Home » Gardening Information » Puzzles
Endangered Species—Gardening to Increase Biodiversity
Plants & Gardens News | Volume 13, Number 1 | Fall 1998
by Janet Marinelli
A few years ago, at a retirement party, I met Clancy, who was leaving his job after dedicating a lifetime to designing bits and pieces of the Big Apple's infrastructure. Clancy had heard that I'd written an environmental book, and he must have been expecting some bra-burning radical. No sooner did we shake hands than he said, "Janet, you know, the best environmentalist is not a banner-waving protester. The best environmentalist is an engineer who believes in God."
Now, anyone who can calculate the stress load on every cable of the Brooklyn Bridge seems like a rocket scientist to me, but still, I had to respectfully disagree with Clancy on this point. I'm convinced that the environmental heroes of the 21st century will be humble gardeners.
Why gardeners? Because, with our centuries of practical experience in growing plants, we're in a unique position to help save thousands of species that are in danger of extinction in the next century, due to habitat destruction and other transformations wrought by humans. Transformations such as the domestication of plants and animals, which has caused a rapid evolution, not only of designer lettuces and genetically engineered cows, but also of invasive weeds that threaten native species. For more than half a century, there has been intense speculation that the disturbances people are visiting on the planet are leading to increasing hybridization in the wild, and that the new habitats we leave in our wake and the countless new hybrids they support are part of an evolutionary upheaval of our own making.
A Sinister Force
Biologists once believed that hybridization between species was rare in nature. The prevailing opinion held that hybrids were seldom found because most organisms produced by the crossing of two species were sterile—the mule being the best-known example. In a 1947 article, however, a botanist named Edgar Anderson marshalled an impressive amount of evidence to the contrary, compiled by geneticists and other scientists, and especially by gardeners. After all, if horticulturists could produce hundreds of new varieties by crossing twoPhalaenopsis orchids, why shouldn't nature be able—and even more apt -- to do this sort of thing in the wild?
Anderson argued that one reason hybrids are so rare in nature is that they require their own peculiar habitats, which under ordinary natural conditions don't exist. He used two species of spiderwort native to the Ozarks to make his case. One, Tradescantia subaspera, thrives in rich woods at the foot of bluffs; the other, T. canaliculata, grows up above in full sun at the edge of the cliffs. Anderson was able to find few first-generation hybrids in the wild, though they crossed readily in an experimental garden. This he attributed to the fact that in all of the Ozark Plateau, there was little of the intermediate habitat—gravelly soil with a light covering of leaf litter in partial shade—required for the hybrid's survival.
Imagine, he mused, what would happen if these first-generation hybrids crossed. Taking into consideration only the three habitat characteristics mentioned above (the actual number of differences between the two species is much greater), he calculated that their offspring, with their reshuffled sets of genes, would require six new habitats in addition to the parents' two. Anderson pointed out that the number of different kinds of habitats demanded by the hybrids would rise exponentially with the number of basic differences between the different species. "With only 20 such basic differences (and this seems like a conservative figure)," he wrote, "over a million different recombined habitats would be needed."
Under natural conditions, this would be virtually impossible. Only some force causing ecological havoc could create the strange new habitats necessary for the survival of the new hybrids.
A Momentous Choice
In a subsequent article, Anderson and co-author G.L. Stebbins, Jr., came to an even more breathtaking conclusion. They pointed to new studies showing that evolution has proceeded not by slow, even steps as once thought, but rather in bursts of activity. This was what happened when overgrazing by the dinosaurs led to, in their words, "the greatest revolution in vegetation that the world has ever seen"—the replacement of conifers and other gymnosperms with the flowering plants that predominate today. In the new habitats thus created, the scientists surmised, some hybrids must have been at a selective advantage. Hybridization in these disturbed habitats in turn produced conditions under which more familiar evolutionary processes such as mutation and selection could proceed at top speed.
This, they concluded, is what's happening right now. At the moment, it is we humans who are "having a catastrophic effect upon the world's faunas and floras." We're doing what the dinosaurs did before us, only faster. According to Anderson and Stebbins, "The enhanced evolution which we see in our own gardens, dooryards, dumps, and roadsides may well be typical of what happened during the rise of previous ecological dominants."
With one big difference: The dinosaurs didn't know what they were doing. We do.
And so we are left with a momentous choice. We can deign to do nothing to stop the growing torrent of extinctions, and let evolution proceed on its current course—come what may for us as well as the other creatures on the planet. This might lead serendipitously to new forms of wild spiderwort or, more ominously, to a further proliferation of highly competitive weeds, such as the new bush honeysuckle hybrid Lonicera x bella, that will invade the few areas left relatively undisturbed by humans, exterminating the ancient species in their path. Or we can summon up the courage to play the leading role in the evolutionary drama for which we, the most aware and self-conscious of nature's creations, seem destined.
Dazzlingly Diverse
I believe that for now, our primary responsibility is to tend the wild gardens I described in the last issue of Plants & Gardens News, preserving and restoring those native species and plant communities that are the glorious incarnation of three and a half billion years of nature's creativity. How can we, in good conscience, let them slip away before our eyes? But we should also be free to let our imaginations run wild, to learn how to be the creators of biodiversity as well as its preservers and restorers.
In fact, we gardeners have been responsible for the expansion of diversity so evident in the development of food and ornamental crops. And in some ways we're producing ever more diversity, ever faster. It took centuries, for example, for us to create the many new vegetable varieties, from broccoli to cauliflower to collards, from a single species of wild cabbage. But in a matter of decades, beginning with the crossing of the yellow trumpet narcissus with the flat-flowered poet's narcissus (of the red-rimmed eye), in the quest for a narcissus with an orange trumpet, we've developed enough new daffodil hybrids to support an entire garden industry.
We're learning how to distinguish the lovely spiderwort from the destructive honeysuckle, the first step towards a new landscape art that promotes a richer evolution of life on Earth. Someday we'll know enough about ecology to be able to create totally new plant communities combining species from around the globe that add to, rather than subtract from, the planet's wonderful diversity of life forms -- and take the next great evolutionary leap in the dazzling history of gardening.
Brooklyn Botanic Garden Director of Publishing Janet Marinelli is editor of BBG's renowned series of quarterly gardening handbooks and the author of Your Natural Home and The Naturally Elegant Home. Janet is a champion of the gardener's role in the preservation of the planet, a philosophy that informs her P&G News column, "Down to Earth." It's a philosophy that also serves as the bedrock for her latest book, Stalking the Wild Amaranth: Gardening in the Age of Extinction. In Stalking the Wild Amaranth, Janet tells of her quest for a landscape art that protects disappearing species, both flora and fauna. It's a gardening journey marked by humor—ecologically sensitive gardening needn't be a dreary affair, Janet insists. "We can do our part," she says, "and still have flair and fun."