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deciding which ones to watch, but tolerance shapes the choice in various ways. “We might logically assume that we are interested in the organisms that we think are going to be most susceptible to the particular technology that we are planning to use. In some cases, however, people doing environmental-impact studies have deliberately chosen tolerant nontarget organisms because they can be sure that they can find them when they go out for monitoring.”

Researchers should also consider the abundance of various organisms when deciding which to monitor, but the same sort of conundrum exists. “Do you choose common species that are logistically much easier to deal with in a monitoring scheme,” Power asked, “or do you choose rare species that you predict are going to be more subject to risk?”

The distribution of the nontarget organisms is another important factor: “Do you choose cosmopolitan species so that you can make some general rules that hold across the United States or across the world, or do you choose organisms that are highly localized in their distribution and yet again may be the ones that are most sensitive to the environmental perturbation that you are putting out there?”

Finally, Power said, population stability can make a major difference in the success of monitoring a particular nontarget species: “Choosing organisms that have relatively stable populations seems to give you the opportunity to detect impacts more easily because if you see wild fluctuations you may be able to correlate them in some way with the impact you are interested in. But it may be the organisms with unstable populations, which fluctuate wildly under normal conditions, that are most sensitive to the risk you are interested in. “I am not really giving any answers here; I am giving you a sense of the dilemma surrounding how we choose what to monitor.”

In addition to deciding which species to monitor, one must decide how long and how thoroughly to monitor. The difficulty here, Power explained, is that a single ecosystem can vary greatly in space and time. If ecosystems were uniform, it would be possible to take one or a few measurements and spot any effects caused by a transgenic crop. But the natural variation of ecosystems makes it necessary to take data from a number of sampling sites over several years to have a reasonable chance of telling the difference between a real effect and chance variations.

Power described a study performed by a British researcher, Mick Crawley, that looked at the potential for transgenic herbicide-resistant canola to be invasive in the UK. “They set up a huge experiment. They planted in 12 sites, and they followed it over the course of 3 years and measured invasiveness in an appropriate way.” The final answer was that transgenic canola was not likely to prove invasive in the UK. Then, Power said, after the study was published, a second scientist reanalyzed its data

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