Humans tend not to be satisfied with a reasonably good yield but always seem to strive for more, even recognizing that it is possible to have “too much of a good thing.” When a crop plant (or the cropland itself for that matter) is forced to produce a maximum possible yield of desired products (e.g., forced increase in the harvest ratio), the plant has very little energy left to defend itself from pests. This is one of the reasons we see an increase in pesticide use. Also, growers seeking the maximum rather than the optimum will often experience overshoot or “boom and bust” crop production patterns, as occurred with cotton in the Canete Valley of Peru in the 1950s (Barducci, 1972) and in Texas in the 1960s (Adkisson et al.,1982).
An experimental study of the effect of increased egg production on disease in a small bird, the great tit, provides an analogous example of the trade-off between productivity and defense (Opplinger et al., 1996). Researchers artificially increased clutch size by removing the first two eggs laid in the nest boxes. With an increase in the clutch size by an average of one egg, the researchers observed an increase in prevalence of malarial parasites in the blood stream from 10 percent in the control females to 50 percent in the experimental females.
Given this trade-off between productivity and defense, what we should consider doing is using the same biotechnology to reintroduce some of the antiherbivore defensive capabilities that most wild plants have developed through natural selection back into crop plants. This strategy may result in lower but more sustainable yields for the crops in question.
Our effort to increase agricultural productivity worldwide, in order to support increasing numbers of people and domestic animals (which, in turn, excrete huge amounts of nutrients to the environment), has caused global eutrophication problems that are perhaps the greatest threat to ecospheric diversity, resilience, and stability. Global warming that results from CO2 enrichment of the atmosphere is just one aspect of this overall perturbation. Nitrogen enrichment is also a serious threat (Henrikson et al., 1997; Vitousek et al., 1997). Excess nitrate fertilizer and other nutrient runoff favor many noxious weeds, exotic pests, and dangerous disease organisms because these organisms are highly adapted to high-nutrient environments.
The well-known red tide phenomenon is a good example of how enrichment can create a pest out of normally innocuous organisms. At their typical low densities, the red tide microorganisms in estuaries cause few or no problems. They secrete a toxin as a defense mechanism but not in concentrations that could affect fish. But when the estuary is enriched by nutrient-filled pollution, the organisms can rapidly multiply and reach densities when their defensive toxins can cause massive fish kills.