- What is biodiversity and what are people concerned about in relation to the widespread cultivation of GM crops?
Biodiversity refers to the number of different species that are present in a given location, along with the numbers of individuals in each species and the genetic variability within the species. More formally, biodiversity is defined as “the variability among living organisms from all sources, including, among others, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems” (Convention of Biological Diversity, 1992).
With the widespread planting of GM crops, the following questions have been asked frequently:
- Is it possible that GM crops cause the reduction of the number of species present or the amount of or genetic diversity within species (wild and/or cultivated)?
- Is it possible that local farmers stop planting traditional varieties because the GM crops provide useful traits (such as insect resistance) not available in the traditional varieties?
- Is it possible that the levels of genetic diversity in the traditional varieties will be reduced through hybridization with the GM crop?
- Is it possible that GM crops invade the natural environment and outcompete with native species?
- Is it possible that GM crops hybridize with wild relatives and make them more weedy or invasive?
- Is it possible that transgenes cause unintended (secondary) effects in natural populations?
- Is it possible that GM crops negatively affect species diversity in centres of diversity?
- Is it possible that GM crops negatively impact on non-target species?
- Is it possible that GM crops negatively impact on the non-living components of the release environment, damaging or polluting the air, soil or water?
These concerns are discussed more fully below.
- Could GM crops reduce species abundance or levels of genetic diversity within species?
This could happen only if GM crops: (1) replace crop varieties (including traditional land races), (2) invade natural habitats and outcompete with the wild species, (3) hybridize with
wild relatives making them more invasive and allowing them to outcompete with other wild species, or (4) reduce the abundance of non-target species by producing compounds that reduce species numbers.
- Could GM crops replace traditional crop varieties?
Traditional crop varieties, also called land races, are crops grown by subsistence farmers. They represent an intermediate stage of domestication between wild ancestors and modern varieties. Land races continue to change over time because of cross pollination and selection by farmers.
Will subsistence farmers stop planting traditional varieties because they perceive that GM crops are superior? Will the levels of genetic diversity in the traditional varieties be reduced through cross pollination with the GM crops?
To answer these concerns, there is a need to understand that:
- Farmers decide on what they will plant, based on what varieties are available to them and which ones they think will benefit them the most. For a farmer to plant a new variety, he needs to feel that his income and food security would be enhanced by growing that new variety.
- Most farmers know how to maintain the purity of traditional varieties if they so wish, even if grown in the same field or next to a field with other traditional and improved types1. This has been well documented.
- Small-holder farmers commonly share seeds, and usually plant mixed-seed populations or different varieties side by side to generate hybrid populations1. They have been doing this kind of informal breeding, either consciously or unconsciously, since plants were first domesticated thousands of years ago.
Based on this, the possibility that GM varieties will replace traditional varieties is no different than that which has been posed by conventionally bred varieties for decades. While there are numerous instances where conventional varieties have replaced traditional ones, the change has generally been made by commercial farmers rather than subsistence farmers. Subsistence farmers rely on the diversity of their land races for yield stability from year to year, and they rarely have the resources necessary to grow modern varieties.
When a farmer saves seed from a cross between his or her traditional variety and another land race or improved variety, levels of genetic diversity can actually increase in his or her field rather than decrease. Genetic variability in a farmer’s field will only be reduced if he or she saves seed from only a very limited number of individuals each season, or he or she stops growing the traditional varieties altogether. But most subsistence farmers are interested in maintaining genetic diversity, not reducing it. Even when commercial varieties are grown for a cash crop, small-holder farmers frequently also plant traditional varieties, because these are preferred for household consumption. Thus, where there is a preference for traditional varieties, they will be maintained in communities.
In addition, genetic variety is valuable for plant breeders, who also maintain a gene bank of different varieties. There has been concern that the agronomic biodiversity of crops will
decrease with the adoption of GM crops, because farmers will stop growing commercial varieties that do not have the new genetic improvements. However, the developers of GM crops have found that it is necessary to have the new genetic improvements in varieties that are best suited to different growing environments. For this reason, the GM traits are bred into numerous commercial varieties to ensure that farmers have access to the new genes in the varieties that are already best suited to their farms.
