In my previous blog posting (https://palmera01.wordpress.com/2011/03/21/take-a-poll-how-many-predators-did-deer-once-have/) I asked people to take a guess at how many predators deer once had. Everyone seems to notice that deer are overrun. As people’s gardens have bulbless herbs, as the shoulders of roads become smothered with broken carcasses, and as a warm spring occurs that is void of forest wildflowers, people have begun to acknowledge that deer overpopulation is problematic. The most meaningful impact deer have had on the hearts and minds of Americans is deer crash incidents. The Wisconsin Department of Transportation’s 2009 crash report revealed that deer are the third most commonly struck object by motorists, with striking another vehicle and striking a non-moving object the only two factors ahead of deer!
But why are deer overpopulated?
Most people think they know the answer: we humans killed all of their predators (such as wolves) and destroyed their forest homes. Without natural predators to control their populations and with all of the forests cut down, deer have no where to go. That is why deer are so overpopulated! Therefore, the solution is to stop cutting down all of the forests and to bring back wolves.
Yet evidence for why deer are overpopulated, particularly in the eastern half of the United States (east of the Mississippi River), reveal a much deeper and more complex problem and radically different solutions – different enough to churn the stomachs of any petunia gardener or a highway truck driver that currently holds the laymen’s assumption stated above.
In this blog post, I’ll attempt to offer some solutions that may actually help because they are based upon observed and known facts rather than well-intentioned assumptions. However, we must first understand some basic information about deer and the reasons they have become overpopulated.
Oh deer! Too many of them, but how?
The white-tailed deer (Odocoileus viginianus) is one of the largest ungulates, or hoofed mammals, in the eastern portion of the United States. Deer are in the same family of animals as cows, horses, elk, and mules. They are exclusively herbivores, eating a wide variety of plants and some fungi (mushrooms).
Deer are often pictured as graceful animals of the prime forest. Figure 1 offers a seemingly convincing image of the forest home of a typical deer.
Figure 1: Deer are often thought of as living in rich prime forests with pristine streams banked by large, tall trees. Image from digitalblasphemy.com
Yet deer are not deep forest animals. Rather, deer are most abundant and successful along forest edges and clearings bordering prairie and old fields (Bowers et al, 2004). In other words, deer often live at the interface of a prime forest (like in the move Bambi) and the open landscaping of a grassy meadow…or a suburban backyard. In fact, the rise of suburban backyards and grassy roadside edges have actually increased deer habitat, essentially creating what Miller calls an “all you can eat buffet’ of garden plants and forest wildflowers in patches of woods that are surrounded by human development (2007).
What about predators? I’ve heard many estimates about the number of different kinds of predators that deer once had (again, east of the Mississippi River). Some say there were 50, others argue just one (the gray wolf). The general argument is that because humans have eliminated wolves, deer have no natural controls, and are therefore overrun.
Yet the elimination of wolves and other natural predators cannot be the most significant cause for the increase in deer populations.
That’s right, you heard me correctly. Why? It is a commonly held belief (and now a myth) that predators control prey populations. This is known as top-down population control, and is easily illustrated by the typical lion chasing a gazelle frequently seen on TV nature documentaries. However, as ecologists (people who study interactions in nature) continue to do research, they are discovering a somewhat perplexing phenomena: bottom-up population control. In essence, prey (i.e. deer) are actually the ones that control predator populations (i.e. wolves)!
One study that confirmed this jaw-dropping observation (and is now frequently cited in ecology textbooks such as Miller, 2007) was the lynx and the hare scenario.
For a long time, it was common knowledge that big carnivores like lynxes hunt down and kill their prey: cute furry hares (hares are similar to rabbits). This would work to the advantage of the lynx, because he (or she) would have food to eat. And clearly, it worked to the disadvantage of hares, because if there are too many lynxes, they could all get eaten! Yet this seemingly logical set of events has not held up via scientific research. Careful observations of snowshoe hair (Lepus americanus) populations over time (in terms of whole decades) reveals a puzzling observation: hare populations naturally go up and down with or without the presence of predators. Take a look at the graph below. At first glance you may think this graph shows how hare populations go up and down in response to predation by lynxes.
Figure 2: Lynx and hair population cycles. Copied from Radcliffe, 2009.
But look carefully! Shortly after the hare gains population numbers, the lynx follows suite. Then, as hares die off, lynxes die off shortly after. As hare populations rise again, there is yet again a brief lag time before lynxes make a comeback. Finally, note that even at extreme highs and lows in in population, lynx are almost always lower in number than hares (the exception being mainly towards the right side of the graph, when both populations dropped off dramatically in this analysis). Finally, studies by Boonstra et al (2001) among others show that other factors (such as vegetation, climatic variation and availableness of shelter) play a part in the population control of hares even in areas where there are little or no predators. Essentialy, it has appeared to reserarches that snowshoe hares “flucutate in 10 year cycles” on an average basis (Boonstra et al, 2001).
Why, then, am I talking so in depth about snowshoe hares and lynxes when we are talking about deer and wolves (and possibly other predators)? This is because I have reasons to believe that historically, deer had a similar population dynamic to hares and other herbivores…and that the advance of human development (e.g. suburbs, lawns, ornamental gardens, etc.) has been a greater factor in the increase of the deer population than that of the loss of predators.
I’ll admit, I’m speculating somewhat. I’m not going to bounce up and say I am the expert know-it-all on this matter (but I’ll at least pretend). I’m basing this speculation though on observations I find too convincing to ignore.
