IS there anything to all of this? Can we really save (or destroy) the planet in our very own back yards?
The basic claims of the horticultural nativists seem to fall into two categories. One is what might be termed direct effects. Several thousand native plant species are considered to be under threat from habitat destruction or competition from alien species. Accidental imports that stowed away in ships' ballast or animal bedding and took root on American soil, or deliberate imports, such as cornflower and English ivy, that jumped the garden fence and ran wild, now make up as much as a quarter of the wild flora of New England; certain regions, such as the Hawaiian Islands, face an even more acute threat from escaped imports. Native plants provide habitat for other plant and animal species, so their loss threatens to have extended ecological consequences. By eschewing particularly invasive and escape-prone aliens, gardeners can diminish the pressure on native species in their vicinity. (On the other hand, an embarrassingly large number of alien and highly invasive plants turn out to contribute valuable food and habitat to endangered fauna, including monarch butterflies and a number of birds.)
And by cultivating rare native plants, gardeners can help to ensure that these species survive even if they vanish altogether from the wild. Steven Clemants, the Brooklyn Botanic Garden's director of science, notes that a number of cultivated native plants have already disappeared from the wild in America; franklinia, a beautiful flowering tree discovered in the late eighteenth century by the American botanist John Bartram along the Altamaha River in Georgia, has never been seen in the wild since. All franklinias in gardens today are probably derived from the specimens Bartram collected.
Of course, there is a danger that enthusiastic native-plant gardeners will create a demand for rare species that will be met not by propagation by ethical nurserymen but by illegal collecting from the wild. That has certainly been the fate of some much-coveted orchids and cacti.
The second argument about backyard biodiversity, which is made far more often and with considerably more passion, is an indirect one -- that the gene pools of even well-established cultivated species are being dangerously diminished by the practices of seed companies. Since the 1940s plant developers have increasingly dropped old standard varieties in favor of hybrids, which are produced by pollinating plants from one breeding line with the pollen from another and then collecting the seeds that result. Conventional plant breeding involves repeatedly selecting individual plants that show a desirable characteristic, such as disease resistance or high yield or earliness. But strains that happen to have one of these desirable traits often have severe shortcomings in other departments that can be weeded out only by selective breeding over many generations. Hybridization allows for a swift melding of the best traits of several different strains. Moreover, there is a long-recognized bonanza inherent in the very act of hybridization -- hybrid vigor. Crossing two genetically diverse parents tends to cancel out debilitating recessive traits that can become fixed in an inbred line. Thus hybrid seeds produce plants with qualities that are far more consistent than, and often far superior to, those of either of the parent lines. Hybrids are the force that fomented the Green Revolution, more than doubling the yield per acre of rice, corn, and other grains.
But the unique combination of traits produced during hybridization is lost in the next generation. Even when a hybrid plant pollinates itself, its genes recombine by the laws of chance; the seeds a hybrid produces carry a spectrum of unpredictable traits. The only way to reproduce the hybrid's desirable characteristics is by recrossing the parent lines each year. In other words, the farmer or gardener has to go back to the seed company each year for fresh seed. That fact has perhaps inevitably reinforced the feeling on the part of grassroots enthusiasts that they are striking a blow for the common man, as well as for biodiversity, by refusing to embrace the hybrid revolution. By growing "open-pollinated" -- that is, nonhybrid -- strains of vegetables and flowers, the seed savers argue, they are maintaining a living, standing library of genetic diversity that may be crucial to the viability of these species. To quote from "Starting From Seed,"
The genetic diversity of food crops everywhere is eroding at an unprecedented and accelerating rate.... without ... infusions of genetic diversity, food production worldwide is at risk from epidemics and infestations.... about six percent of all vegetable varieties are dropped from seed catalogs every year.... the varieties that are dropped often represent the life's work of several generations of gardeners or breeders.
One frequently cited example of the danger of relying on hybrids is the 1970 outbreak of southern corn blight, which destroyed 15 percent of the corn crop in the United States; subsequent investigation showed that 71 percent of the U.S. crop that year had been planted in six hybrid varieties that all carried a previously undetected genetic susceptibility to the disease.
The trouble with this line of argument for backyard biodiversity, however, is that the heirloom open-pollinated varieties grown by home gardeners are hardly the last bastion of genetic diversity. In fact, they are not even a terribly significant one. The mistake of 1970 was not corrected by someone who had ordered Hopi Magenta Parch Corn from the Seeds of Change catalogue; it was corrected by those "agrichemical conglomerates" who have been systematically collecting and preserving samples of corn strains from all over the world on a huge scale. Tapping into their germ-plasm libraries, the companies were quickly able to develop new hybrids that eliminated the defective gene. Indeed, the very business of producing hybrids creates an enormous incentive for the discovery and preservation of what breeders usually term "landraces" -- traditional open-pollinated varieties.