A world with deer (left) versus a world without them (right). The difference is stark and extends from the ground to the canopy -- birds, mice, and chipmunks are more abundant without deer.
But beyond the roads, experts say the deer are also having a major impact on forests, which are unable to replenish themselves to nurture the next generation due to the deer population's eating habits.
To illustrate this decline in forests during the past several years, a group of scientists blocked off a chunk of woods to the deer more than two decades ago.
A slice of untouched, and uneaten, woods
It's called an exclosure, and it's a place where no deer have trod for decades. Back in 1990, scientists at the Smithsonian's Conservation Biology Institute in Front Royal, Va., closed off 10 acres of forest with 8-foot high fences to see how the land would evolve without its furry friends.
"So we're comparing inside the fence to the outside the fence," says Bill McShea, a wildlife ecologist at SCBI. "And there's two things of note. One is, it's green on both sides of the fence but in here it's a lot more diverse than out there."
That is an understatement. The deer side of the fence has a carpet of grass, a shrubby looking thing and some large trees -- things that are either too big for deer to eat or among the very few plants they don't like to eat.
Inside is practically a jungle, with dozens of different almost exotic looking plants are tumbling over one another, many of them young trees.
"In here I can see white ash and hickory and red maples and white maples and serviceberry," McShea says. "A whole bunch of under story and canopy trees that are all now three or four feet tall. We are looking at 20, 30 species. There's a lot of diversity in here. You look out there, and it's a much simpler world."
Deer-eaten forests risk dying
That simpler world is an aging world. Really, it's a dying world as far as forests go.
"The future is not good. There are no teenagers, there's no young adults," McShea says of the trees and other foliage. "Everybody's a mature individual. Whereas, inside this fence you have the complete profile of ages. You have youngsters, you have teenagers, you have middle-aged adults, you have the old trees.
"And when the old trees go -- and they're going to go, because that's what happens with old trees, they fall over -- there is something here to take its place," McShea says. "Out there, I don't see anything out there that's a small tree."
These results of the exclosure, although striking, are what scientists could have predicted. One of the surprising things they found, however, is that deer allow invasive species to flourish.
"The Japanese stilt grass is just coming up now as a highly invasive annual grass," says Norm Bourg, a plant ecologist with SCBI.
The Japanese-origin grass carpets the floor outside the exclosure, but inside, there are many more native species present.
"There's a lot of native species like horse balm," Bourg says, gesturing to the plants beneath his feet. "This is black cohosh, which is a native medicinal plant that you hardly ever see out there."
With fewer native plants outside the exclosure, there are fewer birds there that depend on them for nests and food, and there are also fewer mice and chipmunks when they have to compete with deer.
Deer population is result of re-population
But it wasn't always this way. One hundred years ago, deer were nearly extinct in Maryland and extremely rare in Virginia.
"By that time, you couldn't find a deer or a turkey or a bear in the state," McShea says. "Both the habitat changes and the restaurant trade eliminated most of those animals."
Today's ubiquitous food trend of "buying local" was the norm back then, and hunting was an industry, says McShea.
"They weren't going to put a cow on a train in Texas and ship it to Virginia," McShea says. "If you were going to go to a restaurant, order yourself a steak, for the most part that was a venison steak."
In the early part of the 1900's, newly minted state game departments rushed to the rescue, banning or regulating hunting and setting up parks.
"When they made the Shenandoah Park in the 1930s, they went and got deer from Arkansas and brought them back here to repopulate that area," says McShea. "So growing the deer population was intentional. It's a conservation story and it went just like they planned."
A conservation effort's unintended consequences
The result is that today, there are several million deer, and, as McShea puts it, "the flip side has happened.
"They're hitting too many cars, there's too much gardens being eaten, the forest succession is changing," he continues. "We've got to dial that back a little bit.
Deer aren't evil, McShea is quick to emphasize, but they have no predators now and they need to be managed. States currently rely primarily on scheduled hunts, where the public is allowed to come in and take out deer.
That works well on parkland to some extent, but it doesn't work on private property or in federal parks, which have been slower to adopt aggressive management.
"We have time for that, we don't have to make a decision this year," he says. But we don't have decades, he adds. Trees don't live forever.
CORRECTION: The original version of this story misstated the number of deer vs. vehicle collisions. There were 88,000 deer-vehicle collisions between July 1, 2009 and June 30, 2010.