crops be integrated into their ongoing activities. For example, the Agricultural Extension Service maintains offices in many counties. These extension service personnel have many responsibilities and spend considerable time observing agriculture. They have little spare time, and it would be ineffective to mandate that they spend additional time specifically looking for environmental effects of transgenic crops. But because they are trained observers of agriculture, it is possible that while they are conducting their normal activities they could be on the watch for potential environmental effects of transgenic crops. This may be facilitated by a short half-day or one-day workshop on the potential environmental hazards of transgenic crops, so that these extension service personnel can understand better the types of effects that might occur. Crop consultants are another group of trained observers in agriculture. It may be possible to add a training module to the short courses they routinely take that would prepare them to observe environmental effects of transgenic crops.
The release of transgenic plants is often compared to the introduction of nonindigenous species (Kareiva et al. 1996, Marvier and Kareiva 1999). Lessons from invasion biology and management of nonindigenous plants suggest that the reasons for differences in invasiveness among species are poorly understood (Williamson 1996), and that invasiveness may evolve or be delayed (Blossey and Nötzold 1995, Ellstrand and Schierenbeck 2000), often in the form of a “lag” phase between initial introduction and explosive spread (Williamson 1996). Ultimately, we have little power to predict which species will be a successful invader, which ecosystem may be particularly vulnerable to invasion (Williamson 1996, Williamson and Fitter 1996, Lonsdale 1999), or what the impact of invasions will be (Hengeveld 1999). In fact, for some of the most well-known invasive bird species, such as European starlings and the House sparrow, multiple introduction events were necessary before populations became established (Williamson 1996). Lag times in invasive species may be explained by (1) inherent factors associated with population growth and range expansion, (2) changes in environmental conditions favoring the introduced species, and (3) genetic factors (Crooks and Soulé 1999). Moreover, time-lagged effects are particularly evident after establishment of exotic perennial plants compared to annuals, and in association with range expansion of the invaders (Williamson 1996). Overall, short-term experiments (even if conducted over multiple years or a decade) cannot substitute for long-term time series observations because of the potential for lag times and