Deer Exclosure 001.JPG

The deer exclosure on the left shows the plant life that grows when deer are prevented from browsing. The less vegetated area to the right was open to browsing.

(Clay Wollney)

Plants that grow in low light levels--underneath the canopy produced by the foliage of taller trees--make up the understory of a woodlands. Shade-tolerant plants include a variety of species from low-growing dogwood trees and spicebush to vines like Virginia creeper to simpler plants such as ferns and mosses.

If you've noticed that there isn't as much of an understory growing in Staten Island's woodlands lately there is a reason for it, and it's not global warming. Our newly established deer population needs food, and the understory provides a large part of the fodder they need. As a result, many formerly common plants are becoming a rarity in many of our local parks.

Exclosures are often used to assess the damage done by deer

One commonly used method to assess the impact that deer have on the ecological community of a forest is to use exclusion fences that keep deer out of a small section of woodland. Typically these exclosures are at least 100 square meters in area and are surrounded by fencing eight feet or more in height since deer are excellent jumpers.

Exclosures give wildlife biologist and ecologists an opportunity to observe the plants that would grow if the area was left undisturbed by browsing deer. In most cases there is a dramatic difference in the variety and type of plants that grow within the enclosure and those in the surrounding area where deer are able to browse.

Locally, two deer exclusion fences have been set up at Clay Pit Ponds State Park Preserve. This is a particularly good place for study since the park has one of the highest population density of deer on the island, though deer had been absent for sixty years before their return in the 1990s. The first exclosure was constructed on June 8, 2013 by Eagle Scout Jason Amsterdam and members of Troop #2. The second was installed earlier this year.

The effects of deer browsing have already begun to be seen

Although the fenced-in areas are still relatively new there have been some changes noted already. Emily Becker, the Environmental Educator overseeing the project notes that "I have not seen a raspberry plant growing in the Park since I started working here. Now inside new deer exclosure, which is less than a year old, raspberry plants are beginning to blossom again."

And it's not just the raspberries. Other plants that grew in the exclosures that are missing in the surrounding woods include Virginia creeper, wood ferns and seedling spice bush. The deer have even eaten much of the green briers and poison ivy. In other places around the island, jewel weed aka "touch-me-not" has also suffered greatly from browsing deer. Other wildflowers that have been negatively impacted by deer at other parts of the island include Canada mayflower, false Solomon's seal and wild sarsaparilla and low bush blueberry.

Destruction of sapling trees will have a long-term effect on the woodlands. Since some of today's saplings will become part of future forests their loss will affect the local ecosystems for decades to come.

Browsing encourages invasive species

There are some plants that have survived well despite the hungry deer, but this really isn't good news. Japanese stilt grass, an invasive species, now covers much of the woodlands where ferns used to grow. This is consistent with the conclusions of other studies, such as the work done by Eschtruth and Bates in hemlock forests in New Jersey using exclosures which was published in 2009. In that study the researchers found that over-browsing by deer increased the growth of Japanese stilt grass as well as red barberry and garlic mustard, two other species of invasive plants. Other studies have shown that native plants are half as likely to survive in areas with high numbers of deer compared to locations where deer were excluded.

Other wildlife are affected as well

Other studies that took place over longer periods have further indicated that over-browsing also has an impact on other wildlife. Studies done as early as the 1990s found that browsing by deer removed much of the vegetation that songbirds need for food, cover to escape predators and for nesting. As a result, the abundance of nesting songbirds decreased in number and diversity where deer populations were high compared to similar areas with lower deer populations. Similar effects have also been found for insects, especially caterpillars that live and feed in the understory. By competing with other squirrels and herbivorous animals deer may further impact the populations of other wildlife.