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Alien Invasion!
In any environment there is competition between species to survive. There are adaptations which species have developed side by side with other species' adaptations. Plants develop chemicals which deter herbivores. Animals develop stomachs able to process toxins. By occupying a variety of spaces and differing roles, species reduce competition with others. Some reproduce in the fall instead of spring, or live in a crevasse that no other life can survive in, so that they can continue to survive. In a situation of high competition or long periods of isolated evolution, certain species can become extremely specialized; and are only capable of competing with other species in the immediate area.
Introduced species are plants or animals that are from other places. These species usually have predators in their native habitat, or other environmental conditions which keep their numbers in check. In the new environment that they have been introduced to, there is often none of the controls to keep their population in check as there were in their original environment. Invasive species can upset natural communities drastically and quickly. Species present in an ecosystem are able to compete effectively and survive with each other; but with new species that the inhabitants have developed no defenses to, they can be quickly over-run and destroyed. The species don't have thousands of years to adapt to new species, like they did with other plants and animals in its' community. Sometimes it is only a period of a few short years, and the population of an entire species can be wiped out. The problem with alien invaders is particularly acute in areas that are one-of-a-kind and have threatened or endangered species. The more specialized plants or animals become, the harder it is for them to adapt to change and regenerate their small population size from a limited number of individuals. Hawaii is a horrific example of what can happen to endemic (occurring only there) species when non-natives are introduced. To learn more about the dire situation in Hawaii see the Hawaiian Ecosystems at Risk Project at www.hear.org . Hawaii has more endemic species than any state in the U.S., but the conditions that have made it such a unique place, have also caused Hawaii to have more extinct species than any other state.
The threat of introduced species to the native environment is second only to habitat loss caused by human development. It is easy to see the losses that occur with the building of a new freeway, but environmental losses caused by invasive species are not so obvious. Something you might think is a pretty looking flower could be the environment's worst nightmare. For example, many weeds are so familiar that they might not even be seen as pests at all, like chickory, scotch broom, and mullen. Some invasive weeds that are wreaking havoc in the west such as purple loosestrife, are still grown and sold in nurseries as landscaping plants. Purple loosestrife is one of the wetlands' greatest enemies- taking over thousands of acres, crowding out native species, and making clear, clean waters into murky stagnant pools. Weeds are often unnoticed and the effects of the introductions are silent. Our native species are not going to scream "HELP!" There are still plants that grow and animals that live. Species that annihilate ecosystems can only be controlled if we are aware that there is a problem.
Invasive species have made their way here through accidents- as seeds in crop seed or in ships' ballasts, and some have been purposefully introduced- like the wild horses of the west. Some of the best intentions have turned out in the worst way. There are many invasive species in America that are a problem. The more we know about these pests, the better chance we have at controling them. If we cannot control them, then at least we will know the consequences of certain actions and not make the same mistakes again. I have chosen to focus on knapweed because it is a local as well as a national problem, it is comparatively easy remove without using chemicals, and because it poses a grave danger to native ecosystems.
Knapweed is an invasive weed. Some of the most familiar introduced species are Centaurea
diffusa (diffuse knapweed), C. maculosa (spotted knapweed), and C. solistice
(yellow starthistle). Diffuse knapweed was first found in the United States
in an alfalfa field in Washington in 1907. It is believed that it was introduced
in alfalfa seed that had been imported from Turkey or Europe. Other species of knapweed
had similar beginnings in the U.S. and Canada.
Today knapweed is widespread throughout western America as well as in Canada. Diffuse knapweed growth covers over 3.11 million acres (1,264,000 hectares) in the West, and continues to spread, covering about 18% more land every year. Yellow starthistle and other species also continue to define their own perimeters, spreading at an increasing rate. Knapweed is been successful in competing with native plants for space and water. It is great at establishing itself on disturbed sites such as roadsides, logged areas, new building sites, and overgrazed land. It grows well in arid conditions, and can also tolerate wetter sites, high elevation, cold weather, and continuing disturbance.
Although livestock and wild herbivores have been known to graze on knapweed - especially when there aren't more palatable species
present, rodents eat the seeds, and bees seem to enjoy the flowers; knapweed is limited in its'
usefulness. It is a sparsely foliated plant that doesn't provide much
cover for animals to hide in or get shade from; and most Centaurea species contain a chemical called cnicin which makes the
plant bitter and unpalatable to would be diners on the plant. Many have prickles and some are very sharp, which can cause injury
to the stomachs of animals, and to their skin as the animals walk or scurry through a heavily
infested area.
In the rangelands of the west grazing by livestock can contribute to the spread of knapweed. As knapweed gains a foothold on land because of its' ability to compete effectively with native plants in arid conditions, the more palatable native species numbers begin to fall. The livestock will eat what remains of the grasses, this in turn leaves more room for the knapweed to spread. It is a vicious cycle. Losses of good rangeland to knapweed have been estimated at 80% or more in some areas. Read more about how overgrazing can negatively impact the environment at http://osu.orst.edu/instruct/bi301/genconce.htm .
