Non-native. Alien. Introduced. Non-indigenous.
Noxious. Naturalized. Several terms have been applied
to the concept of organisms "out of place," and the
definitions associated with these terms reflect both the standpoints
(social, political, and economic) of those employing them and
the perceived or documented threats posed to natives and ecosystems
by the organisms to which these terms refer. There are
issues of provenance, severity, and scale embedded in each of
these terms, as well. Does an invasive species have to
come from across the ocean, or simply from another part of the
same continent, or merely from another river system or mountain
range? In many cases, it is not as important where the
organism originates as what it does once in its new environment.
This issue forms the basis for defining an "invasive"
species, though other subtleties of classifying "out of
place" organisms are no less important to consider.
What is an invasive species?
An invasive species is
an organism "that has the potential to substantially alter
the structure and function of natural ecosystems at a landscape
spatial scale" (Peine & Lancia, 1999:268). Others have
offered expanded definitions: colonizers are species that are
introduced and spread rapidly, causing ecosystem-scale changes;
invaders, more strictly, can enter intact vegetation with the
potential to dominate or displace it (Bazzaz, 1986:107).
The process by which this takes place has been given the name
"bioinvasion," or the movement of organisms among ecosystems
resulting in newly established populations where none previously
existed. An invasive species can be a micro-organism which
causes disease in native organisms; a parasite that diminishes
the health (and, ultimately, fitness) of native organisms; a
plant or animal that competes with a native counterpart for nutrients,
space, and other resources, potentially displacing the native;
and/or an organism that alters the composition or function of
an ecosystem, potentially such that native organisms can no longer
sustain their populations.
How does a species become
Demographic change among
populations of plants and animals is nothing new, nor is interspecific
competition or predation, nor is extirpation or extinction that
can result from these processes. The distinction made for
modern-day invasive species issues hinges on "the human-caused
breakdown of biogeographic barriers to species dispersal,"
wherein deliberate and accidental movement of species into new
environments, by humans, occurs "at rates that are without
precedent in the last tens of millions of years. As a result,
taxa that evolved in isolation from each other are being forced
into contact in an instant of evolutionary time" (D'Antonio
& Vitousek, 1992:63).
Human relocation of species
(also known as "species transfers", "taxonomic
redistribution", or even "biotic enrichment")
is only the first step, though, in a species' becoming invasive.
It is not enough for a species to be able to exist in its new
environment, although a close match between environmental conditions
in the species' home environment and the environment to which
it is introduced is fundamental to its survival there.
Beyond this, the organism must be able to establish a viable
and growing population. To do so, the new species must
be capable of outcompeting and/or displacing native organisms
(Herbold & Moyle, 1986; see Parker & Reichard, 1998,
for discussion). This ability lies in part with the organism's
particular ecology and life history, including but not limited
to its reproductive biology and its tendency to associate with
humans (see Ehrlich, 1986).
Successful invasion is,
as well, often facilitated by an important aspect of the new
environment: absence of natural controls that regulated the invasive
species' population in its homeland. Without these diseases,
parasites, and predators, the new species can spread rapidly.
On the other side of the same coin, native species may lack adaptations
that allow them to resist competition from or predation by the
invasive species, affording success to the invader through attrition.
If the invasion (particularly with plant species) is coupled
with other disturbances to the new ecosystem (e.g., earth moving,
plowing, fire, livestock grazing, changes to surface and groundwater
hydrology), native populations may be weakened from the start
and the ecosystem rendered more susceptible to an invasion (just
why disturbances foster invasion, or how invasion occurs in the
absence of disturbance, are issues that remain under debate,
however; see Orians, 1986; Parker & Reichard, 1998).
What are the consequences
of invasive species?
Invasive species have ecological
as well as economic consequences. Agricultural "weeds"
- considered to be any plants other than the crop plant, bearing
in mind that many crop plants are also introduced species - can
contaminate seed crops and reduce their value, pose health threats
to livestock, or necessitate costly repairs to harvesting machinery.
Invasive insects can harm crop plants and reduce their productivity,
contaminate harvest surpluses, and act as vectors of disease
among livestock or in human populations. Control of these organisms
chemically, biologically, and mechanically exacts great expenditures
of time and money in addition to the losses mentioned above;
in the United States alone, these expenses may total many tens
of billion (or, by some estimates, well over 100 billion) dollars
annually (Pimentel, 1986; see Bright, 1998:174-194, for more
The ecological changes that
result from invasion are several and interconnected, ranging
from local to global in scale, although "the fraction of
successful invasions that alter ecosystem processes...is not
known" (D'Antonio & Vitousek, 1992:64). The weakening
or loss of a species in an ecosystem can leave room for other
species to take its place, potentially reverberating up the food
chain or even directly impacting other native species that were
once dependent on the one that was lost (Schlarbaum, et al.,
1999). Genetic diversity within a species as well as biodiversity
at the level of ecosystem may be diminished. At a landscape
scale, demographic changes, particularly in plant cover, can
lead to loss of soil and increased erosion, affecting the physical
structure of the habitat itself. At a continental or global
scale, these processes result in diminished "regional distinctiveness
of Earth's flora and fauna" (Vitousek, et al., 1997) both
through redistribution of species between and among continents
- homogenization - and through losses of native taxa to deleterious
impacts (predation, competition, disease) imposed by introduced
organisms - what might be called depauperization.
