Characteristics, significance, and human dimension of global invasive weeds - Invasive Plants: Ecological and Agricultural Aspects - page 227

constitute at least 27% of the total flora of the state [16]. Currently, it is esti-
mated that there are 4,678 species of invasive plants in other countries that
could still be introduced into the United States. Some of the important invasive
weed species of tropical regions are listed in Table 2.
Characteristics and traits of invasive species
An invasive species is one that both spreads in space and has negative impacts
on species already in the space that it enters. In the United States, an “inva-
sive species” is defined by the Executive Order in 1999 [17] as a species that
is i) non-native (or alien) to the ecosystem under consideration and ii) whose
introduction causes or is likely to cause economic or environmental harm or
harm to human health. The Order further provides that a Federal agency may
make a determination that the benefits of an action, which may lead to the
introduction or spread of an invasive species, clearly outweigh the potential
harm caused by the species and take steps to minimize that harm [17]. This
management plan is focused on those non-native species that cause or may
cause significant negative impacts and do not provide an equivalent benefit to
society.
In general, invasive woody plants tend to have small seed size, a short juve-
nile period, and a relatively short interval between seed crops that produce a
high number of seeds [18]. Invasive plants (regardless of their origin) tend to
have many similar biological attributes relating to high reproductive potential
and stress tolerance. Some of the common traits are: i) rapid seedling growth
and early maturation, ii) ability to reproduce at an early stage, iii) ability to
reproduce by vegetative propagules as well as by seeds, iv) ability to produce
viable seeds, v) seed dormancy ensuring periodic germination, vi) diverse dis-
persal mechanisms and high dispersal rate, vii) high photosynthetic rates, viii)
ability to tolerate wide range of environmental conditions, ix) ability to toler-
ate high habitat disturbance, and x) vigorous growth allowing to compete
aggressively with native species.
Invasiveness of plant species is dependent upon species traits, habitats, envi-
ronmental stress and other biological factors. It has proven difficult to identi-
fy particular traits that are consistently associated with the tendency of plant
species to invade [19]. Knowing history of past invasiveness may be the best
predictor on invasiveness of a species in a new habitat. This suggests that some
plant species are more invasive than others but does not explain which traits
encourage invasiveness (see Chapter 2, this volume).
Pathways of species introduction
Introduction means the movement, by human agency, of a species, subspecies,
or lower taxa (including any part, gamet or propagule that might survive and