Languages:

Genetically Modified Crops

5. What effects could genetically modified crops have on the environment?

  • 5.1 What direct effects could genetically modified plants have on the environment?
  • 5.2 What indirect effects could genetically modified plants have on the environment?
    • 5.2.1 Agricultural practices
    • 5.2.2 Pesticide use
    • 5.2.3 Herbicide use
    • 5.2.4 Pest and weed resistance
    • 5.2.5 Difficult agricultural conditions
  • 5.3 How should these environmental effects be assessed?

5.1 What direct effects could genetically modified plants have on the environment?

The source document for this Digest states:

ENVIRONMENTAL IMPLICATIONS

Agriculture of any type - subsistence, organic or intensive - affects the environment, so it is natural to expect that the use of new genetic techniques in agriculture will also affect the environment. The ICSU, the GM Science Review Panel and the Nuffield Council on Bioethics, among others, agree that the environmental impact of genetically transformed crops may be either positive or negative depending on how and where they are used. Genetic engineering may accelerate the damaging effects of agriculture or contribute to more sustainable agricultural practices and the conservation of natural resources, including biodiversity. The environmental concerns associated with transgenic crops are summarized below along with the current state of scientific knowledge regarding them.

Releasing transgenic crops into the environment may have direct effects including: gene transfer to wild relatives or conventional crops, weediness, trait effects on non-target species and other unintended effects. These risks are similar for transgenic and conventionally bred crops (ICSU). Although scientists differ in their views on these risks, they agree that environmental impacts need to be assessed on a case-by-case basis and recommend post-release ecological monitoring to detect any unexpected events (ICSU, Nuffield Council, GM Science Review Panel). Transgenic crops may also entail positive or negative indirect environmental effects through changes in agricultural practices such as pesticide and herbicide use and cropping patterns.

Transgenic trees involve similar environmental concerns, although there are additional concerns because of their long life cycle. Transgenic micro-organisms used in food processing are normally used under confined conditions and are generally not considered to pose environmental risks. Some micro-organisms can be used in the environment as biological control agents or for bioremediation of environmental damage (e.g. oil spills), and their environmental effects should be assessed prior to release. Environmental concerns related to transgenic fish primarily focus on their potential to breed with and outcompete wild relatives (ICSU). Transgenic farm animals would probably be used in highly confined conditions, so they would pose little risk of environmental damage (NRC, 2002) (Box 22).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications

5.1.1 Gene flow

The source document for this Digest states:

Scientists agree that gene flow from GM crops is possible through pollen from open-pollinated varieties crossing with local crops or wild relatives. Because gene flow has happened for millennia between land races and conventionally bred crops, it is reasonable to expect that it could also happen with transgenic crops. Crops vary in their tendency to outcross, and the ability of a crop to outcross depends on the presence of sexually compatible wild relatives or crops, which varies according to location (Box 23 on page 70) (ICSU, GM Science Review Panel).

Scientists do not fully agree whether or not gene flow between transgenic crops and wild relatives matters, in and of itself (ICSU, GM Science Review Panel). If a resulting transgenic/wild hybrid had some competitive advantage over the wild population it could persist in the environment and potentially disrupt the ecosystem. According to the GM Science Review Panel, hybridization between transgenic crops and wild relatives seems “overwhelmingly likely to transfer genes that are advantageous in agricultural environments, but will not prosper in the wild … Furthermore, no hybrid between any crop and any wild relative has ever become invasive in the wild in the UK” (GM Science Review Panel, 2003: 19).

Whether the otherwise benign flow of transgenes into land races or other conventional varieties would itself constitute an environmental problem is a matter of debate, because conventional crops have long interacted with land races in this way (ICSU). Research is needed to improve the assessment of the environmental consequences of gene flow, particularly in the long run, and to understand better the gene flow between the major food crops and land races in centres of diversity (ICSU, GM Science Review Panel).

Weediness refers to the situation in which a cultivated plant or its hybrid becomes established as a weed in other fields or as an invasive species in other habitats. Scientists agree that there is only a very low risk of domesticated crops becoming weeds themselves because the traits that make them desirable as crops often make them less fit to survive and reproduce in the wild (ICSU, GM Science Review Panel). Weeds that hybridize with herbicide-resistant crops have the potential to acquire the herbicide-tolerant trait, although this would only provide an advantage in the presence of the herbicide (ICSU, GM Science Review Panel). According to the GM Science Review Panel, “Detailed field experiments on several GM crops in a range of environments have demonstrated that the transgenic traits investigated - herbicide tolerance and insect resistance - do not significantly increase the fitness of the plants in semi-natural habitats” (GM Science Review Panel, 2003:19). Some transgenic traits, such as pest or disease resistance, could provide a fitness advantage but there is little evidence so far that this happens or has any negative environmental consequences (ICSU, GM Science Review Panel). More evidence is required regarding the effect of fitness-enhancing traits on invasiveness (GM Science Review Panel).

