Aquatic invasive species have gained attention primarily because of the spread of the zebra mussel, an invasion which will require a $2- billion cleanup over the next two years, and up to $10 billion during this decade. Invasions into natural habitats is the focus of Dr. Dudley's work. The zebra mussel is primarily a problem in "disturbed" systems as opposed to "natural" ecosystems. This raises the question: is there really a clear separation between natural and modified systems? The distinction is actually quite arbitrary because important natural elements, for example, some endangered species, may be present and threatened in systems that have been quite modified by humans.
Some examples of problem aquatic invaders:
Impacts
1 Species/Populations
At this level, exotic organisms can endanger a species' existence
and/or reduce populations. One example of a species/population
level impact is genetic introgression from stocked trout populations
which hybridize with natives, diluting their genetic integrity. In one
case, there may be no natural population of true native lahontan
cutthroat trout; the only true lahontan cutthroat populations have
been bred and introduced into fishless waters for their protection.
Similarly, hybridization has resulted in the loss of genetic
individuality in crayfish populations, for instance, the threatened
Shasta crayfish. Aquarium fish introduced into springs are major
competitors and predators threatening native desert spring fish and
western trout.
Native golden trout are endemic to the Kern River watershed in California and are severely threatened by introduced brown trout. An aggressive, multi-million dollar eradication program has nearly eliminated brown trout in the Kern river system. The program consists of electroshocking (which stuns but doesn't kill the fish), followed by application of a chemical treatment, and finally, replacement of only golden trout. To keep migrant brown trout from coming back into the system, rock structures like waterfalls are built to prevent fish from coming back upstream. By starting at the headwater and moving downstream, the project seeks to create a stream network free of brown trout. So far, the program appears to be successful at eliminating most brown trout and protecting the only endemic habitat of golden trout, but the presence of stragglers requires periodic retreatment.
The history of trout in the American West is characterized by moving them around in coffee cans. There is almost no river system that has not had non-native trout introduced over the past 100 years or so. The taxonomy of aquatic invertebrates is relatively unknown; there are now low and greatly modified invertebrate communities as a result of all the introduced trout. Introduced brook trout and other fish may have eliminated most populations of three species of frogs in ponds and streams. Dave Bradford's work on mountain yellow- legged frogs suggests that trout introductions are the major factor in frog loss, while disease and drought (which dries out satellite populations in shallower ponds) are other factors. There is only a handful of surviving populations of this frog species. They are near extinction and may have lost some genetic variability. One program is using seining (nets) to remove brook trout and then will reintroduce frogs to these ponds.
Non-indigenous species are largely responsible for threatening native species with extinction: about 58% of listed fish species are documented as threatened by exotics, while 15% of protected bird populations are threatened by exotics. Two-thirds of all endangered species face exotics as a major threat. Aquatic habitats function similarly to islands, where exotics are also a major threat; similarities include discrete environmental boundaries, more limited habitat, fewer refuges from exotic invaders, and high dispersal abilities of non-indigenous species. Populations in confined areas tend to be more susceptible to exotic threats and local extinctions than populations that are distributed over larger and more heterogeneous habitat.
2 Communities
Nile perch were introduced into Lake Victoria in Africa as a food
source and ended up eliminating about 200 species of native cyclid
fishes. Two-thirds of the species of cyclids once found in the lake are
now gone and this percentage is rising rapidly as Nile perch spread
throughout the system. Nile perch now make up about 80% of the
fish biomass in Lake Victoria. The perch yield only one-third the
price per pound of the native cyclids, so the impacts have been
enormous on all levels Ð biological, ecological, economic, and
sociological. Fishing communities have collapsed and been replaced
by large industry which can take advantage of the harvest-to-
harvest nile perch. In addition, the native cyclids can be air dried
for storage whereas perch can not, and so are usually smoked,
leading to clearing of land for firewood, resulting in erosion and
other environmental damage. This species introduction clearly has
caused cascading effects through the entire socio-bio-ecosystem in
just ten years, roughly from 1980 to 1990. In general, introduced
fish can have extraordinary effects in a system because they often
can quickly become dominant species in the communities.
3 Ecosystems
An example of ecosystem impacts is the introduction of Bermuda
Grass to the southeastern US for cattle forage, lawns and livestock
feed. It makes a carpet on the stream bottom and increases the
resistance of substrate to disturbance during floods, eliminating the
scoured habitat preferred by native fish and invertebrates (a clean
sand substrate that provides the spawning area for native fish). This
also creates habitat for the fathead minnow and other exotic fish that
compete with natives, and facilitates establishment of watercress,
aquatic buttercups and other weedy vegetation. The result is a
short-circuit of the natural successional process which sets up what
we label an "invasion complex" in which one invading species favors
additional invaders. It doesn't work to look only at single species Ñ
they can act in synergy with other invaders in a manner similar to
the synergism of two "safe" chemicals which can interact to become
toxic.
