Rocky precipices around the world provide a surprisingly sheltered environment for plants and animals
In 1904, Paul Vogler, a Swiss scientist who had noted the decline of the English yew (Taxus baccata) in Prussia, set out to write the botanical obituary for this species in his country. Intensive use of its valuable wood had fueled widespread clear-cutting in the 19th century, which had virtually eliminated this yew from large parts of Europe. Although the surviving trees were restricted to rather inaccessible spots—mostly rocky outcrops on the vertical faces of cliffs—to his surprise and delight Vogler found that the English yew in Switzerland was nevertheless wide-ranging.
In the same year, Max Oettli, a colleague of Vogler, published a report about the plants growing on various crags and bluffs in the southern Alps. He concluded that for a variety of plant species, these precipices served as refuges. Both Vogler and Oettli, however, failed to recognize that their findings might be part of a general pattern. Decades passed with only occasional references in the botanical literature to the rare plants that liked to make a home of cliffs. Few scientists realized that these nearly vertical surfaces support rich communities that are quite distinct from their level-ground neighbors.
Only after World War II did a few people, such as Peter Davis at the University of Edinburgh and, more recently, Peter Wardle of the Department of Scientific and Industrial Research in New Zealand, fully grasp this point and draw attention to it. Yet most other naturalists remained oblivious. Even the well-known ecologist John Curtis of the University of Wisconsin at Madison, writing in his 1959 classic text, The Vegetation of Wisconsin, argued that cliffs "represent a geological feature, not a biotic community type." Curtis, like most other scientists, did not see cliffs as proper places at all. We, too, held this prejudice until the mid-1980s, when a chain of intriguing events led us to alter our thinking—and, ultimately, to adopt the ecology of cliffs as the focus of our research.
At that time, we were trying to assemble a team of ecologists to address a central question: How does environmental stress organize a biological community? We anticipated that if we selected a particular habitat to study with the tools of our specialized subdisciplines, we could achieve a comprehensive answer, at least for one chosen setting.
Clearly, we needed to examine a place with a strong environmental gradient. The Niagara Escarpment of southern Ontario—which we each had seen many times, at least from a distance—appeared to offer an obvious contrast between the open cliffs and the adjacent woodlands. So we selected this locale for our program. Closer inspection of the trees on the 30-meter bluff revealed a striking pattern: The exposed wall of rock supports a sparse cover of eastern white cedar, but almost none of the tree species found in the surrounding forest (which is composed mostly of different kinds of hardwoods, including oaks, ashes and maples) reside on the cliffs.
This incongruity, we thought, signaled that the different types of trees had evolved to exploit specialized environmental conditions. Two years of work, however, showed that all these species preferred the rather lush properties of the adjacent forest; none had specific adaptations for life on the cliffs. What is more, a careful search of the bluffs for seedlings turned up almost no newly established plants. We could think of only one explanation: We imagined that all this land had been cleared sometime in the 19th century and that the more aggressive hardwoods took over the prime spots on level ground, leaving the cedars to fill the gap left on the cliff. Little did we know how wrong our guess would turn out to be.