By John Hadidian, Ph.D., and Sydney Smith
If you stop and think about it, edges are everywhere. The
side of the road, the wall of a room, and the borders of this
page are all edges that are so natural to us that we don't
often think of them as such. Edges in the natural world often
escape our attention, too, but when we do reflect on their
importance, we may see that edges contribute to our
understanding of how things work in nature. An "edge" is most
commonly visualized as a distinct and sharp line, such as the
edge on the blade of a knife, but it may also be a boundary
separating two different surfaces or places. It is in that
context that ecologists have taken the concept of edge and
endowed it with greater meaning (or, perhaps, greater mystique)
by expanding it to a concept called an ecotone. An ecotone is a
transition from one ecological community to another, as, for
example, from a forest to a grassland.
Anyone who has hiked up the slope of even a moderate incline
has probably passed through one or more ecotones. Starting out
in grasslands and slowly progressing through shrub, then
scattered trees, and eventually, a forest, the hiker might pass
through an open scrub land and meadow, finally arriving at the
highest altitude—a place with little vegetation beyond the
lichens dotting the boulders. The hiker may be unaware that he
or she has passed through as many as five ecotones during this
walk, given the gradual nature of the transitions.
Nature also contains abrupt changes in ecotones, such as
cliffs, but natural systems usually change more gradually. Most
abrupt edges and ecotone changes occur in human-altered
landscapes, because our impact on the landscape is much more
dramatic. The transition from a cultivated field to a woodlot
is a clear example of an abrupt edge. This boundary usually
contains more than trees on one side and a pasture or
cultivated field on another. A strip or layer of herbaceous
(soft) plants may occur on the outside of the boundary, where
it is called a saum. Behind it and adjacent to the woodlot may
be a layer of small trees and shrubs, called a mantel. Both
provide habitat for wild animals, as does the woodlot and, to a
lesser extent depending on its exact use, the field or
pasture.
Getting Edge-y
It is, in fact, of considerable importance to the makeup of
a wildlife community exactly how much edge occurs across a
given landscape and how that edge relates to the interior area
or size of the other land types. Some species of wild animals
do extremely well when there is a considerable edge in the
landscape, while others do poorly.
Birds, of course, are one of the most prominent and varied
forms of wildlife that we find in urban and suburban
environments, and their preference for edge varies considerably
as well. The shrubby mantel associated with the transition from
cultivated to woody lands can provide excellent nesting habitat
for a variety of bird species. But it also can attract species
such as brown-headed cowbirds, parasites of sorts that do not
build nests, but rely on host species—warblers, thrushes, and
sparrows—to do the job for them. After finding a complete nest,
the cowbird will lay her own egg, and when the young cowbird
hatches, the more active, larger, and aggressive nestling
competes for food and attention, often with fatal results for
the other offspring. None of this would be possible without
sizable areas of edge habitat: Cowbirds have a much harder time
searching for host nests in larger blocks of woods, which
provide enough space for forest-interior species, such as the
wood thrush and red-eyed vireo, and a much improved chance to
successfully raise young without interference.
Over the Edge
From an example such as this comes the concept of
edge-to-interior ratios as a construct in landscape ecology. To
take a page out of your high school geometry textbook, the idea
is simply that the shape of a given piece of land that occurs
as a single type (say, woods) dictates the amount of edge as a
function of interior: A rectangular block of woods that is 40
acres in size has more edge than a circular piece of land the
same size.
This starts to take on greater meaning when we recognize
that people have a preference for developing land as squares
and rectangles, especially when farming. Cutting woods in
blocks of those shapes gives the areas higher edge-to-interior
ratios than they would otherwise have, which may have a
significant impact on the species inhabiting those areas.
Wildlife species who thrive in edge habitat usually are
considered generalists, those capable of taking advantage of
different food sources or habitat requirements like the crow,
sparrow, white-tailed deer, starling, robin, and squirrel.
Specialists, such as the field sparrow, weasel, or blue
grosbeak, on the other hand, have much more narrow diet and
habitat affinities; thus when they lose their preferred habitat
type in a given area, such as a large forest tract, their
population is diminished.
If we look closely at the generalists mentioned above, we
may see another type of pattern—many edge species are the very
ones at the heart of our wildlife conflicts. Sparrows and
squirrels raid our feeders, and the cacophony of a starling
roost in a residential tree can disturb our restful times.
White-tailed deer, we are finding, especially relish the cover
provided by the rectangular mosaic of woodlots and forage for
food in fields and pastures (not to mention our manicured lawns
"edged" with tasteful landscaping).
On a more immediate level, edges occur at the boundary
between our property and our neighbor's and within our own yard
where we plant our gardens and flower beds. Yards often are
bordered by public open space, a woodlot, or a field that has a
quite distinctive transition from a managed or manicured
landscape to one that is not. Here then, where we live, is the
greatest distinction in urban-suburban edge habitats. It's no
coincidence that this is where homeowners most often experience
conflicts with wildlife. Certain wild neighbors approach this
human-altered landscape with as much zeal as those of us who
make our homes here. The next time a conflict occurs, look for
an edge created by humans. Once we recognize that we are the
ones who originate many of these conflicts, including the
proliferation of edge habitat, we may be more willing to
understand and tolerate the situation.
