By David Spector Gazette Contributing Writer
As I walk down my wooded driveway at this time of year I often get a strong whiff of grape, a smell very much like that of grape jelly or grape juice. Wild grape vines climb many trees along forest edges in western Massachusetts, and the grapes not eaten in the fall release their odor when damaged by freezing in the winter.
Wild grape is only one of many viny plants that inhabit the forest edge here. Virginia creeper and poison ivy crawl along the ground and occasionally climb the trees, several species of Rubus (the blackberry genus) creep on the ground or form dense thorny thickets and an occasional multiflora rose appears.
These vines are planted by fruit-eating birds, such as American robins and gray catbirds, which get nutrients from the fruit pulp and pass the undigested seeds through their guts. A simple experiment can reveal which plants are distributed in this way: Place two poles with a string between them on a lawn and clear a path of bare soil beneath the string. After a year or two, a line of bird-deposited plants will be apparent. If you try this experiment at home, I suggest that you learn what poison ivy looks like in its early stages, so that you can remove it before it becomes well established.
The presence of several vines with similar growth habits in the same habitat raises several basic ecological questions: Why are species distributed the way they are? For instance, why is one tree climbed by grape and another by Virginia creeper? How do ecological interactions between species combine to produce the observed patterns of distribution? Which of those interactions has the greatest effect?
I cannot answer these questions in general for this group of plants, but a decade of observations in the small area to which I pay most attention provides a clue. Over this time Virginia creeper has increased in abundance, poison ivy and Rubus have declined, grape has held steady and rose has appeared briefly but not persisted. Do you see the pattern? Can you tell what interaction produced the pattern?
The changes in relative abundance are due to differential predation, the result of an organism opting to eat one plant rather than another. This assertion may seem surprising; the plants that have decreased are protected by strong toxins or thorns that might deter a predator. The predator, though, is a pair of shears wielded by someone (me) averse to those toxins and thorns. My shears and I function as a selective agent that gives an advantage to some species over others in their competition for space and thus helps to bring about the observed patterns of abundance.
Although this example is artificial, it illustrates one way in which one species, or a category of species such as browsers (e.g., deer), can influence the structure of a community of other species. Humans—with our shears, shovels, hoes and mowers; grazing and browsing sheep, goats, horses and cattle; and complex influences on populations of deer and other herbivores—have become major shapers of local plant communities.
So, as I smell a scent of grape in mid-winter, as I watch the grapes and other vines leafing out and growing again in a few months, and as my shears and I shape the little community of plants along the edge of the woods near my house, I think about choices. The choices that a deer makes to eat, perhaps constrained by millions of years of history, and the choices that I make to trim, according to my convenience and aesthetic preferences, each shape this collection of vines and the odors I experience.
David Spector is a former board president of the Hitchcock Center for the Environment and teaches biology at Central Connecticut State University.
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