Exotic species and
the 100-acre woods

The following article about the fragile future of our regions biological diversity was written by Brian Parsons, natural areas coordinator of Holden Arboretum. It was originally printed in the Fall, 1998, Arboretum Leaves newsletter.

By Brian Parsons

What will the natural landscape of Northeastern Ohio look like in 50 years or even 10 years? Will The Holden Arboretum, or any other land-holding organization, be able to maintain the level of species richness, or biodiversity, that we experience today? How many of us will notice if these levels decline? These questions will be answered by how land managers address and mitigate the influence of edge effect and the corresponding invasion by alien edge species.

Life on the edge

What is an edge in ecological terms? Simply put, an edge is that area where two different plant communities meet and intergrade. Older texts state that the greatest diversity of plant and animal species exists where edges of two different plant communities meet. The greater the contrast between the two communities, the greater the species richness. Texts written over 50 years ago speak of edge in positive terms. Is that still true today?

The ever-expanding edge

As a result of suburban development, natural areas throughout the U.S. are beset by ever-expanding edges. Obviously, suburban development results in an immediate loss of habitat and species diversity on the developed parcel, but what about the undeveloped parcels?

Imagine a 100-acre woodland with typical, rich, biotic diversity. This woodland contains multiple layers within the woody canopy and shrub zone; a rich understory of native wildflowers; and numerous bird and animal species.

Suddenly, the area is developed and reduced to a 20-acre woodland surrounded by houses on all sides. There are many long, well-defined edges between the remnant woods and the new development. What might happen to this remnant woodland?

When succession takes place near a well-established seed source of native plant species, native plant species develop along edges. The first species to colonize these edges generally are those species planted by birdswoody plants with fleshy fruits that birds eat, digest, and ultimately plant.

As succession continues, more species are added, and the edge moves toward the center of the more open community. But in new housing developments, most residents don't let succession occur. They work to push the edge back toward the natural area.

Today what more commonly happens is that the majority of species planted by birds along woodland edges are non­native woody species of trees or shrubs. A walk along any wood line at the arboretum will reveal shrubby combinations of buckthorn, privet, multiflora rose, European cranberry, bush honeysuckles, and vines such as oriental bittersweet and porcelain berry.

Our naturalized aliens

The sciences of agriculture and horticulture have long promoted plant species from other countries for additions to our landscapes and gardens. Older gardening texts tell us that a plant isn't considered established in your garden until it seeds into your landscape.

Statistically, this means we have actually established over 2,000 exotic plant species in our natural landscape on a national level. Throughout the Midwest, states report that approximately one-third of their total flora is composed of non-native naturalized plant species. In Ohio, for example, only 1,800 species are considered native of our total flora of approximately 2,700 plant species.

Plant, animal, and insect species within natural communities co-evolve and develop a series of checks and balances that prevent any one species from dominating a community. Some plant species may be opportunistic and may for a short period dominate a community, but, as succession progresses, balances are restored.

The main problem with exotic plant species is two-fold. Many of these aliens are opportunistic, and no organisms exist within their new community to keep the alien plant in check. Imbalances occur, and these exotics maintain their hold on the community despite the best efforts of succession.

Vertical nichery

Vertical niches develop from the many layers of vegetation in a plant community. Various species inhabit and use as many vertical niches as are present in a habitat. Niches might be used for nesting, food sources, or cover.

The greater the number of vertical niches available, the greater the species diversity. When exotics dominate woodland edges, however, diversity decreases as our flora becomes homogenized and all woodlines look alike.

Into the interior

Edge effect and the influence of exotic plant species do not stop at woodland edges. In time, the forest interior becomes disturbed by natural events as openings are created in the canopy. When trees fall, these new openings become colonized and dominated by exotic plant species if they are the nearest available seed source.

So, here is our 20-acre remnant. Houses now surround it, exotic plant species are established along all of its edges, and its interior is slowly being degraded by exotics, which are added as natural disturbances occur.

We have mentioned only woody plant species. What happened to all the animal species that once lived in our 100-acre wood? Fortunately, animals are mobile, and many of the displaced animals seek refuge wherever possible in the persisting remnant.

But as available niches are filled, the remaining animals either leave to establish new ranges or, which is more often the case, they have nowhere to go and must compete within their own niche for food and cover, resulting in overpopulation.

What effect do the displaced animal species, such as white-tailed deer, have on our remnant woods? As almost all of us have seen by now, deer eat woody plants and eliminate plant layers within woodlands through intensive browsing. When this occurs, the bird species using these layers for food or nesting decline. Many bird species use the lower layers of a woodland for nesting, but a formerly common resident of our mature woodlands, the wood thrush, is declining throughout its range because of the lack of nesting sites and other edge species.

Around our woodland are houses, and as time goes on we see an increase in other edge animal species such as raccoons, cats, and brown-headed cowbirds. All of these species are parasites on woodland nesting bird species. Cats and raccoons eat nesting bird species they can reach, and cowbirds lay their eggs in other birds' nests, resulting in a decrease in native birds and an increase in cowbirds.