Knapweed is most common in disturbed areas where the natural
environment has been changed drastically making is easy for knapweed the gain a foothold
and take over, but can also invade undisturbed native communities and displace indigenous species.
Knapweed's long taproot is very efficient in using the nutrients and moisture available, and they produce
the chemical cnicin, which inhibits other plants growth. The chemical is in the leaves of the plant, and
gets into the ground through the leaching of the chemical from live plants, and from decomposing
plants from previous growth. Only other knapweed plants can grow in heavily infested areas since they are
resistant to their own growth inhibitors.
Cnicin inhibits other plants root systems, making it easier for knapweed to
reach limited water resources with little competition. The chemical can still be active after years of being in the soil.
Cnicin may also inhibit the seed germination of other plants, making it difficult to reestablish native
communities even after the knapweed is gone.
Another concern with knapweed is soil erosion. Unlike many native species with fibrous root systems that
bind the soil together, knapweed has a single, long taproot. In areas dominated by knapweed, soil erosion
can contribute to an already bad ecological situation, causing long term effects which are
difficult to remedy by any solution.
Nothing.
Something.
Anything.
Although the amount of plants, the ways they can spread, and sheer numbers
of seeds can be daunting, everything must begin somewhere. The major problem with knapweed
is its' tremendous reproductive potential, and the way these seeds are spread. The average diffuse knapweed
(C. diffusa) plant will produce about 900 seeds a year at maturity. Some species of knapweed are
capable of producing even more seeds a year. These seeds are light and spread by the wind.
Some of these seeds aren't dispersed immediately, but scattered when the stalks break off
in fall and are blown
to wherever the tumbleweed rolls. Some of the seeds germinate immediately, but some of the
seeds can germinate after being dormant in the soil for up to ten years.
The attributes of knapweed make it difficult to effectively control, and impossible (it
seems) to eradicate. The nature of the far travelling, long dormant seeds means that the only
possibility in getting the invasion under control is a far reaching and long term commitment to
their destruction. It does little good to pull all the plants in one small area so that it is
free of knapweed, if adjacent areas are still infested by it. It is also not effective to destroy
all the living knapweed plants one year, then quit. Dormant seeds in the soil will germinate, and soon the
problem can be just as bad as what it was in the beginning.
If all this information doesn't scare
into thinking it is a hopeless situation, and you would like to try to make a difference
in this invasion there are ways that you can help. If you are acting out of concern as a citizen or
because your land is infested, there are people that you can contact to help or to get help.
Most counties have a noxious weed control board or program, which is a good place to start.
Government organizations such as the Forest Service, the Bureau of Land Management (BLM), the
National Park Service, the Department of Fish and Wildlife, the Department of Natural Resources,
and the numerous branches of the Department
of Agriculture often have volunteer opportunities as well as information for help
on private lands. The universities of your state can be a great source of information
about invasive weeds also. In Washington, the University of Washington and Washington
State University and its' cooperative extensions in local communities tend to be very knowledgable,
or can find someone who is.
In central Washington there are many opportunities to get involved in knapweed control.
Part of the Make a Difference Day in the Wenatchee Valley is knapweed pulling, and last year they
had a wonderful turn out of volunteers who pulled thousands and thousands of plants. The Nature Conservancy
of Washington State is very active in knapweed eradication with many knapweed pulls throughout the
year on TNC lands. If you would like to volunteer for the Nature Conservancy of Washington
Lisa Younger is TNC of Washington's volunteer program
manager, and can be reached by email at lyounger@tnc.org . In the Leavenworth area knapweed pullers
which have included such diverse groups as The Mountaineers and work parties from the Chelan County Jail,
have banned together in eradicating knapweed from the Icicle Canyon area. More about this project
can be seen at the Leavenworth
Ranger District's noxious weed site.
The following is a list of some of the things that can be done to control knapweed. Much of the facts stated are
just that- facts. Some of the recomendations are my own opinion, and I strongly suggest that you take them
with a grain of salt. Before implementing any noxious weed control programs of your own, take the time to
fully educate yourself on whatever method you might choose.
Pulling knapweed by hand with patience, a strong back, and friends has been successful in some situations. Because of the enormous amount of work that goes into pulling the weeds and the doubts of the its effectiveness, there is some disagreement on hand pulling knapweed. On one side some people want to use herbicide, saying it uses less labor, time and money, and kills the young plants as well as the mature plants. On the other side are people who are concerned with the effects of the herbicides on the natural communities surrounding the weeds, the leeching of chemicals into water systems, and human health. If the world was a perfect world and I had my way, pulling the weeds by hand would be my solution. Unfortunately, with the magnitude of the problem, I don't feel that hand pulling alone can work. There are just too many weeds. Take a local situation such as the Icicle Canyon near Leavenworth Washington. If every person who visited the Enchantments, the Chiwakum Mountains, went rock climbing, or stayed in a campground out the Icicle would pull ten knapweed plants everytime they enjoyed the valley's scenic beauty, three million plants a year would be killed. At that rate there wouldn't be any more knapweed to pull in a very short period of time. Well, needless to say this is not how is works in that area. There is a small, dedicated group of volunteers that spend hours and hours pulling thousands of weeds every year; and although the situation has improved, there are still many areas there that are in need of help. When knapweed pulling is used in conjunction with other methods of controlling the weeds, the chances of success are greater than just hand pulling alone.
