The predictions
If weed species are better colonizers, faster reproducers,
and/or better survivors than non-weedy species, I would
predict that weeds would be:
– Vegetatively reproductive
– Hermaphroditic or monoecious
– Self-compatible
– Wind pollinated
– Tolerant of high light levels
– Tolerant of low moisture regimes
– Annuals or biennials
– Perennial grasses or forbs
– Armed, and
– Toxic.
It is commonly assumed that exotic weeds are more
aggressive than native weeds. Several hypotheses have
been suggested (and some tested) to explain this pattern,
including genetic (Baker 1962; Rejmanek 1996; Siemann
and Rogers 2001), release from competition (Crawley
1987; Wolfe 2002), release from predation (Crawley 1987;
Mack et al. 2000; Wolfe 2002), and evolution of increased
competitive ability (EICA) (Blossey and Notzold 1995;
Siemann and Rogers 2001). An alternative explanation is
that the process of colonization and establishment selects
those exotic species that exhibit more traits associated with
the “ideal weed’’. If this were true, then I would predict
that more exotic weed species would exhibit these traits
than native weed species. But not all exotic weed species
are invasive. I would also predict that more invasive,
exotic weed species would exhibit these traits than noninvasive
exotic weed species.