ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

222 INTRODUCTION/ORIGIN AND DIVERSITY OF EAST TEXAS FLORA
tremendous negative impacts in the U.S. (e.g., European zebra mussels, fire ants,
Africanized (killer) bees, gypsy moths, chestnut blight, Dutch elm disease, West Nile
virus). Unfortunately, a number of these are currently affecting East Texas. Of particular
importance for many plant communities is the direct negative impact caused by feral hogs
(Sus scrofa). This species causes various types of damage ranging from exposing soil to
erosion to general destruction of vegetation, hindering longleaf pine restoration efforts (by
uprooting plants while searching for longleaf seedlings), disturbing habitats in and around
small streams where they wallow, changing successional patterns, affecting water infiltration
rates (Synatzske 1997; Halstead 2002), and damaging populations of herbaceous
species with underground storage structures.
CHARACTERISTICS OF INVASIVE EXOTICS
Most introduced plants do not become problematic invaders because they face a variety of
environmental hazards in their new surroundings, and most are unable to persist and naturalize
(Williamson 1996; Williamson & Fitter 1996; Mack & Erneberg 2002). Of the more
than 30,000 introduced plant taxa commercially available in the U.S. today, it is estimated
that fewer than 3,000 (10%) have become naturalized (Mack & Erneberg 2002), and even fewer
have become problematic invaders. Whether or not a particular species becomes invasive
depends upon complex interactions between several factors: 1) the characteristics of the
invading species (i.e., invasion potential—including competitive ability, reproductive rate,
persistence of seeds, disease and herbivore resistance, climatic compatibility, ecological distinctiveness,
genetic variability, etc.); 2) propagule pressure (how many seeds or other
propagules are introduced); and 3) the properties of the ecosystem that is being invaded (e.g.,
level of disturbance, resistance to invasion, vacancy of niches, etc.) (Williamson 1996;
Lonsdale 1999; Lambrinos 2002). One of the most important ecosystem factors is disturbance—disturbance
reduces the ability of native species to compete with potential invaders
(Lonsdale 1999). Unfortunately, most habitats worldwide—and virtually all in East Texas—
are now disturbed.
Because so many interacting factors are involved, it is very difficult to predict with certainty
whether a species will become invasive or not. This low level of predictability has long been
noted (e.g., Elton 1958) and is a serious problem. The literature of invasion ecology is full of
examples of presumed-harmless species intentionally introduced for specific purposes (e.g.,
erosion prevention, forage, biological control) but which become horrible pests as the result
of unanticipated consequences. Some species, because of frost sensitivity, sterility, or similar
known limiting factors, can be assumed to be relatively safe. Others (e.g., species known to
be problematic in other parts of the world) may have a much greater probability of becoming
invasive. However, for many species, the invasion potential is uncertain. Thus, individuals
intentionally introducing plants (i.e., those in the horticultural trade) should take precautions
and be alert for evidence of high invasion potential. A useful, albeit rough, rule of thumb
regarding the frequency of invasion has been called the “tens rule” (e.g., Williamson 1996).
By this it is meant that roughly 10% of plant species imported become feral (found in the
wild), 10% of these become established (naturalized, with self-sustaining populations), and
10% of these become pests (invasive). Given the many thousands of species that have been
introduced into the U.S., it is fortunate that the percentage is not higher.
One curious aspect of invasive species is that there often appears to be a time lag between
their actual introduction and their widespread impact (J. Taylor, pers. comm.; Simberloff
2000a, 2004). Sometimes a species is seen only rarely and appears innocuous for a significant
period of time, after which its spread appears rapid—the invader seems to undergo a
population explosion. These species are sometimes referred to as “sleepers” (Simberloff
2004). The cause of this phenomenon is not clear, but possibilities include adaptation to the
local environment or quirks of dispersal (e.g., getting into roadside environments where highway
mowing equipment can result in extremely rapid and widespread dispersal). Insight on