One potential effect of transgenes escaped from cultivation, either in crop plants themselves or introgressed into wild relatives, is release of the plants from ecological constraints that currently limit or control their distribution and abundance. Release may occur only within the community that the species presently occupies, or it may allow range expansion into new communities. Experience with ecological range expansions of invading plant species suggests that when ecological release occurs, the consequences can be severe. To assess properly the likelihood of ecological release, the factors that currently limit species' distributions and abundances must be determined by manipulative experiments. The effect of transgenes on these factors must then be investigated throughout the life cycle of the species and on a case-by-case basis. For ruderal annual species, seed survival and seedling establishment phases may be particularly important.