- Could GM crops invade natural environments and outcompete with native species?
Whether a GM crop persists in the natural environment and competes with native species is dependent on the invasiveness of the crop species and how much the GM trait affects that invasiveness2. If the specific kind of trait that is expressed in a GM crop allows it to grow better in response to challenges from the local environment, it may provide an advantage in a non-agricultural situation. If this should happen, then the crop might outcompete with wild species, resulting in a decrease in biodiversity.
To date, the traits that have been engineered into GM crops (insect or virus resistance, herbicide tolerance, or compositional changes) have not noticeably enhanced the ability of these crops to invade unmanaged habitats and outcompete with wild species. However, all GM crop approvals include an evaluation on a case-by-case basis as to whether the new traits pose a potential threat to native species. Tiered risk assessment procedures have been developed to do this by evaluating the biology of the crop and the nature of the engineered trait.
- Could GM crops hybridize with wild relatives making them more weedy or invasive?
If a GM crop is capable of crossing with a wild relative, then an engineered trait might be transferred to progeny of the wild relative. As was previously mentioned, whether an escaped transgene persists and has an impact on the abundance and competitiveness of a native species will be strongly associated with the nature of the new genes, and how invasive the recipient species was before the genes became incorporated2.
If the new trait causes an advantage that allows the recipient populations to increase in number and spread more easily from the release environment, then other native species could be outcompeted. However, none of the traits that have been engineered into GM crops to date including insect, virus and herbicide resistance has the potential to enable a GM event to outcompete with wild species in nature.
With regard to agricultural practices, the new trait, such as herbicide tolerance, might allow wild species to become tolerant of the current measures used to control individuals in an agronomic field. If this should happen, then the wild species could present a new weed problem. If herbicide resistant weedy species become abundant, farmers will have to return to the method of weed control they used before the GM crop or utilize herbicides with different modes of action. Development of resistance to herbicides is not an issue specific to GM crops. Herbicide resistance evolves naturally in wild species even without the presence of GM crops. This resistance means farmers need to use diverse weed control measures to limit the development of herbicide resistance and to remove herbicide resistant weeds. The same diverse weed control practices are needed for the cultivation of GM crops3
Future GM crops will need to be evaluated on a case-by-case basis as to whether gene flow from them could significantly alter the competitiveness of native species. Tiered risk assessment approaches have been developed to assess these risks.
- Could transgenes cause unintended (secondary) effects in natural populations?
These unplanned effects result from what are called “epistatic effects” or “pleiotropic effects”. Epistatic effects occur where the transgenes interact uniquely with the genes of the native species to produce an unexpected characteristic. Pleiotropic effects occur when the transgenes influence more than the target trait.
During the selection of events for commercial use, researchers compare the events to the conventional recipient to observe agronomic, phenotypic and compositional changes that might result from unintended changes. Events with different, unwanted phenotypes, composition or agronomic performance are eliminated during the selection process. The presence of unintended effects cannot not be completely excluded, but an effort is made to help ensure that the changes are those intended by the genetic improvement, and not just random changes to the plant genome.
- Could GM crops negatively affect species diversity in “centres of diversity”?
A centre of diversity for a particular plant species is the area where a high degree of genetic variation of that species is found. The centre of diversity of a crop can also be its centre of origin. Centres of diversity are considered to be important storehouses of genes valuable for breeding new characteristics into crops.
Concerns have been raised that the planting of transgenic crops in “centres of species diversity” could result in losses of genetic variability4. This could result from cross pollination between transgenic crops and close relatives that reduces the level of genetic diversity in the centre of diversity, or from transgenic crops outcompeting with wild types in centres of diversity and causing local varieties (and any rare genes they may contain) to become extinct.
