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Weed resistance could mean herbicide is futile
Danita Delimont/Alamy
THIS WEEK
I’m a lizard… get me out of here! Bob Holmes
Ryan Calsbeek and Robert Cox of Dartmouth College in Hanover, New Hampshire, cut no corners in their experiment. They excluded predators from two small, uninhabited islands in the Bahamas by wrapping the islands – about 1000 square metres each – with netting to keep out predatory birds. Meanwhile, they enhanced predation on two other islands by introducing lizard-eating snakes. To vary the amount of competition, they seeded one of each pair of islands with high densities of Anolis sagrei lizards, and the other with lower densities. But first, they got to
WRAPPING entire islands in the Bahamas with netting, introducing snakes to other islands and putting hundreds of animals on treadmills: one of the most ambitious ecological field experiments ever conducted has resolved a long-standing question about the evolution of lizards. Lizards of the genus Anolis are found throughout the American tropics, where they vary widely in size and shape depending on ecological conditions. It has long been thought that predation is the most important evolutionary pressure for mainland lizard populations, whereas on islands “They wrapped the islands – competition between lizards about 1000 square metres themselves is more important. each – with netting to keep Until now, though, no one had out predatory birds” tested this experimentally.
–The right stuff–
know the animals. Before release, they marked and measured each one and tested its stamina by running it on a treadmill. “Your Lance Armstrong lizards can run for about 7 minutes. Your overweight field-biologist lizard runs for about 2 minutes,” says Calsbeek. The animals were tested daily for several weeks. Four months after release, the researchers returned to the islands and recaptured every remaining lizard, noting which
had survived and which died. Larger, longer-legged and higherstamina lizards had survived better than smaller, wimpier ones on higher-density islands where competition was more intense. However, these traits did not affect the chance of survival in the face of predation (Nature, DOI: 10.1038/nature09020). This supports the idea that competition, not predation, is the primary selective force in these island lizards, says Calsbeek. n
Evolution of weed resistance deals a blow to popular herbicide THE world’s most popular herbicide is losing its knockout punch. More and more weeds are evolving resistance to glyphosate – originally marketed by Monsanto as Roundup – but the problem could have been forestalled by farming practices enriched by a better understanding of evolution. This is a serious problem. “Glyphosate is as important to world food production as penicillin is to human health,” says Stephen Powles, a plant scientist at the University of Western Australia in Perth. In 1996, Monsanto began selling crop varieties genetically modified to contain a gene for glyphosate resistance. This enabled farmers to spray glyphosate – lethal to plants yet
12 | NewScientist | 15 May 2010
non-toxic to animals – on their fields to kill weeds without damaging the crops, even during the growing season. Today nearly 100 million hectares worldwide are planted with glyphosate-resistant crops. In much of the south-eastern US, as well as Brazil and Argentina, farmers grow glyphosate-resistant corn, soybeans and cotton year after year and have come to rely almost exclusively on this herbicide. This has encouraged at least nine species of weed to evolve their own glyphosate resistance, to the point where some farmers can no longer control weed infestations. The solution, as any evolutionary biologist will tell you, is for farmers to vary weed-control practices so that
Scott Olson/Getty
Insight
Genetically modified cotton fields hide herbicide-resistant weeds
weeds face a number of evolutionary pressures instead of just one. Monsanto recommends precisely this in its instructions to farmers. But farmers have been reluctant to reduce their use of an effective herbicide for an intangible future benefit, especially when few have experienced
glyphosate-resistant weeds. Where diverse weed control is practiced, however, resistance has not yet developed. In most of Canada, for example, farmers grow glyphosate-resistant canola in rotation with wheat and barley. They vary the herbicides used depending on the crop grown, and glyphosateresistant weeds are unknown. To keep resistant weeds from spreading may require intervention from governments or farmers’ associations. “You’re going to need some sort of collective management,” says David Ervin, an environmental economist at Portland State University in Oregon. GM crops and herbicides are already widely used, but they can still be regulated, or as Ervin puts it: “While the cat is out of the bag, it’s possible to control the range of the cat.” Bob Holmes n
THIS WEEK
I’m a lizard… get me out of here! Bob Holmes
Ryan Calsbeek and Robert Cox of Dartmouth College in Hanover, New Hampshire, cut no corners in their experiment. They excluded predators from two small, uninhabited islands in the Bahamas by wrapping the islands – about 1000 square metres each – with netting to keep out predatory birds. Meanwhile, they enhanced predation on two other islands by introducing lizard-eating snakes. To vary the amount of competition, they seeded one of each pair of islands with high densities of Anolis sagrei lizards, and the other with lower densities. But first, they got to
WRAPPING entire islands in the Bahamas with netting, introducing snakes to other islands and putting hundreds of animals on treadmills: one of the most ambitious ecological field experiments ever conducted has resolved a long-standing question about the evolution of lizards. Lizards of the genus Anolis are found throughout the American tropics, where they vary widely in size and shape depending on ecological conditions. It has long been thought that predation is the most important evolutionary pressure for mainland lizard populations, whereas on islands “They wrapped the islands – competition between lizards about 1000 square metres themselves is more important. each – with netting to keep Until now, though, no one had out predatory birds” tested this experimentally.