Management and genetic methods are being developed to minimize the possibility of gene flow. The complete isolation of crops grown on a commercial scale, either GM or non-GM, is not currently practical although gene flow can be minimized, as it currently is between oilseed rape varieties grown for food, feed or industrial oils (GM Science Review Panel). Management strategies include avoiding the planting of transgenic crops in their centres of biodiversity or where wild relatives are present, or using buffer zones to isolate transgenic varieties from conventional or organic varieties. Genetic engineering can be used to alter flowering periods to prevent cross-pollination or to ensure that the transgenes are not incorporated in pollen and developing sterile transgenic varieties (ICSU and Nuffield Council). The GM Science Review Panel and other expert bodies recommend that GM crops that produce medical or industrial substances should be designed and grown in ways that would avoid gene flow to food and feed crops (GM Science Review Panel).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Gene flow

BOX 23
An ecologist's view of gene flow from transgenic crops
Most ecological scientists agree that gene flow is not an environmental problem unless it leads to undesirable consequences. In the short term, the spread of transgenic herbicide resistance via gene flow may create logistical and/or economic problems for growers. Over the long term, transgenes that confer resistance to pests and environmental stress and/or lead to greater seed production have the greatest likelihood of aiding weeds or harming non-target species. However, these outcomes seem unlikely for most currently grown transgenic crops. Many transgenic traits are likely to be innocuous from an environmental standpoint, and some could lead to more sustainable agricultural practices. To document various risks and benefits, there is a great need for academic researchers and others to become more involved in studying transgenic crops. Similarly, it is crucial that molecular biologists, crop breeders and industry improve their understanding of ecological and evolutionary questions about the safety of new generations of transgenic crops.

The presence of wild and weedy relatives varies among countries and regions. The chart shows examples of major crops grouped by their ability to disperse pollen and the occurrence of weedy relatives in the continental United States. This simple 2 x 2 matrix can be useful in identifying cases where gene flow from a transgenic crop to a wild relative is likely. For crops where no wild or weedy relatives are grown nearby - as with soybean, cotton and maize shown here in green - gene flow to the wild would not occur. Rice, sorghum and wheat have wild relatives in the United States and a relatively low tendency to outcross, which could allow transgenes to disperse into wild populations. The crops that have a high tendency to outcross and have wild relatives in the United States are shown in red. There is a high potential for gene flow between these crops and their wild relatives, so care should be taken in growing transgenic varieties that might confer a competitive advantage on their hybrids.

examples of major crops grouped by their ability to disperse pollen and the occurrence of weedy relatives in the continental United States

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops , Box 23

5.1.2 Non-target species

The source document for this Digest states:

Trait effects on non-target species

Some transgenic traits - such as the pesticidal toxins expressed by Bt genes - may affect non-target species as well as the crop pests they are intended to control (ICSU). Scientists agree that this could happen but they disagree about how likely it is (ICSU, GM Science Review Panel). The monarch butterfly controversy (Box 24 on page 71) demonstrated that it is difficult to extrapolate from laboratory studies to field conditions. Field studies have shown some differences in soil microbial community structure between Bt and non-Bt crops, but these are within the normal range of variation found between cultivars of the same crop and do not provide convincing evidence that Bt crops could be damaging to soil health in the long term (GM Science Review Panel). Although no significant adverse effects on non-target wildlife or soil health have so far been observed in the field, scientists disagree regarding how much evidence is needed to demonstrate that growing Bt crops is sustainable in the long term (GM Science Review Panel). Scientists agree that the possible impacts on non-target species should be monitored and compared with the effects of other current agricultural practices such as chemical pesticide use (GM Science Review Panel). They acknowledge that they need to develop better methods for field ecological studies, including better baseline data with which to compare new crops (ICSU).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications
Subsection Trait effects on non-target species

BOX 24
Does Bt maize kill monarch butterflies?
John Losey, an entomologist at Cornell University, published a research paper in the scientific journal Nature that seemed to prove that pollen from Bt maize killed monarch butterflies (Losey, Rayor and Carter, 1999). Losey and his colleagues found that when they spread the pollen from a commercial variety of Bt maize on milkweed leaves in the laboratory and fed them to monarch butterfly caterpillars, the caterpillars died.

