own natural areas and the other wild things that live there, or even about your own garden, this image should make you run to hide under the bed in terror.
It’s all in the numbers
It is possible to find deer management plans (intended to limit deer populations) described from most states in North America. The reason is that throughout the 20th century, deer (of several different species) have increased hugely in numbers. This is because of their tremendous reproductive capacity.
Deer have evolved to be a prey species. The dynamics of the predator-prey relationship are well understood. As this treatise states, “Species compete, evolve and disperse simply for the purpose of seeking resources to sustain their struggle for their very existence. ” The figure shown here uses a fox and rabbit as examples, but numerous studies have shown that many biological interactions are based on the same mathematics. Note the steep rise of the prey species (rapid reproduction). As the predator population increases with this availability of food, the population of the prey drops precipitously. After the corresponding drop in numbers of predators, the reproductive capacity of the prey once again kicks in to bring the population back up. The web of life consists of many such interactions in a dynamic balance. (Herbivory – the predation of plants by animals – has also been shown to follow this pattern. When herbivores such as deer exhaust the vegetation available to them, this is known as “exceeding the carrying capacity” and also leads to a population collapse.)
Deer (in all this discussion, we are referencing the white-tail deer, though there are other species) have only one reproductive cycle per year. A doe can have two fawns at one birth, though triplets have been reported. Poorly fed does may have fewer. The female fawn will bear in the succeeding year, but the first doe, her mother, is also bearing again. This means that the population growth is not merely replacement, but additive.
The figure above shows that a single doe can produce a population of 32 females in 5 years. (Multiply by 2 to include males – a total of 64 deer from one fertile doe!) The solid line points to the maternal doe; dotted lines show the births from the new does in each generation.
1.The does are well fed and can support the birth of two fawns yearly.
2. Each birth produces one male and one female.
3. There are no deaths.
Here is the same model, shown graphically. Again, note that this is does only. If males (bucks) are born at the same rate, the numbers will be doubled, but they do not change the rate of increase. As shown, there are 32 does at 5 years. By 10 years, if no change in the assumptions, 1,000! At this point, the original doe and some of her daughters and grand-daughters may have died or stopped reproducing, but their contributions are no longer significant.
At this point, the predator-prey model would predict that the wolf population will now have increased to exploit this food supply. Oh, oops. No wolves.
Of course, it is unlikely that deer populations in most Michigan locations, even in the protected environs of Ann Arbor, quite fit those assumptions. Various estimates are given on the life span of deer. A maximum lifespan has been reported to be 20 years, but estimates for most situations are for 6-10 years. Averages are lower where hunting occurs, especially for bucks. Some number of deer are killed by vehicles, there is some winterkill, and there have been a couple of instances of disease that reduced herds. In addition, there are coyotes who probably don’t take down mature deer but are known to take fawns. The Rochester Hills staffer who spoke at the Ann Arbor February 5 meeting said that coyotes were apparently affecting deer populations there, but offered no details.
Still, it is obvious that this terrific reproductive potential is a major factor in the impact of deer on our environment.
History of deer in North America
In the early part of the 20th century, there was a deer deficit. Deer have historically been a key factor in the abundance that this continent had to offer both to its earlier inhabitants (North American Indians) and to European settlers. As an account from Minnesota tells it, both the delicious meat (used in making jerky) and the skins were important in the early days, and survival by humans in wild territory was made possible by taking deer. The history of deer populations in Michigan and reaction to them is similar to other locations. Deer increased as logging made habitat more favorable for them. (Deer do best in clearings where there are seedlings, young trees and “forbs”, or herbaceous plants.) They became seen as an inexhaustible resource and were hunted for export to other states as well as for food locally. Legislation to prohibit market hunting was finally passed in 1895. Hunting bag limits were imposed. Nevertheless, the deer population was dangerously low – it was estimated that only 45,000 deer remained in Michigan in 1914. Further hunting restrictions, including a “buck law” (only bucks could be taken), allowed the herds to recover. At the same time, settlement and agriculture had converted much of the landscape, and wolves were being exterminated over the North American continent. After all, they were eating our deer. The DNR also began to create habitat protection for deer. By the 1940s, there were 1.5 million deer and some of them were starving.
The tendency of deer to increase their numbers well beyond the carrying capacity of their environment is termed an “irruption”. This early (1943) paper (large pdf) by Aldo Leopold, one of the fathers of environmental conservation, lays out the story. A reserve owned by the University of Michigan was stocked with 4 does and 2 bucks. Within 6 years, there were 160 deer, and they had exceeded the food supply in a 1200 acre reserve. Overbrowsing was observed, so the herd was “shot down” to 75, and later to 50, deer. Leopold declared that the herd was now in equilibrium with its range – and that it was being held at that level by additional shooting. Note that the graph also shows a hypothetical loss of carrying capacity if the herd had been allowed to continue expanding, and a likely drop in deer population through starvation.
An even more famous example of a deer irruption was also detailed by Leopold. The Kaibab plateau, where deer were protected from hunting and from predators, suffered a population collapse after deer consumed all available food. This observation, common in ecology texts for decades, has recently been reaffirmed.
Washtenaw County as the ideal deer habitat
We probably don’t need to worry about deer starving to death very soon in Washtenaw County (though it is possible if current population trends continue). We have constructed the ideal deer habitat. Not only has urban sprawl created thousands of semirural estates with landscaping, we have lush agricultural areas and lots of parks and natural areas, most restricted from hunting. Here is what the DNR’s analysis says about our county (the entire county is a Deer Management Unit):
The landscape supports a patchwork of cover types, with agriculture, forest, and grass/shrubland being most dominant. Urban development is concentrated in the City of Ann Arbor; however this DMU supports other largely developed areas and suburban and ex-urban communities; this is a populous county and development is ubiquitous throughout the DMU. …
Although much of the private lands toward the south central parts of the DMU are in agriculture, private and public lands in the area support cover habitat for deer (e.g., woodlots, shrub/brush, and wetland). Deer throughout the Washtenaw DMU have ample access to food, water, and cover and can meet all life requisites in every portion of the DMU. However, in many cases, they may be meeting these requirements in areas closed to hunting. (emphasis added)
Here’s the problem: through our own intervention to create a beautiful and lush living space, we have created a refuge for deer, with little hunting, no wolves, and tasty food, well tended. Being deer, they have responded with explosive growth in our living space. (Anecdotally, the areas west of us where hunting is allowed are not overpopulated.)
So while it may take a while yet before our local deer exceed their food supply (biological carrying capacity), they will have long since exhausted our patience. The concept of “cultural carrying capacity” expresses the total impact of deer-vehicle collisions, agricultural losses, landscape damage, and now even concern over disease-carrying deer ticks.
I’d say that Ann Arbor’s deer have reached that level already.
Note: A list of posts on this subject and some other useful resources on deer management in Michigan are found on our page, What Do We Do About the Deer?.
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