The impact of any particular
invasive species depends on many factors, including the number
of individuals that constituted the original introduction, the
number of locations in which introductions occurred, the mobility
and fecundity of the invasive species, and the species' habitat
specificity, or what conditions it requires to become abundant
and widespread. The narrower, or more specific, this set
of conditions, the more restricted the consequences will be in
scope, though effects may be locally more pronounced; the broader,
or less specific, the species' habitat requirements, the greater
the magnitude of disruption in both severity and geographic extent.
In turn, the degree to which a species becomes invasive will
be proportional to the difficulty of reversing consequential
environmental degradation and controlling further spread.
What can be done about
"Control of exotic
species is an extremely complex matter that involves societal
values, policy decisions, economic analyses, ecological interrelationships,
population dynamics, and technological capabilities" (Peine
& Lancia, 1999:268). Use of chemicals to control invasive
plants, trapping or killing of invasive animals, or introduction
of biocontrol agents (natural predators of an invasive species,
usually from the species' homeland) are effective to varying
degrees and at different geographical scales, and carry with
them their own sets of consequences: incidental killing of native
species, pollution of soil or water, selection for resistant
individuals, proliferation of new invaders, for example (see
Schlarbaum, et al., 1999; Dahlsten, 1986).
Relative to the ease with
which inadvertent extirpations and even extinctions of natives
occur due to bioinvasions, elimination of the invaders once they
have become established in their new environment is difficult.
Accomplishing this would require location and elimination of
all reproductively viable individuals of the invasive species
in the invaded area. This might be possible in some cases,
such as with large animals, or on islands (physical or ecological)
where biogeographic barriers have limited the spread of the invader
and serve to prevent reinvasion during and after the eradication
process. In cases of other organisms such as microbes,
insects, small and/or aquatic animals, and plants that grow rapidly,
root deeply, and/or produce numerous seeds (which can lie dormant
for years), "it is unlikely that an exotic can be exterminated
and reinvasion prevented, except perhaps in very localized areas,
because costs can be prohibitively high. The alternative
is a control effort that requires long-term commitment to constrain
populations to acceptable levels and spatial distribution"
(Peine & Lancia, 1999:269).
Stabilization of an environment in its invaded state is cited
as a first step in the control process, including preventing
both further introductions of non-natives and additional losses
of native species. Also advocated are reintroductions of
natives whose populations have been diminished or eliminated
by invasive species; if an ecosystem has been altered significantly,
however, this may not always be an option nor be guaranteed success
(Schlarbaum, et al., 1999). A third approach involves redirecting
patterns of disturbance so as to favor recolonization by native
species over that by invasive species, potentially entailing
large-scale manipulations of landscape processes such as fire
and flooding. Any effort aimed at controlling invasive species
and/or restoring native species and ecosystems entails decisions
about what the end product should look like. Because invasions
and changes to Sonoran Desert ecosystems have histories extending
back a century or more, it is difficult to know exactly to what
appearance, structure, and function - or to what point in time
- an area in question should be restored; alternatively, deciding
what aspects of the altered ecosystem to accept and maintain
in the interest of what remains may be the only viable approach
to managing an advanced invasive species problem (Patten &
What is the status of
invasive species in the Sonoran Desert region?
In a survey of scientists
working in the Sonoran Desert bioregion, "planting of exotic
grasses" and "biological invasions" ranked 7th
and 10th in the top ten threats to biodiversity in this region.
Most of the other eight threats - including urbanization, population
growth, water diversion, livestock grazing, groundwater depletion,
conversion to farmland, and recreation - have indirect consequences
either or both for placing stress on native biota and encouraging
the spread of non-native organisms. In an assessment of native
species and habitats at risk in the Sonoran Desert bioregion,
6 of 12 classes of species have invasive species as their major
threat; for 9 of 14 subregions and habitat types, invasive or
introduced species are cited among the major, worsening threats
to native species in these areas. Across all classes of biota,
it is estimated that several hundred non-native organisms are
now present, and in many cases well established, in the Sonoran
Desert bioregion; many are extensively distributed, and the most
severe are causing ecosystem-level changes that are or are potentially
irreversible (for more information, see Nabhan & Holdsworth,
(Salsola sp.) near an alfalfa field (Medicago sativa)
edged by barley (Hordeum murinum) and Johnson grass (Sorghum
halepense) near Maricopa, AZ (©1999,K.Mauz)