An awareness and understanding of crop diversity and centres of diversity is used to help ensure that approved transgenic crops are not released and used in a way that will reduce genetic diversity in the agronomic and wild species. This genetic diversity is a valuable tool for plant breeders as it contains genes that could be used in future crop improvements.
Importantly, most crop progenitors are dispersed over a wide geographical range and their numbers are large, making it unlikely that wild type species will become extinct as a result of cultivation of transgenic crop varieties. Care is taken to recognise vulnerable and endangered species in release environments and to ensure that the impact on these is well understood and documented prior to approval for GM production. In addition, crops with multiple centres of diversity (wheat, sorghum, pearl millet, barley, pea, lentil, chickpea, flax, maize and lima bean), and those with no discernible centres of diversity (radish, cole crops and bottle gourd) have fewer concerns about loss of wild type diversity 4.
Paradoxically, cross pollination between crops and wild species could result in higher levels of genetic diversity in the wild relatives when the genes of the wild species and the crop are blended together. As the hybrid population evolves, alleles may be lost, but the recipient population is likely to carry high levels of diversity for many generations.
Overall, the impact of cross pollination from a transgenic crop is fundamentally no different than that from conventionally bred crops, but the impact of the transgenes must be evaluated to fully understand and manage unwanted effect.
- Could GM crops negatively impact on non-target species?
Non-target species are organisms which were not the intended targets, but that may be adversely affected by GM crops. These include pollinators, other beneficial species, soil organisms, endangered species, etc.
Indirect effects on non-target species are a concern for crops expressing compounds that protect them from pests, such as those that express Bt proteins. This concern applies equally to GM crops and the use of conventional pesticides.
The potential for negative impact on non-target organisms is assessed for all new GM crops before they are approved for commercial release and general use. The assessments are undertaken using a tiered approach that identifies when risk levels require further research. Risk assessment evaluates potential impact from direct exposure to gene products and from indirect exposure through feeding patterns and accumulation of gene products in the air, soil or water residues of the release environment.
Widespread commercial use of Bt genes for insect protection in GM crops has been carefully evaluated for non-target effects. From numerous field studies, the overall conclusion has been that there are no consistent differences in the composition of non-target organisms between GM and non-GM fields. Non-target studies have indicated that the growing of Bt cotton has an overall beneficial effect on biodiversity when compared to insecticide applications5.
Future GM crops with novel pest protection genes must continue to be evaluated on a case- by-case basis before deployment, to assess the potential impact of these crops on non-target organisms. Tiered approaches have been developed to assess these risks and help to ensure that sufficient review is undertaken where potential risks are identified.
- Could GM crops negatively impact on the non-living components of the release environment, damaging or polluting the air, soil or water?
All GM crops are reviewed to ensure that their use will not result in practices that could negatively impact on the non-living components of the release environment. This review looks at the nature of the new gene products and the agricultural management of the GM crops to compare the impact of these on the release environment, in comparison to the impact of the production of conventional crops. Crops that require more fertilizer or increased tillage may be responsible for increased water pollution or soil structural damage. This may be deemed less acceptable than conventional varieties, because these
impacts on the abiotic components of the release environment can cause changes in the biodiversity of the area.
- Where to find additional Information:
1Breeding by indigenous farmers and Maintenance of traditional varieties and land-race purity:
Bellon, M. R., and J. Berthaud. 2004. Transgenic maize and the evolution of landrace diversity in Mexico: The importance of farmers’ behavior. Plant Physiology 134:883– 888.
Bellon M.R. 1996. The dynamics of crop infraspecific diversity: a conceptual framework at the farmer level. Economic Botany 50: 26–39.
Brush S.B. 1991. Farmer-based approach to conserving crop Germplasm. Economic Botany
Jarvis, D. I., and T. Hodgkin. 1999. Wild relatives and crop cultivars: Detecting natural introgression and farmer selection of new genetic combinations in agro- ecosystems. Molecular Ecology 8:S159– 173.
