Herbicides are used in very large amounts and on a worldwide scale. They are active against pest plants and when used at commercial rates appear to have few significant effects on animals. Herbicide pollution of the environment did not, until relatively recently, arouse the passions associated with insecticides. However, conservationists now worry about the loss of 'weeds' that are the food hosts for larvae of butterflies and other insects and whose seeds form the main diet of many birds. A recent development has been the genetic modification of crops such as sugar beet to produce resistance to the nonselective herbicide glyphosate (see Section 184.108.40.206). This allows the herbicide to be used to effectively control weeds that normally compete with the crop without adverse affect on the sugar beet itself.
Fat hen (Chenopodium album), a plant that occurs worldwide, is one weed that can be expected to be affected adversely by the farming of genetically modified (GM) crops; but the seeds of fat hen are an important winter food source for farmland birds, including the skylark (Alauda arvensis). Watkinson et al. (2000) took advantage of the fact that the population ecologies of both fat hen and skylarks have been intensively studied and incorporated both into a model of the impacts of GM sugar beet on farmland populations. Skylarks forage preferentially in weedy fields and aggregate locally in response to weed seed abundance. Hence, the impact of GM sugar beet on the birds will depend critically on the extent to which high-density patches of weeds are affected. Watkinson et al. incorporated the possible effects of weed seed density on farming practice. Their model assumed: (i) that before the introduction of GM technology, most farms have a relatively low density of weed seeds, with a few farms having very high densities (solid line in Figure 15.3a); and (ii) the probability of a farmer adopting GM crops is related to seed bank density through a parameter p. Positive values of p mean that farmers are more likely to adopt the technology where seed densities are currently high and there is the potential to reduce yield losses to weeds. This leads to an increase in the relative abundance of low-density fields (dotted line in Figure 15.3a). Negative values of p indicate that farmers are more likely to adopt the technology where seed densities are currently low (intensively managed farms), perhaps because a history of effective weed control is correlated with a willingness to adopt new technology. This leads to a decreased frequency of low-density fields (dashed line in Figure 15.3a). Note that p is not an ecological parameter. Rather it reflects a socioeconomic response to the introduction of new technology. The way that farmers will respond is not self-evident and needs to be included as a variable in the model. It turns out that the relationship between current weed levels and uptake of the new technology (p) is as important to bird population nonpests become pests when their enemies and competitors are killed mortality of nontarget species in general unintended effects of the genetic modification of crops with herbicide resistance
Figure 15.3 (a) Frequency distributions of mean seed densities across farms before the introduction of GM sugar beet (solid line), and in two situations where the technology has been adopted: where the technology is preferentially adopted on farms where weed density is currently high (dotted line) and where it is currently low (dashed line). (b) The relative density of skylarks in fields in winter (vertical axis; unity indicates field use before the introduction of GM crops) in relation to p (horizontal axis; positive values mean farmers are more likely to adopt GM technology where seed densities are currently high, negative values where seed densities are currently low) and to the approximate reduction in weed seed bank density due to the introduction of GM crops (T, third axis; realistic values are those less than 0.1). Note that the parameter space that real systems are expected to occupy is the 'slice' of the diagram nearest to you, where small positive or negative values of p give quite different skylark densities. (After Watkinson et al., 2000.)
density as the direct impact of the technology on weed abundance (Figure 15.3b), emphasizing the need for resource managers to think in terms of the triple bottomline of sustainability, with its ecological, social and economic dimensions.
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