Genetically engineered crops include varieties of corn, cotton and potatoes. These plants have a bacterial gene from Bacillus thuringiensis (Bt) inserted into their genome. The Bt gene codes for the synthesis of a toxin that kills insect larvae. Other crops are genetically modified to withstand a specific herbicide. While these crops can potentially feed the world’s growing population, they also pose serious risks to the natural variety of organisms, or biodiversity.
Herbicides are toxic to many species. When a herbicide is applied across agricultural landscapes, harmful chemicals enter natural ecosystems. Many believe that herbicide-resistant crops encourage increased use of herbicides, and when more herbicides are used, even more chemicals end up in natural systems. These chemicals kill native plants that feed animals and sicken amphibians directly, causing a decrease in biodiversity.
When genes from genetically modified crops enter the environment, they have the potential to disrupt natural plant communities, threaten biodiversity and enter human food supplies. In September 2000, StarLink, a variety of Bt corn unapproved for human consumption was discovered in taco shells in the United States. During the following months, StarLink was also discovered in various yellow-corn products, some outside the country. At first, some growers were suspected of ignoring agreements not to sell StarLink to mills. However, interviews with the growers revealed that many had either not received clear instructions about not selling StarLink to mills, or were told that the unapproved variety would be approved by harvest time. The exact points at which StarLink entered the supply line remain unknown, and according to a series from Cornell Cooperative Extension’s Genetically Engineered Organisms Public Issues Education Project, it may have made its way into more than half of the United States' corn supplies.
Areas where crop species originate are particularly vulnerable to out-crossing with local varieties. In Mexico, where over 100 unique varieties of corn exist, genetically engineered corn is prohibited. Despite the ban, genes from genetically engineered corn have been found in Mexican corn. Plant geneticists at U.C. Riverside have shown that gene flow from many conventionally-bred crops increases weediness in wild relatives and there are a few instances in which crop plants have become weeds. Increased weediness is a concern when genetically engineered plants are able to out-compete other species by producing more seed, dispersing pollen or seed further, or growing more vigorously in specific environments. Transgenic sunflowers can produce 50 percent more seed than their traditional counterparts and some researchers are concerned that genetically modified plants may gradually displace valuable genetic diversity.
The toxins produced by genetically engineered crops threaten biodiversity, and according to the Sierra Club, genetic engineering should be considered environmentally dangerous. A Cornell University study shows that the Bt toxin kills the larvae of beneficial, non-target species, such as moths and butterflies. Similar studies indicate a reduction of other beneficial species, including lacewings and ladybugs. The toxin also persists in the root systems of Bt corn and in plant residues long after crops are harvested and may have detrimental consequences for millions of microorganisms that live in the soil and maintain its fertility. When Bt toxin binds to soil particles, it can persists for two to three months. This can have negative impacts on aquatic and soil invertebrates, as well as nutrient cycling processes that occur in bacterial species.