Biology Reference
In-Depth Information
which seem to be correlated with rapid growth to maturity and seed pro-
duction. Small seed mass also correlates with large seed number, wider
seed dispersal, high germination, and a short chilling period needed
to break seed dormancy. These characteristics, derived from an analysis
of invasive and noninvasive species of pines, have proven useful in iden-
tifying invasive plants among a wide range of woody angiosperms
(Rejmánek 1996a).
Experimental studies of growth rates of invasive and noninvasive
species of pines, carried out by Grotkopp et al. (2002), concluded that the
best distinguishing factor was growth rate per unit mass of plant tissue.
This, in turn, was closely related to specific leaf area—the area of leaf sur-
face per unit of leaf mass.This study also supported the importance of small
seed size and short generation time as predictors of invasiveness of pines.
Based on an analysis of a global data set of agricultural weeds, invasive
species were found to show statistical tendencies to be pollinated and dis-
persed abiotically (Daehler 1998). For species known to be invaders of
natural vegetation, however, species capable of nitrogen fixation and
species with a vine growth form were more frequent than expected
(Daehler 1998; Py
v
ek 1997).
Altogether, these adaptations suggest a pattern of growth that enables
invasive species to preempt resources, either in time or space. Rapid ger-
mination and growth enable invasives to preempt resources early in a
growing season. Clonal growth and abundant seed production permit
plants to capture resources in horizontal space and, through their climb-
ing ability or rapid vertical growth, to gain the light resource by overtop-
ping competitors.
Another major set of factors related to the success of alien species are
those to which members of invaded communities are not adapted. For
example, chemical inhibition has been suggested as a mechanism under-
lying the success of some invasive plants (Callaway and Aschehoug 2000).
Diffuse knapweed (
Centaurea diffusa
), native to Eurasia, has become a
serious invasive weed in parts of western North America.This plant pro-
duces root exudates that inhibit the growth of perennial bunchgrasses in
North America, although not of related Eurasian bunchgrasses. Similarly,
herbivores and predators that invade island areas previously free of such
animals encounter plants and prey lacking adaptations to avoid their easy
exploitation.
A number of other correlations exist with respect to invasive plants,
although these do not yet give a clear indication of an underlying adap-