The blow beneath our feet

So exotic trees and shrubs surround us, our lower understory layers are disappearing, and there are fewer and fewer nesting birds. Have we left out any exotic influences? Unfortunately, the final blow to our woodland is happening to the forest floor, our once-full ground layer.

Here groups of plants, namely all the members of the Lily family, have been systematically eliminated by deer. Vegetation that thrives on disturbance and is graze-resistant is favored, such as cat-briar and brambles. Shade-resistant herbaceous exotics join the battle.

Invasive herbaceous exotics such as garlic mustard or lesser celandine sweep through, shade out, and replace our native wildflowers. Canary reed grass, common reed, and purple loosestrife dominate the watershed. The battle is over, and our 100-acre woods is a woodland in name only.

Streets are named to honor the fallen. Trillium Trail and Wood Thrush Way become choice addresses. The once-teeming diversity has been reduced by hundreds of species. Biodiversity declines. Is this the rain forest? No, this story happens in northeastern Ohio every year, and is coming to a woodland near you.

Is all lost? Are we powerless to stem the tide of exotic and mitigate edge effect? Only with careful planning and considerable effort can we exclude exotic plant and animal species and prevent edge effect from severely reducing diversity

The size and shape of a forest remnant has a great bearing on edge effect. Long, skinny preserves with a
high edge-to-interior ratio have a greater chance of failure than do preserves with a low edge-to-interior ratio. Edge effect has been estimated to extend anywhere from 10 to 100 meters into a natural area. The farther the edge reaches, the greater the opportunity for our tale of woe to come true.

Fighting back

Edges can be planned and managed so as to prevent domination by exotic plant species. More native plant species can be planted and restored by the homeowner. Also, the homeowner can weed, mow, or spray to exclude exotic woody plant species. Deer can be controlled through hunting or excluded by fencing, and our lower woody layers can be preserved.

Invasive herbaceous edge species such as garlic mustard and lesser celandine can be pulled or sprayed and not allowed to seed on the site. Plant layers can be saved through considerable effort and planning by the landowner.

Greater efforts have to be made to screen new plant species and cultivars for their potential to harm natural areas. There is a fine line between the horticultural merit and the natural-area threat of any nonnative plant species. Nurseries, plant breeders, and homeowners should evaluate plants on the basis of their weed potential and the type of fruit they bear. Not all non-native plant species are bad. However, some genera, such as Viburnum, have a proven capacity to invade natural areas and need to be closely evaluated prior to introduction to our landscapes.

What about the bird species? What is their fate? Unfortunately, our 20-acre remnant still remains an ecological trap that can doom migrant bird species to nesting failures and ultimately a reduction in species numbers. Forest fragmentation is the greatest threat to area-sensitive, forest-interior birds. Thus, we need more large preserves and land conserved by the state, counties, townships, cities, developers, and other land-holding agencies. Conserved areas need a low edge-to-preserve-interior ratio to offer these bird species a chance.

Perspective on remnants

Does our 20-acre woodland have any merit? Although just a remnant, it still offers shade, protects part of a watershed, and provides homes for some animal and plant species. It also presents an opportunity for restoration, and perhaps it can serve as a link or corridor to a larger, better-planned preserve.

But this remnant should not be rationalized and considered a complete ecosystem. In truth, it is damaged. Its biotic diversity has been severely reduced and may never return to pre-development levels. This remnant woods teaches a lesson, one which planners, developers, zoning boards, and all land managers need to learn if we are to maintain the levels of diversity we now enjoy and the levels we may need in the future.

Experts say that for every species lost, ten associated organisms disappear. Nobody knows what level of biotic diversity is needed for our growing populations to survive, but does it make sense for us to continue down the same path time and time again? Lessons need to be learned.

We need to change our thinking and our planning of communities. We need to carefully screen new plant introductions for their potential threat to natural areas. We need to aggressively exclude exotic species from natural areas.

We need to support legislation to declare more aliens as nuisances. We need to support land-managing organizationsagencies such as The Nature Conservancy, the Ohio Division of Natural Areas and Preserves, The Holden Arboretum, The Cleveland Museum of Natural History, and county park systems with our votes and dollars so they can purchase enough land to provide buffers and create preserves with low edge-to-interior ratios.

We need to protect more 100-acre woods while we still have them. We need to give the edge back to our native species.


The Holden Arboretum is a 3,100-acre living museum of woody plants located in the Chagrin Highlands east of Cleveland at 9500 Sperry Rd., Kirtland, OH. It is the largest arboretum in the United States, and its research and conservation programs make it a leading horticultural institution. For information about educational programs and membership, call 440-946-4400.


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Exotic herbaceous plants
Exotic trees, shrubs, and vines

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Beech-maple woods in Geauga County.


The main problem with exotic plant species is two-fold. Many of these aliens are opportunistic, and no organisms exist within their new community to keep the alien plant in check. Imbalances occur, and these exotics maintain their hold on the community despite the best efforts of succession.


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