Deer and the Web of Life

“I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.” – Aldo Leopold, A Sand County Almanac.

As we said at the very beginning, much of the controversy surrounding the solution to Ann Arbor’s deer problem is based on values.  One set of values that is crucial to this question is how one views the importance of the natural world, and how one judges its components.  There are some major philosophical and functional questions here, and often they are not examined so much as taken for granted.

Does “nature” have an inherent value, or is it only valuable in economic terms and its usefulness to humans?

Is there a moral basis for preserving wilderness, or is it to be judged solely on recreational use and conservation of certain resources (lumber) for future exploitation?

Are some species more worthy than others?  Or is every species deserving of protection?

These questions first emerged in America toward the end of the 19th century, as the expansion of civilization across the continent was drawing to its finish. President Grant signed some of the first legislation establishing Yellowstone National Park.  (The second national park was Mackinac National Park in Michigan.) John Muir, who died in 1914, was a major leader in the preservation of wilderness and conservation of wild species and the founder of the Sierra Club.  He was influential in Theodore Roosevelt’s moves to strengthen and expand National Parks.  He saw natural areas as having a transcendent value, a source of spiritual inspiration, rather than as treasures to be exploited. The questions are still relevant today, as every fight between mining operations, lumber interests, developers or ranchers and conservationists demonstrates.  (Note the recent battle over the fate of wolves in Michigan.) Aldo Leopold, who was most active during the first half of the 20th Century, furthered the discussion with his early work on ecology.  We quoted his careful work on “deer irruptions” previously.  Leopold brought some rigor and analysis to the understanding of how wild systems work, but he also enunciated a philosophical position which he called the “land ethic”.  He had a strong empathy for all parts of the natural system, beginning with the soil and incorporating the plants and the animals.  In the foreword to his landmark book, A Sand County Almanac, (1949) he says,

“That land is a community is the basic concept of ecology, but that land is to be loved and respected is an extension of ethics.”

Unfortunately (from my perspective), that is an ethic that is not universally shared.  Even many supporters of maintaining biological diversity often justify it on an utilitarian basis, e.g. “there might be a new antibiotic or a cure for cancer there”.  For Leopold, and for me, every part of the natural community (what he calls the “land”) is of value in itself. There is an aesthetic but also a reverence for all these members of the natural community and the beautiful fabric that they weave.

Favored Species

But of course any of us do discriminate among species.  When I was a child, I couldn’t figure out why God created the mosquito or the rattlesnake.  Now I ruthlessly weed out “invasive” plants (usually plants that are not native to our area and are weedy in behavior, crowding out other plants).  I certainly favor songbirds over starlings (another invasive).  But if we care about the natural world, we recognize that most species, especially those native to the community, have a place in the “web of life”, where there are many dependencies.  Even the less glamorous deserve some measure of respect. What has been distressing in the current discussion about deer is that some people evidently do place the welfare of individual deer above that of all the rest of the natural world.  In the A2 Open City Hall survey (we summarized it here) the question was asked,

In general, do you believe deer are a more valued species than other urban mammals (coyote, rabbit, squirrel, skunk, woodchuck, etc.)

to which 26% of respondents answered “Yes”.  The question does not ask about birds, plants, reptiles or amphibians, much less invertebrates like insects.

A Question of Balance

A simplified food web illustration. Arrows point to the one who eats.

A simplified food web illustration. Arrows point to the one who eats.

The notion of the “balance of nature”, in which the various members of a native community interact with each other to form a more or less stable condition, is based on the concept of the food web.  Plants form the basis of several food chains (combined in the web) in which energy is captured by photosynthesis and passed along from animal to animal, where prey animals are ultimately consumed by predators.  (Prey animals may be herbivores, omnivores or carnivores.)  Predator-prey relationships exist at each step (see our explanation), including between herbivores and plants.  This means that an excess of any predator can depress the ability of critical parts of the food web to supply the needs of the rest of the web. As we noted previously, without a predator (and you’ll notice the wildcat is missing in this web), deer will employ their tremendous reproductive capability to exceed the ability of the plants in their community to support them.  But the rest of the food web suffers, too.  This has been especially evident in the forests of North America.  A post originating from the august Nature Conservancy states,

In our opinion, no other threat to forested habitats (than the white-tailed deer) is greater at this point in timenot lack of fire, not habitat conversion, not climate change.

