Rice grown in rainforests by slash and burning technology has
no typical generalist weeds in the beginning (De Rouw et al. 2014).
It would be interesting to see how long it takes for these areas to be
infested with cosmopolitan species if continually used for rice
growing. A quite recent example for the adaptation of weeds to
crops are results of research projects in Alaska which could
demonstrate how an endemic flora can change and how new nonnative
weed species can settle on newly cleared agricultural fields
due to man's activity in a rather short time span (Conn et al., 2010).
Within 23 years of agricultural practice, seven new weeds could
establish on Alaskan fields that were not present before. Weedcontaminated
crop seed was mentioned as one major source of
weed infestation. Animals feeding on weed seeds such as birds
migrating over long distances can be another source. The example
of Australian rice weeds as described in the introduction to this
review proves that weeds can establish easily on fields within less
than a century.