Tracking refers to monitoring the movement and dispersal of organisms and their genes over time. If crop plants do not survive well beyond cultivated fields, tracking is not necessary. But if cultivated plots of crop plants have close relatives growing nearby, outcrossing of the engineered genes may be a concern. It is commonly recognized by breeders and agronomists that natural mechanisms for such outcrossing do exist. However, it is only in certain cases that a biosafety concern is raised (see "Scientific Issues for Environmental Risk Assessment," page 28).

Expanding the geographic range or duration of "sampling" beyond small-scale field tests poses significant difficulties for a comprehensive monitoring program. Assumptions about the best monitoring design and methodologies must be made on the basis of incomplete or insufficient information, despite what is often characterized as long-term experience with specific organisms and a full understanding of their growth characteristics. Episodic events at disparate intervals may produce very large differences in monitoring data. For example, the dispersal distance for oilseed rape pollen from commercial fields was measured at more than 150 meters as opposed to less than 10 meters from experimental plots. For events that have a very low probability of occurring, spatial and temporal expansion of monitoring protocols may be necessary to see gene flow when it happens.

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