Last month, protesters outside Rock Creek Park carried signs reading “Shame on NPS,” “Don’t Kill the Deer” and “Birth Control, Not Bullets.” But if the opponents of the National Park Service’s effort to use sharpshooters to thin the herd in the park had looked into the woods and seen an understory thinned or absent altogether , would they have known what they were — or, rather, weren’t — seeing? Few of us do.

Even if we spend a fair amount of time out in the woods, most of us have never seen a forest east of the Mississippi that’s not shaped by deer. We see large numbers of deer in the woods and feel good about it, thinking it’s a measure of how healthy the ecosystem is. It is, but in the opposite way we think.

A forest with too many hungry deer is not healthy. Deer reduce the density of plants in the understory, and as preferential browsers they also affect species composition and diversity. A forest with too many deer contains both fewer and different plants than it otherwise would. Inevitably, these changes reverberate until they’ve affected every plant, animal and bird in the ecosystem.

Although deer population dynamics is a complex subject, the short version is that deer are good at two things: eating and making more deer. They rarely volunteer to stop doing either. Eventually, when they become severely malnourished, their population crashes — only to rebound again within a few years. Apart from the ecological consequences of this, standing back to “let nature take its course” is not a particularly humane option. And even if many of us might prefer otherwise, the most effective solution to the problem is, indeed, bullets, not birth control.

Here are the facts: Deer contraception, the alternative often favored by opponents of culling, is great in theory. In practice, however, it’s difficult, expensive and temporary. There is no convenient oral deer contraceptive that we can sprinkle about the woods or pour onto piles of corn. Nor is there a permanent, single-dose contraceptive that can be delivered by a dart gun.

In the 1990s, many wildlife biologists had high hopes for an experimental immunocontraceptive called porcine zona pellucida (PZP), which could be delivered with a dart gun. The only problem was that it required two doses the first year, plus an annual booster shot. The only contraceptive currently approved for use on wild whitetails is an immunocontraceptive called GonaCon, which gives slightly better results than PZP. But GonaCon needs to be injected by hand, and not just once. In a Maryland field study of single-dose injections, 88 percent of vaccinated does were infertile during the first summer after vaccination. During the second summer, however, that number dropped to 47 percent. In a New Jersey study, those numbers were only 67 percent and 43 percent, respectively.

Earlier studies with captive deer were more promising; researchers speculate that the difference was likely due to better nutrition and, hence, better immune responses among captive deer. The authors of the New Jersey study noted rather dispassionately, “As a practical matter . . . most overabundant deer populations that would be candidates for treatment with contraceptive agents would likely have experienced suboptimal nutrition.”

This means for contraception to be effective we need to repeatedly, on schedule, dart, capture, inject and release every single doe. Some, however, die from the stress of being captured and drugged. Others watch, learn and get better at avoiding their would-be captors. Labor-intensive from the start, the job becomes progressively more difficult. It also typically costs $500 to $1,000 per captured doe. Given the average deer’s three- to five-year life span, multiply those figures by at least three. Even if only dealing with a few hundred deer, the numbers add up quickly.

But that’s for the few deer we manage to inject. Before long, their fertile friends, joined by more deer wandering in from nearby, will render the entire effort meaningless. Meanwhile, even the deer we’ve managed to inject will still be present and hungry.

The consensus among wildlife biologists is clear: Deer contraception is only a viable option for small, isolated populations such as on an island or in a fenced enclosure. As much as we might wish otherwise, it does not provide an easy answer to the problem of overabundant deer in our nation’s cities and suburbs.

If we feel uneasy about lethal control measures, we should feel even more uneasy about the only real alternative: collisions with motor vehicles, disease and starvation. As one ecologist told me, “Just because we’re not shooting them doesn’t mean we’re not killing them. And just because we’re not shooting them doesn’t mean they’re not suffering.”

Al Cambronne is the author of “Deerland: America’s Hunt for Ecological Balance and the Essence of Wildness.”