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Exotic Species and Sustainable Ecosystem
Management
Tom L. Oudley 1 and Carla M. O'Antonio2
I
Bennudagrass can fonn mats on sandy stream and bank
substrates in the Sonoran Desert, and is resistant to natural
flooding. It also provides a refuge to macrophytes from such
distmbance, altering the benthic successional sequence and
reducing open sand substrates available for native invertebrates
and fish, particularly Agosia chrysogaster which builds nests in
clean sand. Tamarisk also invades desert riparian areas, and is
known to increase soil salinity, reduce sutface water and run-off
for downstream uses due to high transpiration rates, and provide
poor habitat for insects and native birds. In some streams which
experience frequent naturn1 flooding, however, it may not
dominate because it appears to be more susceptible to such
distutbance than some native riparian species but in regulated
streams and those with infrequent flooding, it remains a major
problem.
In many cases, control of exotic species is prohibitively
expensive or impossible. Their influences are with us
indefinitely because their propagules are widespread, and
invasions tend to be promoted by continuing human land-use
changes. These problems are global in nature, and rival other
global concerns such as climate change in terms of their impacts
on naturnl ecosystems.
The goal of ecosystem management is to simultaneously
promote ecological integrity and sustainable resource
production; however, one factor often not considered in resource
pIanning is the influence of introduced species on natural
ecosystems. While some exotic species are relatively benign
invaders, those of concern reproduce and disperse readily, but
also exlubit one or more of the following traits: (1) directly
intetfering in SUIVival of native species, with repercussions for
other interacting species; (2) changing the rate of resource supply
to the native community; (3) altering natural distutbance
regimes. We use the examples of bennudagrass (Cynodon
dactylon) and tamarisk (Tamarix spp.) in desert riparian areas,
and perennial grasses in Hawaiian forests, to illustrate why these
invasions have important implications for biodiversity,
ecosystem productivity, and/or human welfare.
The grasses Schizacrium condensatum and Melinus
minutiflora were introduced to Hawaii i as forage, but invaded
dry forest areas and the added fuel changed the fire record from
27 fIreS of an average of 4 hectares in the 47 years prior to
grass invasion of Hawaii i Volcanoes National Park, to 58 fires
of 205 hectares in the 22 years after invasion. Native trees and
shrubs are virtually eliminated, and total soil nitrogen declines
40% after two bums. Fire control expenses and the costs of
exotic eradication efforts comprise over 80% of the Park
Resources Management budget.
1
Pacific Institute, 1204 Preservation Park Way, Oakland, CA
94612.
2 Dept. of Integrative Biology, University of Califomia, Berkeley,
CA 94720.
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