Here in northern Wisconsin, it’s a cold, gray day in late January. It’s time for some green leaves, blue skies, and bright sunshine. For now, we’ll have to settle for these photos, which also happen to have great scientific value. They’re part of an ingenious technique botanists have developed to measure canopy foliage density and leaf area indices.
Using a specialized digital camera with a ultra wide-angle lens that covers a full 180 degrees, they aim straight up from the forest floor and take hundreds of hemispheric canopy photos. When captured on a cloudless day with bright sunlight filtering through the canopy, these photos can be quite beautiful. But as much as one might enjoy their aesthetic value, they’re studied most closely by a piece of software that analyzes the light patterns and uses certain mathematical relationships to obtain a measure of foliage density at that particular location. Average out the results for multiple photos, and you get a single foliage density value for each stand.
It’s just one of many tools ecologists use to measure the long-term impacts of overabundant deer on the forest ecosystem. I recently talked with Dr. Timothy Nuttle to learn more about his latest research, which has to do with deer and birds. “Short-term,” he told me, “we know that too many hungry deer can eliminate the habitat where ground-nesting and midstory birds eat, nest, and rest. But here’s the question: How does herbivory by whitetails affect bird communities in the long term? As whatever trees are left behind by deer eventually grow and mature, how do those effects extend to birds in the canopy?”
“The details are complicated,” he told me. “But we found that one simple relationship
seems key. Both deer and caterpillars like to eat leaves from the same trees, and for the same reasons. They’re more delicious and digestible than the leaves on other trees. If deer eat those leaves first, before the seedling or saplings can even turn into trees, then caterpillars don’t get them. Fewer caterpillars in the canopy, fewer birds. So apart from deer eating the places where birds would nest and rest, the closest link between deer and birds is caterpillars.”
In one recent paper, Nuttle and his co-authors describe “a five-step trophic ricochet” that occurs in a forest with too many deer: “top-down release of ungulates has shifted forest tree communities to less palatable species that present a less dense food resource (foliage) for canopy herbivores (caterpillars) and their predators (insectivorous birds). Furthermore, this browsing legacy persists long after ungulate density has been equalized and trees have escaped browsing by growing into the canopy.”
How long? Decades, and in some cases even centuries. It’s an ugly but inescapable conclusion. Still, the photos are beautiful.
© 2012 Al Cambronne
Very cool and interesting. Even though I kill as many deer as I can each season (I’ve never made the five-deer limit in MN, the best season was a three deer year in WI years ago) these forest ecology issues are the reason why I’m really angered by some of the attempts, largely by Republican legislators, to increase the deer herd in some parts of MN, which has been lowered by liberal bag limits and a couple moderate to severe winters in some parts of the state. The herd will certainly go up after this mild winter.
If there’s a flaw in hunters, or at least a significant minority of us, it’s that we want too many game animals in the woods for the ecosystem…in contrast to what a lot of “antis” believe, that we kill game animals because we hate them and want to kill them all.
Amen to that. But in all fairness, it’s not just hunters who want to see more deer. So do nonhunting wildlife watchers. The ecologist I mentioned in the post was good for several great quotes. One went something like this: Most people see large numbers of deer and think it’s a measure of how healthy that ecosystem is. It is, just in exactly the opposite way they think. A forest with too many deer is not a healthy forest.” But it’s tough to convince some people that it’s even possible to have too many deer. And most of us, even if we spend a fair amount of time out in the woods, have never seen a forest that’s NOT shaped by deer.
Interesting post, Al! I’m familiar with the broad outlines of these issues, but didn’t know about the specific analytic technology.
Yes, it was new to me, too. Quite interesting, and some fun images. But apparently these studies also involve lots of low-tech measuring and analysis that’s quite painstaking. They count trees and measure their trunks, note how many of each species, snip foliage and weigh it, count and identify lots of disgusting caterpillars, etc.
I take what I said back only in minor part – western MN is being managed for more deer, and that’s ok. There’s not very many deer there, a lot of that is because there are too few places to hide (all row crops). The best “big plan” would be to have fewer row crops and more wetlands – and some restored prairie areas.
It is the legitimate business of the DNR to manage for, and promote hunting….what I have a problem with is some of these (usu right-wing) yahoos want to do that at the expense of other ecological concerns and possibly favoring one game species over another and hurting the environment at the same time. Most of Northern MN is not a good environment to have lots of deer. There was a lot of support for reducing the herd there from a broad range of constituencies. The vast majority of the people involved in the process were hunters as well. Now these legislators accuse the DNR of mismanaging the deer herd.
Yes, once politicians get involved, that adds a whole additional layer of complexity. They can be very “helpful” when they think it will get them some more votes. We’ve seen a lot of there here in WI, and also in PA. Maybe in MN, too, the way it sounds.
Scientists have the habit also of disagreeing with themselves, or changing their minds as they learn more. What seems like an ideal population size at one time can be too few or too many another.
There also that there might not be a “right” number, but wildly fluctuating historical numbers covering multiples of decades and even longer. I remember reading that up in Alaska they found they were able to maintain two very different populations numbers where ungulates and predators seemed to stabilize, and once the population became unstable it would revert to the much lower numbers and seemingly get stuck there. Few caribou and few wolves.
The only place I’ve ever seen an ungulate overpopulation is in places where hunting isn’t allowed. To have such a problem as too many is the stuff of my dreams.
Actually, scientists have already been learning quite a lot about this topic, and there’s a clear consensus that it really is possible to have too many deer. But how many is too many? Depends on who you ask, and it depends on the location and the habitat. And you’re right, those population numbers do fluctuate. But one interesting thing I’ve been learning about is that when a forest has way too many deer for just one decade, the effects will still be evident several decades later. And sometimes serious overpopulations can nudge the ecosystem into an alternate steady state, so that the nature of that forest will be changed for decades or even centuries.
You’re right, though, that the most common place where ungulates are overpopulated is in areas where there’s inadequate hunting pressure. And when I’ve been out hunting myself, I’ve never, ever encountered the problem of seeing too many deer.