Sep. 4, 2000 The use of genetically modified herbicide-tolerant (GMHT) crops may severely reduce bird populations on a small percentage of farms, while having little effect on most others, predicts a new study in the 1 September issue of the international journal, Science. Overall, the consequences should depend upon which farmers adopt the new crop types, the study's authors conclude.
The possible effects of GMHT crops on wildlife in the countryside has been the subject of ongoing debate, and the British Government has introduced a moratorium on the use of these crops until the issue is resolved.
Lead Science author Andrew Watkinson, from the University of East Anglia, in Norwich, England, and his colleagues have created a model that simulates the growth of weed populations within crops. Using the model, the team investigated the consequences of the changed herbicide use likely to be associated with GMHT crops. The results showed that weed seed populations can be expected to decline by at least 90% in some cases.
An important part of the study links the decline in weed numbers to bird numbers, predicting that such a decline in seed abundance should seriously reduce the numbers of skylarks using these fields.
The controversial field trials currently underway in the United Kingdom are intended to investigate the consequences of GMHT crops for biodiversity.
"The field trials will be very valuable, but will not tell us what will happen to bird populations. They are carried out on too small a scale. One considerable advantage of the methodology we have adopted is that it enables us to make predictions now rather than having to wait for the results of a three year trial," Watkinson said.
Several decades of intensified agriculture in Europe have had a particularly serious effect on birds, whose populations in the United Kingdom have declined by up to 90 percent in the last 25 years, according to Watkinson.
"It seems likely that the widespread introduction of herbicide-tolerant crops will result in further declines for many farmland birds unless other mitigating measures are taken," Watkinson said.
The model developed by Watkinson's team examines the management of herbicide-resistant sugar beet and its effects on a major annual weed of that crop (Chenopodium album, more commonly known as Lamb's Quarters in North America and Fat Hen in Britain) and the seed-eating skylark Alauda arvensis.
"These results probably apply widely to other crops, weeds, and seed eating birds," noted Watkinson.
The study showed that a key issue in predicting the impacts on bird numbers was the pattern of farmers' uptake of the new GM technology. Most fields have very low seed densities. It's the smaller proportion of fields with high seed densities that is particularly important for bird populations.
The researchers predict that the severity of the bird declines will depend upon which farmers are most likely to adopt the GMHT crops. If their use is restricted to intensive farms with low seed densities then the effect will be minor. However, if the herbicide-hardy crops are adopted by a wide range of farmers--especially farmers with very weedy fields--then the bird declines are likely to be more severe, according to the study.
In their Science paper, Watkinson and his colleagues emphasize that their findings don't just apply to the effects of genetic engineering. The same approach could be used to predict the consequences of other changes in farming practice, they say.
A commentary by Les Firbank, of the Centre for Ecology and Hydrology, in Cumbria, England, and Frank Forcella at the USDA Agricultural Research Station, in Morris, Minnesota, and the University of Minnesota, in St. Paul, Minnesota, accompanies the Science paper.
Firbank and Forcella write that the model provides a "welcome conceptual framework," but that further work will be necessary to resolve some of the model's simplifications. According to the commentary, some data from the United States, where GMHT crops are currently growing, suggest that weed control with GMHT crops may not be as effective as some of the model results indicate.
Such differences emphasize the need for field trials to complement theoretical studies like this one, Firbank and Forcella point out.
The other members of Watkinson's team are Robert Freckleton, and William Sutherland, of the University of East Anglia, in Norwich, England, and Robert Robinson, of the British Trust for Ornithology, in Norfolk, England. Their study was funded by the University of East Anglia and the Natural Environment Research Council.
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