My struggle with weeds over the years has made me aware of the damage they can inflict in gardens, farms, and native ecosystems. I have learned to be vigilant and untrusting of even the smallest, innocent-looking weed seedling and yank it out upon first sight. I have grown curious as to why many weeds spread invasively whereas most crops and native species do not. Both crops and weeds have pollen that can spread widely, right? So why the difference in invasiveness? To answer this question, we need to consider where weeds originate, why they persist and reproduce, and how domesticated crop plants differ from weeds. These issues have become important in the debate about the potential impact of genetically engineered (GE) crops. Some people worry that the presence of GE pollen in the environment will create a new breed of invasive, uncontrollable weeds that will overrun pristine environments or irrevocably alter the genetic makeup of native species.
The weeds in my garden share similar characteristics. Consider yellow star thistle, a non-native plant with gray-green lizard-shaped woolly leaves that is toxic to horses. If ingested they can become ill with a neurological disorder resembling Parkinson's disease in humans. Over the last 150 years this weed has spread over twelve million acres in California. Yellow star thistle possesses an important invasive trait: it is able to complete its life cycle quickly. It germinates with the first rains of fall, sending its roots down to depths of six feet or more where it sucks up all the moisture so that there is none left for the slower-germinating native species. Second, yellow star thistle is an alien weed that has evolved adaptations that allow it to survive and spread. Introduced from Europe, it traveled to California from Chile as a stowaway in alfalfa seed, where it was then inadvertently planted with the hay crop. Third, yellow star thistle produces a large amount of seed and has aggressive vegetative structures. In midsummer, one yellow star thistle plant produces seedheads bearing long tan spikes that can yield 100,000 seeds. These examples illustrates the ways in which weeds can "outsmart" their domesticated cousins and create havoc.
In contrast to invasive weeds, domesticated crops are tame. Consider a field of sweet corn-tall plants with gaudy tassels perched on top of single stems, and large ears. These traits--large fruit, reduced branching, gigantism, reduced seed dispersal, and a lack of genetic diversity--are all signs of domesticity. In the California Central Valley, this corn is grown on farms located quite close to the inner coast range areas. Because of their proximity, it would seem that the crop plants could escape to these foothills. They have not. The foothills are abundantly covered with weedy oats, bromes, and starthistle, but domesticated crops from Central Valley farms are notably absent. There is no corn, no soybean, no alfalfa, no cotton, no tomatoes, no safflower nor rice growing in these areas. Although these domesticated plants are also aliens--tomatoes and corn from Central and South Americas, cotton from what is now Pakistan, safflower and alfalfa from the Near and Middle East, and rice from China, any residual weediness has been eliminated through many years of breeding. This is one of the reasons that the genetically modified corn and cotton, grown here for 150 years, have not established in the foothills. GE cotton and corn, the primary transgenic crops grown in the Valley, are not likely to survive either. After all, a GE crop is still a crop and crops make lousy weeds. The traits that make these plants good for farmers make it hard for them to survive in the wilderness.
The Impact on Pollination
What about GE pollen? Will it drift over to the nearby foothills to create a new kind of weed that will pollute native ecosystems? What if transgenes move from a GE crop to a weedy relative? …