Louette, D., A. Charrier, and J. Berthaud. 1997. In Situ Conservation of Maize in Mexico: Genetic Diversity and Maize Seed Management in a Traditional Community. Economic Botany 51:20–38.
Gepts, P. 2004. Introduction of transgenic crops in centers of origin and domestication. In Controversies in Science and Technology: From Maize to Menopause, ed. D. L. Kleinman, A. J. Kinchy, and J. Handelsman. Madison: University of Wisconsin Press. Pp. 119–134.
Gepts, P., and R. Papa. 2003. Possible effects of (trans)gene flow from crops on the genetic diversity from landraces and wild relatives. Environmental Biosafety Research 2:89– 103.
Perales, H., S. B. Brush, and C. O. Qualset. 2003. Dynamic management of maize landraces in central Mexico. Economic Botany 57:21– 34.
2Determining the impact of a GM trait on weediness/invasiveness:
Conner, A. J., T. R. Glare, and J.- .P Nap. 2003. The release of genetically modified crops into the environment: Part II. Overview of ecological risk assessment. Plant Journal 33:19– 46.
Hancock, J.F. 2003. A framework for assessing the risk of transgenic crops. BioScience
Hancock, J. F., and K. Hokanson. 2001. Invasiveness of transgenic versus exotic plant species: How useful is the analogy? In The Bioengineered Forest: Challenges for Science and Technology, ed.
Raybould, A., and I. Cooper. 2005. Tiered tests to assess the environmental risk of fitness changes in hybrids between transgenic crops and wild relatives: The example of virus resistant Brassica napus. Environmental Biosafety Research 4:127– 140.
Romeis, J., D. Bartsch, F. Bigler, M. P. Candolfi, M. P. C. Gielkens, S. Hartley, R. L. Hellmich, J. E. Huesing, P. C. Jepson, R. Layton, H. Quemada, A. Raybould, R. I. Rose, J. Schiemann, M. K. Sears, A. M. Shelton, J. Sweet, Z. Vaituzis, and J. D. Wolt. 2008. Assessment of risk of insect- resistant transgenic crops to nontarget arthropods. Nature Biotechnology. 26:203– 208.
Nickson, T. E. 2008. Planning environmental risk assessment for genetically modified crops: Problem formulation for stress tolerant crops. Plant Physiology 147:494– 502.
3Prevalence of gene flow between GM crops and wild species:
Ellstrand, N. C., H. C. Prentice, and J. F. Hancock. 1999. Gene flow and introgression from domesticated plants into their wild relatives. Annual Review of Ecology and Systematics
Watrud, L. S., E. H. Lee, A. Fairbrother, C. Burdick, J. R. Reichman, M. Bollman, M. Storm, G. King, and P. K. Van de Water. 2004. Evidence for landscape- level, pollen mediated gene flow fromgenetically modified creeping bentgrass with CP4 EPSPE as a marker. Proceedings of the National Academy of Sciences 101:14533– 14538.
Chapman, M. A., and J. M. Burke. 2006. Letting the gene out of the bottle: The population genetics of genetically modified crops. New Phytologist 170:429– 443.
4Transfer of herbicide resistance into native species and Evolution of herbicide resistance:
Duke, S. O., and S. B. Powles. 2008a. Glyphosate resistant weeds and crops. Pest Management
Duke, S. O., and S. B. Powles. 2008b. Glyphosate: A once- in- a- century herbicide. Pest
Management Science 64:319– 325.
Warwick, S., A. Légère, M. Imard, and T. James. 2008. Do escaped transgenes persist in nature? The case of an herbicide resistance transgene in a weedy Brassica rapa population. Molecular Ecology 17:1387– 1395
Service, R. 2007. A growing threat down on the farm. Science 316:1114— 1117.
Cerdeira, A. L., and S. O. Duke. 2006. The current status and environmental impacts of glyphosateresistant crops. Journal of Environmental Quality 35:1633– 1658.