First, it is rare in natural ecosystems for predators to be the primary population control mechanism for prey. There are other studies that reveal a similar pattern to the above mentioned hair-lynx scenario, but they are not as clean cut as that example and are beyond the scope of what I wish to discuss in this blog post that’s already a few pages long. Bower et al (2004) and Miller (2007) both seem to acknowledge that deer were much lower in population over 100 years ago, both giving estimates of around 500,000 deer in the United States. In addition, deer in the eastern half of the USA likely only had one predator that could been intrepid and mobile enough to attack a live deer: that animal may have been the gray wolf. BUT, observations of wolves and other top predators in North America quickly reveal that they usually only go after young or sickly deer and not healthy strong deer.
Let me summarize this. Lack of viable predators + present day populations well beyond the 500,000 estimte + x+y = Deer overpopulation.
I will tell you what x and y equal in just a few moments. First, let me reiterate the current deer population status. There are now hundreds of millions of deer, and it is likely there are now more deer than there are people living in the USA. Therefore, when European colonists first arrived, deer were a rare sighting.
Second, we have increased the habitat that deer thrive on. Deer are edge species, animals that succeed best at the interface of one habitat type (deep interior forest) and another (large open fields and lawns). The rapid creation of cities and suburbs into wildlands has increased the total amount of edge habitat (as opposed to undisturbed interior habitat) by millions of acres. When you have more places for deer to live in, mama does start giving birth to more Bambis. This is what x is in our above equation.
Third, deer have seen a geographic shift. It is likely that prior to European settlement, deer resided mainly in the Midwestern states in a region that many natural historians refer to as the “oak openings.” These former lands would have been a pre-Colonial textbook definition of natural deer habitat, as these “openings” were made up of large swaths of open prairie dotted with woodland patches and trees (see Figure 3).
Out east, most areas were heavily wooded for tens of thousands for years, except for areas near major rivers and coastal zones.
This constitutes as y in our above equation. Therefore, let’s compare. I believe most people follow this equation: Lack of Predators + loss of forest homes = deer overrun with no where to go. What I am suggesting is this:
Lack of viable predators + present day populations well beyond the 500,000 estimate +increase in habitat + geographic shift = deer overpopulation
What we need to do, deer.
If I’m right, then this puts our assumptions and our present course of action regarding the deer overpopulation problem in need of serious review. I know too many people, including close family and friends, that are opposed to the hunting and culling of deer herds because they feel it is animal cruelty. Yet I would argue that if we follow this anthropomorphic view of deer as having every right to live as much as humans do, then we will continue to see the loss of forest plants, increased erosion, and fatal car accidents, among many other consequences (see Figure 4).
Figure 4: (A) Forest with a good amount of under story herb growth and tree seedlings. (B) My mother standing in a forest where deer are oveerun. Notice the lack of understory plants and the amount of exposed soil.
So yes, I am arguing that we should cull deer and promote hunting of deer. However, we must also start thinking more long term. If we are going to preserve our natural heritage and conserve our natural resources, we need to look more at land management as a whole. Again, deer like suburban edge habitat, and in these areas it is nearly impossible to manage the land for interior habitat. Therefore in urban, suburban, and large agricultural areas, we need to cull deer in order to ensure conservation of forest herbs and other native plants. Do we want our parks and our woodlots to look full and green (Figure 4A) or do we want to see bare, eroded soil (Figure4B)? We must decide if it is worth the risk to our suburban natural areas to care for deer in a similar way we care for humans.
In areas that have not yet seen as much development, we must consider reforesting as much if the area as possible. Perhaps bringing our lawnmowers inside and tossing the books on planting exotic plants is an initial first step we can take. If we can get our rural people to preserve large, contiguous intact tracts of mature forest, we can limit the habitat for deer and bring back down there populations. In fact, at this point this method may be the only way to do it!
Finally, we need to decriminalize hunters and sportsman as heartless red necks. These people often bare the brunt of urban ‘yuppies’ calling them out for having a lack of “sophistication” and “intellect” or as people who get a sick thrill from shooting animals. In fact, hunters and sportsman are likely to have better connection with – and ultimately a better understanding, of nature. Therefore, animal rights activities and urban residents among others should reconsider their opinions of hunters and sportsman.
We must decide how we want to tackle the deer problem. It involves making changes for a large part of the American population. It also involves us educating ourselves about the issue before we give our opinion or express our worries about deer population management. This is not necessarily an issue that just concerns environmentalists such as myself. Road kill, chewed up vegetable gardens, and massive erosion should give even the laymen a clue as to the importance of this environmental issue. Let us make the right decision and consider how we manage our land – both urban and rural.
Volunteers reseeding an eroding hillside with native plants at Schenely Park in Pittsburgh, Pennsylvania. Among other things, deer overbrowse of forest vegetation can lead to serious soil erosion problems like this one.
Wisconsin Department of Transportation, (2010). Motor vehicle-deer crashes in 2009
Bower, N., Bower, R., & Kaufman, K. (2004). Mammals of north america. New York, NY: Houghton Mifflin.
Miller, G. T. Jr. (2007). Essentials of ecology, fourth edition. Thomson & Brooks/Cole.
Radcliffe, E.B. (2009). Introduction to population ecology. IPM World Textbook, Retrieved from http://ipmworld.umn.edu/chapters/ecology.htm
Boonstra, R., Boutin, S., Krebs, C.J., & Sinclair, A.R.E. (2001). What drives the 10-year cycle of snowshoe hares?. BioScience, 51(1), 25-35.
Apfelbaum, S.I., & Haney, A. US. Environmental Protection Agency, 1993 Proceedings of the Midwest Oak Savannah Conferences. (1993). Characterization of midwest oak savannas