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

220 INTRODUCTION/ORIGIN AND DIVERSITY OF EAST TEXAS FLORA
Reichard & White 2001; Rossman 2001; Sakai et al. 2001; Todd 2001; Baskin 2002;
Lambrinos 2002; Low 2002; Matlack 2002; Pimentel 2002a; Tellman 2002a; Daehler 2003;
de Poorter et al. 2003; Simberloff 2004). Luken and Thieret (1997) examined the assessment
and management of plant invasions and gave a selected list of species interfering with
resource management goals in North America. Particularly problematic for native plants are
those nonindigenous species that aggressively invade native ecosystems, reproduce extensively,
and occupy the habitat of indigenous species. In some cases, invasive species can come
to dominate communities and occur in near monocultures, completely changing the species
composition, structure, and aspect of an ecosystem. Native species are thus excluded by the
dense concentrations of the invader, a phenomenon sometimes referred to as swamping
(Williamson 1996). Invasive species can also cause other, less obvious problems, including
reducing the supply of water in streams, lowering of water tables, serving as vectors for diseases,
altering native plant regeneration patterns, modifying the cycling of nutrients or other materials
(e.g., nitrogen or salt), releasing toxins that prevent other plants from growing (allelopathy),
or changing the fire ecology of an area. A specific example is buffel grass (Pennisetum ciliare),
which was intentionally introduced from Africa and India in the 1940s by the Soil
Conservation Service (SCS) as a forage species and for erosion control; it was formally
released from a SCS nursery in San Antonio in 1946 (Tellman 1997; Búrquez-Montijo et al.
2002). This species invades desert grasslands and shrublands and can dramatically change
the fire ecology of an area—the “great quantities of tinder-dry biomass” (Enyedy 2002) produced
by huge numbers of buffel grass plants contribute to devastating fires that threaten the
whole ecosystem in areas of the Sonoran Desert in Arizona and Mexico (Tellman 1997;
Búrquez-Montijo et al. 2002; Enyedy 2002). In some places, cacti and other native plants
have been almost eliminated (Tellman 1997).
Globally and nationally, the detrimental effects of alien plant and animal species (e.g., competition,
predation, herbivory, parasitism, disease, etc.), though sometimes underestimated, are
now collectively considered as the second most important threat (following only habitat alteration)
affecting imperiled species (Wilson 1992; Simberloff 2000a; Wilcove et al. 2000;
Pimentel 2002b). On the local scale in East Texas, after habitat alteration, invasion by exotics
also appears to be the most serious threat facing native plants. Further, it is a potentially longlasting—and
in many cases uncontrollable—hazard to natural ecosystems (Coblentz 1990).
As pointed out by Cronk and Fuller (1995), invasive exotics are a “lasting threat because
when exploitation or pollution stops, ecosystems often begin to recover. However, when the
introduction of alien organisms stops the existing aliens do not disappear; in contrast they
sometimes continue to spread and consolidate, and so may be called a more pervasive threat.”
In fact, some authorities (e.g., Vitousek et al. 1996) consider biological invasions to have
become so widespread as to constitute “a significant component of global environmental
change.” They further note that humans are “redistributing the species on the earth at a pace
that challenges ecosystems, threatens human health and strains economies.” The latter point
is important since it is now obvious that the effects of invasive plants are not limited to their
impact on native plants. They are capable of becoming serious agricultural pests (thus causing
huge economic losses), damaging aquatic ecosystems (and preventing boating, fishing, recreation,
etc.), and affecting human health (e.g., allergies).
The economic cost of exotic species (including animals and fungi) is high, with national
cost estimates ranging to billions of dollars per year (Pimentel et al. 2000; Sakai et al. 2001).
In fact, several recent estimates of damages resulting from this problem in the U.S. range from
over $120 billion dollars annually (Simberloff 2000a) to $137 billion annually (Pimentel et al.
2000, 2002). Together, the damages and costs of controlling invasive plants alone are estimated
at $34 billion (Pimentel 2002b). In U.S. agriculture, “weeds [the majority introduced] cause
an overall reduction of 12% in crop yields” (Pimentel et al. 2002). As a result, a number of
control efforts are being undertaken at the federal as well as state levels (Floyd 2002).
According to Sakai et al. (2001), “In response to this problem, Executive Order #13112 of