Six independent teams of researchers conducted follow-up studies on the effects of Bt maize pollen on monarch butterfly caterpillars, published in 2001 in the Proceedings of the National Academy of Sciences of the United States of America. Although these studies agreed that the pollen used in the original study was toxic at high doses, they found that Bt maize pollen posed negligible risk to monarch larvae under field conditions. They based their conclusion on four facts: (a) the Bt toxin is expressed at fairly low levels in the pollen of most commercial Bt maize varieties, (b) maize and milkweed (the normal food of monarch butterfly caterpillars) are generally not found together in the field, (c) there is limited overlap in the time periods when maize pollen sheds in the field and monarch larvae are active and (d) the amount of pollen likely to be consumed under field conditions was not toxic. These studies concluded that the risk of harm to monarch butterfly caterpillars from Bt maize pollen is very small, particularly in comparison with other threats such as conventional pesticides and drought (Conner, Glare and Nap, 2003).

Many scientists are frustrated by the way the monarch butterfly controversy and other issues related to biotechnology were handled in the press. Although the original monarch butterfly study received worldwide media attention, the follow-up studies that refuted it did not receive the same amount of coverage. As a result, many people are not aware that Bt maize poses very little risk to monarch butterflies (Pew Initiative, 2002a).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops , Box 24

5.2 What indirect effects could genetically modified plants have on the environment?

    • 5.2.1 Agricultural practices
    • 5.2.2 Pesticide use
    • 5.2.3 Herbicide use
    • 5.2.4 Pest and weed resistance
    • 5.2.5 Difficult agricultural conditions

5.2.1 Agricultural practices

The source document for this Digest states:

Indirect environmental effects

Transgenic crops may have indirect environmental effects as a result of changing agricultural or environmental practices associated with the new varieties. These indirect effects may be beneficial or harmful depending on the nature of the changes involved (ICSU, GM Science Review Panel). Scientists agree that the use of conventional agricultural pesticides and herbicides has damaged habitats for farmland birds, wild plants and insects and has seriously reduced their numbers (ICSU, GM Science Review Panel, Royal Society). Transgenic crops are changing chemical and land-use patterns and farming practices, but scientists do not fully agree whether the net effect of these changes will be positive or negative for the environment (ICSU). Scientists acknowledge that more comparative analysis of new technologies and current farming practices is needed.

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Indirect environmental effects

5.2.2 Pesticide use

The source document for this Digest states:

Pesticide use

The scientific consensus is that the use of transgenic insect-resistant Bt crops is reducing the volume and frequency of insecticide use on maize, cotton and soybean (ICSU). These results have been especially significant for cotton in Australia, China, Mexico, South Africa and the United States (Chapter 4). The environmental benefits include less contamination of water supplies and less damage to non-target insects (ICSU). Reduced pesticide use suggests that Bt crops would be generally beneficial to in-crop biodiversity in comparison with conventional crops that receive regular, broad-spectrum pesticide applications, although these benefits would be reduced if supplemental insecticide applications were required (GM Science Review Panel). As a result of less chemical pesticide spraying on cotton, demonstrable health benefits for farm workers have been documented in China (Pray et al., 2002) and South Africa (Bennett, Morse and Ismael, 2003).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Pesticide use

5.2.3 Herbicide use

The source document for this Digest states:

Herbicide use

Herbicide use is changing as a result of the rapid adoption of HT crops (ICSU). There has been a marked shift away from more toxic herbicides to less toxic forms, but total herbicide use has increased (Traxler, 2004). Scientists agree that HT crops are encouraging the adoption of low-till crops with resulting benefits for soil conservation (ICSU). There may be potential benefits for biodiversity if changes in herbicide use allow weeds to emerge and remain longer in farmers' fields, thereby providing habitats for farmland birds and other species, although these benefits are speculative and have not been strongly supported by field trials to date (GM Science Review Panel). There is concern, however, that greater use of herbicides - even less toxic herbicides - will further erode habitats for farmland birds and other species (ICSU). The Royal Society has published the results of extensive farm-scale evaluations of the impacts of transgenic HT maize, spring oilseed rape (canola) and sugar beet on biodiversity in the United Kingdom. These studies found that the main effect of these crops compared with conventional cropping practices was on weed vegetation, with consequent effects on the herbivores, pollinators and other populations that feed on it. These groups were negatively affected in the case of transgenic HT sugar beet, positively affected in the case of maize and showed no effect in spring oilseed rape. They conclude that commercialization of these crops would have a range of impacts on farmland biodiversity, depending on the relative efficacy of transgenic and conventional herbicide regimes and the degree of buffering provided by surrounding fields (Royal Society, 2003:1912). Scientists acknowledge that there is insufficient evidence to predict what the long-term impacts of transgenic HT crops will be on weed populations and associated in-crop biodiversity (GM Science Review Panel).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Herbicide use

5.2.4 Pest and weed resistance

The source document for this Digest states:

Pest and weed resistance

Scientists agree that extensive long-term use of Bt crops and glyphosate and gluphosinate, the herbicides associated with HT crops, can promote the development of resistant insect pests and weeds (ICSU, GM Science Review Panel). Similar breakdowns have routinely occurred with conventional crops and pesticides and, although the protection conferred by Bt genes appears to be particularly robust, there is no reason to assume that resistant pests will not develop (GM Science Review Panel). Worldwide, over 120 species of weeds have developed resistance to the dominant herbicides used with HT crops, although the resistance is not necessarily associated with transgenic varieties (ICSU, GM Science Review Panel). Because the development of resistant pests and weeds can be expected if Bt and glyphosate and gluphosinate are overused, scientists advise that a resistance management strategy be used when transgenic crops are planted (ICSU). Scientists disagree about how effectively resistance management strategies can be employed, particularly in developing countries (ICSU). The extent and possible severity of impacts of resistant pests or weeds on the environment are subject to debate (GM Science Review Panel).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Pest and weed resistance

5.2.5 Difficult agricultural conditions

The source document for this Digest states:

Abiotic stress tolerance

As we saw in Chapter 2, new transgenic crops with tolerance to various abiotic stresses (e.g. salt, drought, aluminium) are being developed that may allow farmers to cultivate soils that were previously not arable. Scientists agree that these crops may be environmentally beneficial or harmful depending on the particular crop, trait and environment (ICSU).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental implications, Subsection Abiotic stress resistance

5.3 How should these environmental effects be assessed?

The source document for this Digest states:

Environmental impact assessment

There is broad consensus that the environmental impacts of transgenic crops and other living modified organisms (e.g. transgenic seeds) should be evaluated using science-based risk assessment procedures on a case-by-case basis depending on the particular species, trait and agro-ecosystem. Scientists also agree that the environmental release of transgenic organisms should be compared with other agricultural practices and technology options (ICSU and Nuffield Council).

As we saw above, food safety assessment procedures are well developed and the FAO/WHO Codex Alimentarius Commission provides an international forum for developing food safety guidelines for transgenic foods. By contrast, there are no internationally agreed guidelines and standards for assessing the environmental impacts of transgenic organisms (ICSU). Scientists agree that there is a need for internationally and regionally harmonized methodologies and standards for assessing environmental impacts in different ecosystems (ICSU; FAO, 2004). The role of international standard-setting bodies in providing guidance for risk analysis is described below.

According to the ICSU, regulators in different countries typically require similar types of data for environmental impact assessments, but they differ in their interpretation of these data and of what constitutes an environmental risk or harm. Scientists also differ on what the appropriate basis for comparison should be: with current agricultural systems and/or baseline ecological data (ICSU). An FAO expert consultation (2004) agreed that the impacts of agriculture on the environment were much greater than the measurable impacts of a shift from conventional to transgenic crops, so the basis of comparison is important.

Scientists also disagree about the value of small-scale laboratory and field trials and their extrapolation to large-scale effects, and it is unclear whether modelling approaches that incorporate data from geographical information systems would be useful in predicting the effects of living modified organisms (LMOs) in different ecosystems (ICSU). The scientific community recommends that more research is needed on the post-release effects of transgenic crops. There is also a need for more targeted post-release monitoring and better methodologies for monitoring (ICSU; FAO, 2004).

Source & ©: FAO "The State of Food and Agriculture 2003-2004"
Chapter 5: Health and environmental impacts of transgenic crops 
Section Environmental impact assessment


FacebookTwitterEmail
Themes coveredLeaflets