Another example of ecosystem impacts is invasion by tamarisk, a tree found in Southwestern US riparian areas, which results in a series of effects on an ecosystem. Tamarisk is effective at picking up salts from the soil and depositing them on the surface by dropping leaves. This duff on the substrate makes it difficult for anything to establish under it and insects and birds decline for lack of habitat and food resources. Tamarisk is dependent on disturbance to get established; seeds are deposited on banks and grow rapidly. Until it matures, tamarisk is not tolerant of flooding and so thrives in controlled (dammed) systems which eliminate natural flood cycles. On Sycamore Creek in Arizona, numerous young cottonwood saplings survived a larger-than-normal flood, while very few tamarisk survived this natural disturbance agent. Similarly, flooding of Coyote Creek in southern California killed 63% of tamarisk plants present and only 14% of the native riparian trees. As flood frequency decreases, and trees can grow to a larger size, tamarisk become more resistant to the occasional floods and can take over.
This raises the issue of global climate change and how a predicted drier western US could favor expansion of exotics over natives. More variability in rain events and longer droughts may provide a longer window of opportunity for trees like tamarisk to establish and attain resistant size. Also, as water becomes ever scarcer, currently unregulated streams may become dammed and regulated and thus made susceptible to invasion by tamarisk.
In the research of Sarah Kupperburg in California, she found that bullfrog populations increase and native red-legged frogs decrease during drought periods and the reverse occurs during higher flow periods. Longer periods without flooding also tend to favor development of silty, slower stream habitats that favor transplanted Eastern fish species over native Western species. Several biologists have suggested that by using flow releases from reservoirs, especially short-period high-volume releases to scour stream channels, we could reduce habitat for exotics and maintain the type of habitat needed by natives. Can we use flow management in this way as a tool for protecting native species of both flora and fauna? In general, change in the natural disturbance regime favors establishment of exotics. Flooding at least once every four years appears to be necessary to prevent the establishment of tamarisk. An interesting direction for future research is to examine ways in which we can manipulate altered ecosystems to retain the important elements of natural disturbances.
Ecosystem management approaches for aquatic systems
We need to look at the problem of non-indigenous wetland species from a watershed perspective. Jurisdictional boundaries are inappropriate ways to organize our approaches to ecosystem problems because there are many interacting downstream impacts of upstream actions. The Natural Communities Planning Program in the San Diego region is an example of an eco-regional approach. Instead of fighting battles spot by spot, they have backed off, looked at a map, and are attempting to determine areas suitable for protection or development, and what species are involved. Another example is Aquatic Diversity Management Areas (ADMA), promoted by fish ecologist Peter Moyle, which are based on identifying important co- occurring groups of species, rather than focussing only on single- species management, and concentrating protection on certain key representative areas of each multi-species assemblage as a form of triage which directs efforts efficiently to protect the maximum representation of our natural heritage.
How to use limited resources in the context of improved environmental protection is an important management question. Environmental groups worry about shifting control to regional councils, as is being encouraged by Secretary of the Interior Bruce Babbitt, because when local groups control the planning process, their track record for environmental protection is not that good. Local groups interests are often influenced by local development pressures. Furthermore, national interest must be represented in decision-making at the local level. The Endangered Species Act (ESA) is an effective tool for promoting national interest. Some believe the ESA's emphasis on single species has siphoned off money, attention and interest from a real ecosystem management approach, stressing single species protection instead of natural community and unique habitat protection. Others feel that plants get short shrift as we put the majority of our resources into a few mammal species, although some of the more spectacular cases of radical single-species protection involve rare plant genotypes. We also tend to throw a lot of money into highly degraded areas that perhaps should be written off and the focus shifted to preserving more intact ecosystems. The ADMA system has five categories based on degree of degradation and gives each a different level or process of protection. Dudley again emphasized the distinction between protecting terrestrial and aquatic systems. For aquatic systems, we absolutely must take a watershed approach or management efforts cannot work, due to the extremely interconnected relationships in these systems. With terrestrial systems, it is sometimes possible to take a smaller area and protect it. However, we should bear in mind that in most cases, aquatic and terrestrial ecosystems cannot truly be considered independently from each other.
General Patterns of Aquatic Invasions:
Some specific examples of aquatic system invasions in the Western US: