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to answer a different question: What if only some of the 12 sites had been sampled, or what if the data had been taken over a period shorter than 3 years? Would the answer have been the same? The reanalysis found that the final answer would have been little changed by taking data at fewer sites but would have been quite different if the study had gone on for just 1 or 2 years instead of 3. Such volatility suggests, Power said, that the year-to-year variation in weather or other factors may make monitoring sensitive to the period over which it takes place. In contrast, the site-to-site variability was apparently not so great in Crawley's study that it would have made much difference to look at fewer than the 12 original sites. “Of course,” Power said, “that really depends on the variability of the sites that were chosen for this particular study, and I am not convinced that would be true for all available laboratory sites.”

If there were no constraints on time and resources, it clearly would make sense to maximize both the length of the monitoring and the number of sites, but such constraints always exist. “Obviously there is a conflict here between the desire to monitor until we are absolutely sure that there isn't going to be an impact and the desire to get the technology out to users," Power said. "That is a real conflict, and it incorporates both scientific-ecological decision-making and socioeconomic decision-making. I cannot answer the question, but what I can say is that it is important to think carefully about what should be required for field experiments, about what is a realistic but effective design for making sure that year-to-year variability has been accounted for.”

A related issue is that if ecological monitoring is to discover changes in ecosystems caused by the cultivation of transgenic crops, it will be vital to know what those ecosystems were like before the introduction of the transgenic plants. “If you monitor something,” Schaal commented, “you need to collect a series of different data points to tell whether anything is changing. The collection of these data is critical because you cannot tell whether something has changed if you don't have a baseline.”

And because of the natural variability in ecosystems, such a baseline must be more than just a snapshot—that is, more than just data on the ecosystem at one moment. Unless a researcher understands, for example, how much the population of a particular insect normally varies from year to year, it would be impossible to know how to interpret a 30% drop in the insect's numbers the year after a crop of Bt corn was planted. “One of the main difficulties and challenges in impact assessment,” Power said, “is going to be in separating these impacts from natural spatial and temporal variability.” And the problem will only get worse, she predicted, as the global warming trend continues to alter weather and temperature patterns, making year-to-year variability in ecosystems even greater.

Besides providing a basis of comparison for what happens when