Invasive Species
--From Nabhan, Gary Paul and Andrew R. Holdsworth.1998.
State of the Sonoran Desert Biome: Uniqueness, Biodiversity,
Threats and the Adequacy of Protection in the Sonoran Bioregion.
Sponsored by The Wildlands Project. p. 36
Felger (1992) estimated that there are at least 146 non-native
plant species naturalized in the Sonoran bioregion, which
he believed to be no more than 6% of the total desertscrub
and thomscrub flora of the biome. (This compares to Nabhan's
(1982) estimate that 9%, or 328 species, of Arizona's flora
of 3666 plants are exotics). If we add 5 to 10 naturalized
mammals, 5 to 10 birds, 1 reptile, 2 amphibians, 45 to 50
fish, and an unknown number of invertebrates, the total number
of exotics naturalized and competing with natives in the Sonoran
bioregion is well over 200 species. Table 6 gives a short
list of naturalized animals, while Table 7 highlights the
most extensive or problematic plant invasions.
These tables suggest the astonishing range of exotic species
introduced to the Sonoran bioregion, intentionally or unintentionally,
which have invaded other habitats and changed biotic interactions
there. Microbes such as Giardia and scabies should also be
considered for their effects on native wildlife such as desert
bighorn, particularly where they are transmitted at artificial
water developments (Broyles 1996).
Constant reminders of the magnitude of change caused by these
exotics are in order, since Burgess et al. (1991) claim that
once begun, most of the invasions are irreversible, and that
several exotics formerly considered minor are showing signs
of becoming increasingly invasive. For example, the salt cedar
or tamarisk, after its introduction to North America for erosion
control in the 1850s, spread to over 4,000 hectares of riparian
habitat. Naturalized in the Colorado River watershed by the
1920s, it arrived in the Phoenix area of the Gila and Salt
Rivers by the 1930's, where it became named and dreaded by
the Gila River Indians. A mature tamarisk consumes as much
as 800 liters of water per day -- 10 to 20 times the amount
used by native species which it tends to replace, such as
mesquite (Cooperrider et al. 1995). In one study of winter
bird populations along the Lower Colorado River, biologists
found 154 birds per 40 hectares of native vegetation compared
to 4 birds per 40 hectares in comparable tamarisk-dominated
areas (Anderson et al. 1987; Johnson 1986). While stands of
tamarisk may have considerable insect diversity and high productivity
of White-winged doves, they often choke out native vegetation
to the extent that animals which specialize in natives are
displaced from the scene. Whereas invasions of tamarisk and
many other species are irreversible for biological or geographical
reasons, the most destructive / invasive ungulate, the Feral
Burro, is legally unmanagable due to the Wild Horse Act.
Felger (1992) .
(This compares to Nabhan's (1982)
(Broyles 1996).
Burgess et al. (1991)
(Cooperrider et al. 1995).
(Anderson et al. 1987;
Johnson 1986).
|
