Ravi Shankar’s poem “Crossings” describes something quite familiar to us all: the edge of the forest. The speaker is struck by the clear division between a forest and a field, by how different it feels, even, to step from one to the other, from the cathedral-like hush of the forest under the canopy to the wide-open world of a field. What Shankar is describing here is, in fact, an ecological phenomenon, one called an ecotone.
An ecotone is a transition between two biomes and can be regional (such as between an entire forest and grassland ecosystems) or local (such as the line between a forest and a field). The name comes from the Greek words oikos, meaning household or place to live (“ecology” is the study of the place you live!) and tonos, or tension. So an ecotone is a place where two environments are in tension.
The most interesting part of an ecotone is how it allows for blending of the different organismal communities. On either side of the boundary, species in competition extend as far as they can before succumbing to other species. The influence of these two communities on each other is called the edge effect. Some species actually specialize in ecotonal regions, using this transitional area for foraging, courtship, or nesting.
Terrestrial environments are not the only ones in which we can experience Shankar’s “threshold” between biomes. There are also land-to-water ecotones, such as marshes or wetlands, and strictly aquatic ecotones, such as estuaries, where a river meets the sea. Perhaps in these more dramatic transitions it is easier to see how some species can thrive in this unique habitat.
Even without knowing the biology behind ecotones, it is possible to sense the tension inherent in this boundary. When hiking on a hot summer day, when the trail leads into a forest it’s like an exhalation. We, as animals, sense the natural world much more acutely than society would like us to believe. Ravi Shankar, using the skills of the poet to express what the rest of us cannot verbalize, notes this feeling, writing:Even planes of motion shift from vertical navigation to horizontal quiescence: there’s a standing invitation to lie back as sky’s unpredictable theater proceeds. Suspended in this ephemeral moment after leaving a forest, before entering a field, the nature of reality is revealed.
“Crossings,” by Ravi Shankar. Read it here.
“Ecotone.” Wikipedia. Link.
Senft, Amanda. 2009. Species diversity patterns at ecotones. (Master’s thesis). University of North Carolina. Link.