I have spent hours pulling knapweed myself, and
find it very satisfying to go back to a place I have pulled and see no knapweed where there once was
a "forest" of the stuff. The plants are
biennials, and will live only long enough to reproduce. Until the plant is old enough to do this, it
remains a rosette of leaves with no stalk. These rosettes are difficult to pull without disturbing the
soil around the plants too much by digging them up. It is much easier to wait until the plant sends up
the reproductive stalk, then pull it. The best time to pull the plants is after they send up a long stalk from
the basal rosette of leaves, but before it flowers so that no seeds can be produced. A word of warning though, if you do decide to hand pull knapweed
please wear gloves and wash your hands afterwards. Be careful not to get sap from the plant into any cuts
or abrasions on your skin. Centaurea species may contain carcinogenic properties, or might be a trigger
to activate cancerous cells in the human body. Research on knapweed seems to be fairy scant on this
subject, and the only information on this that I have heard is what might be called a rumor; but it
is a good idea to be overprepared for something that might never happen, than find yourself in an
irreversible situation. Wear gloves!
One of the problems with removing knapweed is that it disturbs the soil and leaves an empty space, creating a perfect environment for other knapweed seeds to blow in and reinvade the site. By reseeding when the knapweed is killed, or in the immediate future afterwards, the chances of sucessfully controlling the population can be increased. Seeds which are native to the area are best when considering a site next to a native community of plants. Another option is reseeding with non-native species of grass. Crested wheatgrass, Agropyron cristatum, has shown the most promise out of the grasses which have been studied (which are few). It has the ability to outcompete knapweed for moisture, reducing both the rate at which seeds germinate and seedlings develop. Crested wheatgrass is not native to North America though, and consideration must be given to replacing one exotic species with another.
Many of the species of knapweed can not tolerate continuous cultivation or irrigation. Beneath three inches of soil many of the seeds will fail to germinate, and plowing can destroy the live plants. Conditions that are too wet seem to decrease the population of knapweed in some areas. The reason for this may be because of the live plants' intolerance to too much moisture, because the competitive advantages that knapweed has over other plants is reduced if moisture is abundant, or because of the seeds germinating at the wrong time of year to grow sucessfully.
A high intensity fire can kill most knapweed and destroy the seeds. However a low intensity fire
will probably allow the long taproots to remain living within the ground and resprout. There is also
a good chance of reinfestation in disturbed areas such as burns. High intensity burning could work
well with reseeding after the burn.
Mowing is not very effective. It may look a little better but those plants are still there. Over a long period of mowing several times a year it could be possible to exhaust the plants and finally have them die, but knapweed is a plant that lives to flower and set seed. The plants will fight you every step of the way.
There are at least 9 exotic species which have been introduced in North America to battle knapweed.
None of them have the ability to control and successfully eradicate the problem by themselves. Some
of the more successful biological controls of knapweed are the Banded Gall Fly (Urophora affinis),
the UV Knapweed Seed Head Fly (Urophora quadrifasciata), and the Broad-Nosed Seed-Head Weevil
(Bangasternus fausti). Each of these, when conditions are optimum, can reduce knapweed seed production
by 95%. Unfortunately most species of Centaurea only need about 0.01% of the seed they produce
yearly to sustain a population at its' current level of infestation. The Sulfur Knapweed Moth
(Agapeta zoegana) is also fairly efficient in reducing the numbers of small plants, and preventing
large plants from flowering. Some people to contact who know much more about these
biological control agents and releasing them are:
This section on chemical control is not meant to be the easy way out of a bad situation. It is only
a tool to use under careful judgement. I wish I could say that these
chemicals are safe, but they destroy life, and do it well. Humans are life too, be careful.
Glyphosate is effective during the year of application. It is a non-selective herbicide which kills
knapweed and anything else you happen to put it on. The good thing about glyphosate is that it binds
with clay particles and biodegrades in the soil rapidly. Glyphosate must be applied directly to the
surface of the target plant for these reasons. The half life of the chemical is about 60 days in the
soil. Glyphosate is routinely used by the Forest Service and the BLM for chemical control of
invasive plants.
Picloram is effective for 3 to 4 years (in optimum conditions such as arid land) and kills knapweed
seeds and live plants. It is a relatively non-selective herbicide which can kill other
broad-leaved perennial plants. It does not bind with soil particles and can leach into groundwater.
Its' half-life is 55 to 100 days in the soil, and from 2 to 41 days in water. Picloram is
considered to be one of the most effective weapons against knapweed, but the advantages of using
it must be weighed against the risks in an intelligent treatment program.
It is my hope that this information is helpful. Invasions by non-native species is a great threat to our natural environment. It is an enormous problem that cannot be solved by only one person, or by only one solution. I believe that the situation can improve but only if people know that there is a problem, and that something can be done. It is self defeating to tackle knapweed alone, or with only a few others. Cooperation from individuals in communities, private landowners, and government institutions are essential to be effective.
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