5Centers of Diversity and GM crops
Gepts, P. 2004. Introduction of transgenic crops in centers of origin and domestication. In Controversies in Science and Technology: From Maize to Menopause, ed. D. L. Kleinman, A. J. Kinchy, and J. Handelsman. Madison: University of Wisconsin Press. Pp. 119– 134.
Gepts, P., and R. Papa. 2003. Possible effects of (trans)gene flow from crops on the genetic diversity from landraces and wild relatives. Environmental Biosafety Research 2:89– 103.
Hancock, J.F. 2012. Plant evolution and the origin of crop species. CABI Publishing. Oxon, UK Hancock, J.F. 2012. Plant Evolution and the Origin of Crop Plants. Wallingford, UK: CABI
Harlan, J. R. 1992. Crops and Man. 2nd ed. Madison, WI: American Society of Agronomy
Harlan, J.R. 1976. Plant and animal distribution in relation to domestication. Philosophical
Transactions of the Royal Botanical Society, London 275, 13-25.
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Harlan, J.R. 1967. Agricultural origins: centers and non-centers. Science 174, 468-474. Rissler, J., and M. Mellon. 1996. The Ecological Risks of Engineered Crops. Cambridge: MIT
6 Bt crops and non-target species:
Bambawale, O., A. Singh, O. Sharma, B. Bhosle, R. Lavekar, A. Dhandapani, V. Kanwar, R. Tanwar, K. Rathod, N. Patange, and V. Pawar. 2004. Performance of Bt cotton (MECH- 162) under integrated pest management in farmers’ participatory field trial in Nanded district, central India. Current Science. 86:1628– 1633.
Cattaneo, M., C. Yafuso, C. Schmidt, C. Huang, M. Rahman, C. Olson, C. Ellers- Kirk, B. Orr, S. Marsh, L. Antilla, P. Dutilleul, and Y. Carrière. 2006. Farm- scale evaluation of the impacts of transgenic cotton on biodiversity, pesticide use, and yield. Proceedings of the National Academy of Sciences, U.S.A. 103:7571– 7576.
Duan, J. J., M. Marvier, J. Huesing, G. Dively, and Z. Y. Huang. 2008. A meta- analysis of effects of Bt crops on honey bees (Hymenoptera: Apidae). PLoS One 3 (1): e1415. Doi:10.1371/journal.pone.0001415.
Huang, J., R. Hu, C. Fan, C. Pray, and S. Rozelle. 2002. Bt cotton benefits, costs, and impacts in
China. AgBioForum 5:153– 166.
Huang, J., R. Hu, S. Rozelle, and C. Pray. 2005. Insect- resistant GM rice in farmers’ fields:
Assessing productivity and health effects in China. Science 308:688– 690.
Marvier, M., C. McCreedy, J. Regetz and P. Kareiva. 2007. A meta- analysis of effects of Bt cotton and maize on nontarget invertebrates. Science 316:1475– 1477.
Pray, C., J. Huang, R. Hu, and S. Rozelle. 2002. Five years of Bt cotton in China— the benefits continue. Plant J. 31:423– 430.
Whitehouse, M., L. Wilson, and G. Fitt. 2005. A comparison of arthropod communities in transgenic Bt and conventional cotton in Australia. Environ. Entomol. 34:1224– 1241.
Wolfenbarger, L. L., S. E. Naranjo, J. G. Lundgren, R. J. Bitzer, and L. S. Watrud. 2008. Bt crop effects on functional guilds of non- target arthropods: A meta- analysis. PLoS One 3 (5): e2118. Doi:10.1371/journal.pone.0002118.
- See also
Barrett, S.C.H. 1983. Crop mimicry in weeds. Economic Botany 37: 255-282
Harlan, J.R. 1965. The possible role of weed races in the evolution of cultivated plants.
Euphytica 14: 173-176.
Harlan, J.R., deWet, J.M.J. and Price, E.G. (1973) Comparative evolution of cereals. Evolution