Gene flow occurs widely in nature through the movement of pollen. No crop can be 100% contained unless it is grown inside an enclosed environment such as a greenhouse. It is important to evaluate the risk of pollen from GE crops spreading to where it is not intended and to assess if there would be significant effects to the environment it this did occur.
The largest cause of unwanted gene flow is pollen transfer from GE fields to nearby non-GE fields growing related crops. Pollen transfer between crops can be limited by providing adequate spacing between related plants and timing pollination so that it does not occur when neighboring crops are flowering. The risks involved need to be evaluated for each plant based on the characteristics of the plant's pollen. Please see the co-existence page for more information about pollen control between GE and non-GE crops.
Pollen from agricultural crops often reaches wild plants growing nearby. If the plants are related, fertilization can occur and the wild offspring may carry the gene that was engineered into the agricultural crop. One concern is that herbicide resistant genes will be found in weeds making them difficult to control. Most of the GE agricultural plants in the United States do not have closely related wild relatives. If hybridization does occur, the plants can still be controlled with other herbicides or by other means. Field trials of glyphosate resistant creeping bentgrass, which was being developed for golf course use, showed gene flow to related plants at large distances leading to further field trials being blocked by court order (Watrud et al., 2004). In that case, the seeds and pollen of the GE plants were light and could travel long distances, and wild relatives of bentgrass were present.
Only one native Hawaiian plant has a high risk of hybridizing with GE agricultural crops (Munster and Wieczorek, 2006). G. tomentosum, a native cotton, is related to and is known to hybridize with cultivated cotton. To decrease the possibility of gene flow, USDA-APHIS requires an isolation distance, currently suggested to be ~12 m, between cultivated transgenic cotton and wild or non-transgenic species. This spacing is based on the distance that viable cotton pollen travels in the environment.
An unlikely method for gene flow is non-sexual transfer of genes between organisms, known as horizontal gene transfer. Bacteria and viruses are known to exchange DNA easily and there are concerns that bacteria will absorb and carry the transgenes to other plants and organisms. While bacteria normally do not integrate plant DNA, there is concern that the methods used to transfer genetic sequences into the plant's DNA will result in DNA segments that can be passed more easily. The main fear for horizontal gene transfer is that antibiotic genes that have been used as part of the genetic engineering process may transfer antibiotic resistance to harmful bacteria (See Health Concerns page).
The third method for genes to escape would be if the genetically engineered plants themselves became established in the environment. Most crop plants have significant limitations in their growth and seed dispersal habits that prevent them from surviving long without constant nurture by humans. Non-food plants, such as grasses, may pose a greater risk if they can form hybrids with wild plants or if they have a characteristic that enhances their survival. Plants are evaluated for their potential to become weeds as part of the permitting process. Please see the Regulation of Agricultural Biotechnology page.
Herbicide and insecticide resistance
The use of herbicide resistant GE crops can reduce the amount of herbicides sprayed onto fields and increase the use of environmentally friendly, no-till methods. However, frequent use of a single herbicide will eventually cause resistance weed strains to evolve. Weeds that have more natural resistance will survive and reproduce, changing the genetics of the weed population over time. The varieties of weeds growing in some areas have shifted to those with greater natural resistance, however this shift is also due to indirect causes such as reduced tilling (Owen, 2008). Crop management techniques such as crop rotation and rotation of herbicides can decrease the rate that these problems occur.
Insects possess a remarkable capacity to adapt to selective pressures. There is concern that large-scale adoption of Bt crops will result in rapid build-up of resistance in pest populations. Agricultural practices affect the the rate and impact of this problem (Tabashnik et al,. 2008; Liu et al, 2008). Modification of the Bt toxins or stacking of genes can be used to further delay resistance to Bt (Kish, 2008).
Impacts on “non-target” species
Some environmentalists maintain that once modified crops have been released into the environment, they could have unforeseen and undesirable effects. Although transgenic crops are rigorously tested before being made commercially available, not every potential impact can be foreseen. One area of risk that is frequently cited is the effect on non-target organisms. This issue came to the forefront following a study that showed negative effects when monarch caterpillars ingested Bt corn pollen. Monarchs were studied because they are related to the target insect, the European corn borer. Follow-up studies have evaluated if monarchs are harmed under normal feeding conditions (Sears et al., 2001). These studies have shown that there is low risk to wild monarch butterflies because the amount of pollen ingested in the wild is lower than in the study and the varieties of Bt crops currently in use are less toxic to monarchs.
Ongoing research is looking at the effects of Bt on bees, beetles and other arthropods. Overall, there may be a beneficial effect to non-target organisms if Bt fields are compared to fields that are being sprayed with insecticides (Marvier et al., 2007).
Loss of biodiversity
Environmentalists, scientists and farmers, are very concerned about the loss of biodiversity in both our natural environment and in food crops. The success of GE crops could result in large areas of land being planted with a single crop variety that shares the same susceptibility to disease or environmental change. Increased adoption of conventionally bred crops raised similar concerns in the past century, which led to extensive efforts to collect and store seeds of as many varieties as possible of all major crops. These “heritage” collections in the USA and elsewhere are maintained and used by plant breeders.
Modern biotechnology has dramatically increased our knowledge of how genes express themselves and has highlighted the importance of preserving genetic material. Agricultural biotechnologists work to make sure that we maintain the pool of genetic diversity of crop plants needed for the future. While transgenic crops help ensure a reliable supply of basic foodstuffs, U.S. markets for specialty crop varieties and locally grown produce appear to be expanding rather than diminishing. Thus the use of genetically modified crops is unlikely to negatively impact biodiversity.