The hypotheses
In general, weeds are successful because they out compete
non-weedy vegetation. Weeds may be better colonizers,
faster reproducers, and/or better survivors than non-weedy
species.
To be successful as a colonizer, a plant must be capable
of increasing its numbers at low densities. A plant capable
of vegetative reproduction may have a selective advantage
over a plant that only reproduces sexually because even a
single individual would be able to reproduce and colonize
new habitats (Baker 1962, 1965). If a plant is incapable of
vegetative reproduction and only reproduces sexually
through cross-pollination (allogamy), then having both
sexes on the same plant [i.e., either a monoecious plant or
a plant with perfect flowers (hermaphrodite)] may confer a
selective advantage over a unisexual (dioecious) plant.
Any two plants would be capable of reproduction and
colonization (Baker 1962). If a plant is bisexual, then selfcompatibility
may confer a selective advantage over selfincompatibility
because just a single plant is capable of
reproduction and colonization (Baker 1962, 1965). If the
plant is not self-pollinating (autogamous), then a generalist
pollination system (e.g., wind pollination) may confer a
selective advantage over a specialized pollination system
because the specialist pollinator may not be present in the
area being colonized (Baker 1965). Plants that colonize
disturbed areas are subject to further selective pressures.
These habitats tend to have high levels of light and,
consequently, low levels of soil moisture. Therefore, a
plant that is tolerant of high light levels would have a
selective advantage over a plant that is shade tolerant, and
a plant that is adapted to low soil moisture would have a
selective advantage over a plant that is adapted to more
moist habitats (Baker 1967).
To be a fast reproducer, a plant has to have a short
generation time and/or high fecundity (Baker 1965). By
definition, an annual plant has a generation time of 1 year,
a biennial has a generation time of 2 years, and a perennial
has a highly variable generation time. Short generation
time may confer a selective advantage to annuals over
biennials, and biennials over perennials (Baker 1965). If a
plant is perennial and life form is correlated with
generation time, then an herb