–The right stuff–
know the animals. Before release, they marked and measured each one and tested its stamina by running it on a treadmill. “Your Lance Armstrong lizards can run for about 7 minutes. Your overweight field-biologist lizard runs for about 2 minutes,” says Calsbeek. The animals were tested daily for several weeks. Four months after release, the researchers returned to the islands and recaptured every remaining lizard, noting which
had survived and which died. Larger, longer-legged and higherstamina lizards had survived better than smaller, wimpier ones on higher-density islands where competition was more intense. However, these traits did not affect the chance of survival in the face of predation (Nature, DOI: 10.1038/nature09020). This supports the idea that competition, not predation, is the primary selective force in these island lizards, says Calsbeek. n
Evolution of weed resistance deals a blow to popular herbicide THE world’s most popular herbicide is losing its knockout punch. More and more weeds are evolving resistance to glyphosate – originally marketed by Monsanto as Roundup – but the problem could have been forestalled by farming practices enriched by a better understanding of evolution. This is a serious problem. “Glyphosate is as important to world food production as penicillin is to human health,” says Stephen Powles, a plant scientist at the University of Western Australia in Perth. In 1996, Monsanto began selling crop varieties genetically modified to contain a gene for glyphosate resistance. This enabled farmers to spray glyphosate – lethal to plants yet
12 | NewScientist | 15 May 2010
non-toxic to animals – on their fields to kill weeds without damaging the crops, even during the growing season. Today nearly 100 million hectares worldwide are planted with glyphosate-resistant crops. In much of the south-eastern US, as well as Brazil and Argentina, farmers grow glyphosate-resistant corn, soybeans and cotton year after year and have come to rely almost exclusively on this herbicide. This has encouraged at least nine species of weed to evolve their own glyphosate resistance, to the point where some farmers can no longer control weed infestations. The solution, as any evolutionary biologist will tell you, is for farmers to vary weed-control practices so that
Scott Olson/Getty
Insight
Genetically modified cotton fields hide herbicide-resistant weeds
weeds face a number of evolutionary pressures instead of just one. Monsanto recommends precisely this in its instructions to farmers. But farmers have been reluctant to reduce their use of an effective herbicide for an intangible future benefit, especially when few have experienced
glyphosate-resistant weeds. Where diverse weed control is practiced, however, resistance has not yet developed. In most of Canada, for example, farmers grow glyphosate-resistant canola in rotation with wheat and barley. They vary the herbicides used depending on the crop grown, and glyphosateresistant weeds are unknown. To keep resistant weeds from spreading may require intervention from governments or farmers’ associations. “You’re going to need some sort of collective management,” says David Ervin, an environmental economist at Portland State University in Oregon. GM crops and herbicides are already widely used, but they can still be regulated, or as Ervin puts it: “While the cat is out of the bag, it’s possible to control the range of the cat.” Bob Holmes n