Think about it.  Most deer, even those in our luxury setting, are hungry all the time.  They need a high-quality diet.  Estimates of how much vary, but about seven pounds a day seems fairly conservative.  And they want it to be high in protein and low in toxic compounds.  They are not grazers of grass, but nibblers of buds and tender young foliage.  In fact, much of what they eat is what we would seek out if we had to live in the wild and had no animals to eat.  Buds, fruit, tender growth, nuts, acorns, even mushrooms.  They have also been observed to eat baby birds.  (Protein is hard to come by.)  They go through a habitat relentlessly, scooping up everything edible – by their standards. There are numerous studies of how deer have affected the Northern forests.  I like this one from Wisconsin.  It is based on rigorous studies using exclosures and plant assessment techniques. From the article:

Deer account for at least 40 percent of the change seen in the forests over the past half-century or so. “The study links microcosm to macrocosm. We have exclosures in the same region where we have documented long-term changes in the plant community over the past 50 years. These are giving us the same message.”

Some previously common plant species (trees and shrubs) have become so rare that they are no longer included in the assessment studies.

A Formidable Influence

Aldo Leopold killed a wolf early in his career and then regretted it.  In the fires of the eyes of the dying wolf, he saw a truth.  He realized that without the wolf, the mountain where it lived would suffer.

“I have watched the face of many a newly wolfless mountain…have seen every edible bush and seedling browsed…every edible tree defoliated to the height of a saddlehorn.  Such a mountain looks as if someone had given God a new set of pruning shears and forbidden Him all other exercise…I now suspect that just as a deer herd lives in fear of its wolves, so does a mountain live in mortal fear of its deer.”

Deer are extremely adaptive, have a high reproductive capability and an unending appetite.  They are a very old species on this continent.  In her recent book The Sixth Extinction, Elizabeth Kolbert relates how the introduction of humans to the North American continent finished off many large mammals (megafauna), including mammoths.   But “white-tailed deer have a relatively high reproductive rate and probably remained plentiful even as the number of mammoths dropped.”  This is referring to the likelihood that deer were able to supply meat to the new predator on the continent and yet survive and prosper.  Unfortunately, we are no longer filling our role as the top predator in the food web.  The balance of nature is off kilter.  I don’t think that we want to bring back the wolves in sufficient numbers to keep them in check. For the sake of the deer as well as all the other species dependent on the wilderness,  we need to fill our place in the natural order.

UPDATE:  An important study of the effects of deer in an Ann Arbor natural area has now been released.   The study, White-tailed deer (Odocoileus virginianus) browse damage in Ann Arbor, Michigan Bird Hills Nature Area, Winter 2015,  is by ecologist (and specialist in deer/vegetation interactions) Jacqueline Courteau and her independent study student at the University of Michigan, Moriah Young; it is based on data collected in February through April 2015.  (The authors have released the study for use and quotation, but it is not yet published in a journal.)

The study finds significant damage in tree saplings and shrubs.

This survey of 142 tree saplings (less than 2 meters tall) and shrubs in Bird Hills Nature Area shows that 80% have been browsed by deer, and 51% have half or more branches browsed. This level of browsing could interfere with forest regeneration and diminish the flowers and fruit available for birds, butterflies, and bees.

The study uses direct, painstaking counts of deer browse on specific woody species.

The discussion also makes an important point about deer population counts.  As it notes,

A small population of deer that browse repeatedly in a small area due to barriers (roads, fences, etc.) or because of the lack of predators may damage vegetation as much as a larger population that ranges more widely.

 There are numerous figures and tables with specifics about the sampling areas.


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One Comment on “Deer and the Web of Life”

  1. mhirzel Says:

    This is just excellent. I do hope it will be widely read by people of Ann Arbor…. especially those who currently oppose culling the deer in the city, and do not understand why it must be done.

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