According to local ecological experts, an increasing deer population is causing significant damage to ecosystems in Pittsburgh’s forests and parks. Overpopulation has led to overgrazing, which is wiping out native plant species and creating or exacerbating challenges for invasive species.
To the untrained eye, Frick Park might feel like a peaceful respite, but if you walk around with naturalists, it might start to feel more like a brutal deer-eat-plant-eat-plant world. Brandon McCracken, senior manager of ecological restoration at the Pittsburgh Parks Conservancy, said while he’s grateful for volunteers who help pull invasives, we’re losing the war to maintain ecological diversity and all the climate and health benefits that come with it.
As the deer population has risen, McCracken explained, their available food source has dropped — the forest’s ecosystems are now shaped predominantly by what deer eat and don’t eat. Non-preferred plants like stiltgrass and bittersweet are thriving and spreading rapidly, while preferred plants are being completely eradicated.
There is no clear division between plants that deer will or won't eat, McCracken said, but rather a hierarchy:
“You have the deer’s most liked plant at the top, then you have three more that they like pretty well, and then at the bottom are the plants that they won’t even touch unless they have no food. They’ve been working their way down the hierarchy for years, so the plants at the top don’t exist here anymore.”
Deer overbrowsing has led to forest floors in Frick Park that are barren or lacking ecological diversity. Many young trees are eaten before they get the chance to grow, or don’t get the chance to sprout, and that can have long term implications.
“Where will the next generation of the woods come from?” McCracken wondered.
He explained, it’s not only deer alone, but also the rapid spread of invasive plants outcompeting natives that contributes to the health of ecosystems.
He added that many plants are considered invasive simply because they are so prolific in certain settings. Mugwort, for example, is normally harmless in healthy ecosystems. However, if a forest lacks biodiversity, mugwort can dominate acres of land.
Other plants are considered invasive because they actively inhibit other plants from growing. One of the region’s most prevalent invasives, Japanese stiltgrass, is a prime example.
“[Japanese stiltgrass] is carpeting large swaths of forests, like at Hartwood acres,” said Ryan Utz, professor of Ecology at Chatham University. “Most of the forest floor is now covered in stiltgrass, and nothing grows through stiltgrass.”
Another invasive species, the Norway maple tree, casts so much shade that not much can grow beneath it. This tree is well adapted to grow in cities, easily springing up in Pittsburgh’s suburbs as its bundles of seeds spread rapidly.
"That's why our ecological restoration projects are so important," McCracken added. "By reintroducing native plants and removing invasives, we are taking positive steps toward healthier urban forests. We have also built deer exclosure areas, large fenced spaces that keep deer out and give young trees a chance to thrive."
“I would encourage everyone to learn their plants,” says Utz. “If you learn your plants, you'll very rapidly come to the conclusion that our forests are changing quickly.”
Utz also recommends that property owners consider replacing invasive species in their yards with plants that are more beneficial for the environment and that deer won’t touch. More information can be found on the Pennsylvania Department of Conservation and Natural Resources website.
