Healing the Wounds: An Example from the Sky Islands - Design with the Desert: Conservation and Sustainable Development - page 201

BOX 10.1 INVASIVE SPECIES

INVASION OF EXOTIC SPECIES
Biological invasion is considered to be one of the symptoms of global environmental
change. Exotic species (from other continents or geographically distant floras) and
some native species can become invasive in habitats to which they are not part of
the local biota. Invasive species are normally defined as a non-native plant or animal
or other organism whose introduction causes or is likely to cause economic or environmental
harm or harm to human health .
Invasive species displace native species when introduced to habitats where they did
not evolve as part of a functionally organized community. Their success is often linked to
the lack of natural enemies from their origin ecosystem(s) that are not present in the new
habitats to keep them in check. In some cases, aggressive invasive species can literally
transform the invaded habitat changing its ecological structure and function. A good
example of this would be downy bromegrass ( Bromus tectorum ) in the Great Basin desert
where it serves as a fire source and after fire becomes the dominate plant in what was
a sagebrush steppe plant community. In the warm deserts, red bromegrass ( B. rubens )
and/or buffelgrass ( Pennisetum cilare ) provide fine fuel, where in the native state, such
fuel is rare. After a fire in the invaded sites, these exotic grasses can play a similar role
in transforming the Sonoran desert from a community dominated by shrubs and cacti
to one largely devoid of these growth forms. The transformed habitat does not support
functional groups of native organisms and may exhibit changes in abiotic processes
such as modified runoff and stream flow patterns and accelerated soil erosion.
Some exotic plant species were intentionally introduced, such as saltcedar
( T. ramossisima ), Russian olive ( Eleaganus angustifolia ), and buffelgrass for examples. Many
potentially exotic species exist as landscape plants in urban and human residential
settings in rural areas. Some of these exotic species invade wildlands such as fountain
grass ( Pennisetum setaceum ), and some native species are invasive to urban landscapes,
such as the shrub, desert broom ( Baccharis sarathroides ). Some invasive animals in the
southwestern deserts include crayfish, a fish named gizzard shad, bull frogs, and New
Zealand mud snails. These animals invade aquatic habitats and can greatly interfere
with native fishes and other native aquatic species. There are invasive organisms, that
are unicellular, some vector diseases, and golden algae, produces a toxin to fish.
The majority of introductions were accidental. About 10% of the introduced plants
adapt their new habitat and spread (naturalize) while about 1% of the introduced species
become ecosystem transformers. Transformer invasive species literally change the
structure and function of the habitat they have invaded. Without their natural enemies
from their original habitats, invasive species tend to out-compete native species for
environmental resources (water and soil nutrients) and have life cycles favoring their
growth compared to native species. The ecological traits of invasive plants that make
them competitive are largely physiological and reproductive rather than morphological.
Invasive plant's effects on reduced ecological biodiversity constitutes a critical concern
to managers of desert ecosystems and associated biotic communities of North America.

John H. Brock
Professor Emeritus, Arizona State University Polytechnic