This article or section should be merged with Invasive species

Invasive plants are defined by their ability to invade and disrupt an ecosystem. While normal species stay within a set range and have predators or other limits on their growth, invasive species overrun these barriers and can easily take over an exotic ecosystem. Collectively they are one of the greatest threats to biodiversity and stable ecosystems today. The reason invasives are so successful is multifaceted and is still an inconclusive issue. Such invasions cost billions to economies all over the world; everything from loss of grazing land to the destruction of keystone native species are caused by these rogue invaders. Invasive plants can be spread in many ways, by accident and on purpose. Many of these plants have been spread through impure seed or through ornamental gardening. Others can appear in foreign areas by hitching a ride on a vehicle or in cargo.

There are dozens of examples of plant invasiveness throughout the world. One example is Bromus tectorum (Drooping brome grass), which spreads rapidly after burning, and crowds out plants vital to grazing, while itself being of low nutritive value to grazing animals. In the southern United States, Pueraria lobata (Kudzu) was originally planted to stop roadside erosion, and now covered large areas with its leafy vines. It has been known to swallow up entire fields and forests if left unchecked. All over the world, invasives are crowding out native species and destroying ecosystems.

But why should we be truly concerned about invasive species? First of all, invasives cost the United States $173 million a year in terms of management and economic loss. More importantly, invasive species have the ability to wipe out whole ecosystems, terrestrial and aquatic. Though much research has been done, the exact cause of invasion has yet to be pinned down. There are several possible mechanisms by which the invasive species become invasive, and more often than not it requires a special set of circumstances for an invader to become established, then reproduce enough to become a threat to the native habitat. This article will examine a few of these mechanisms to give a better understaning of what we know about how invasive plant species act in the wild ^1-3.

Contents

1 Mechanisms 1.1 Species-based mechanisms 1.2 Life history 1.3 Superior competition 1.4 Alleopathy 1.5 Ecosystem based mechanisms 1.6 Unused resources 1.7 Unfilled niches 1.8 Ecosystem instability 2 Human Impact 3 Conclusion 4 See also 5 References

Mechanisms

As the literature stands now, there are several proposed mechanisms to explain invasiveness. These mechanisms generally fall into two different categories; one for mechanisms which focus on the invasive species,and the other which focuses on the invaded ecosystem. More likely, it is a combination of several mechanisms that cause an invasive situation to happen.

Species-based mechanisms

Species-based mechanisms address many of the basic techniques of plant competition. While all plants are able to compete in some manner just to survive, invasive species appear to have specific traits or combinations of specific traits that make them especially dangerous. In some cases it can be as simple as having the ability to out compete native species. Other situations are more complex, such as allelopathy, which is a mechanism where the invader directly or indirectly prevents other plants from growing nearby.

Life history

The life history of an organism describes the different stages of life an organism will go through during its lifetime. Such traits are tempting to study because life history is a quantifiable trait that could lead to very predictive models. Several traits have been singled out by researchers as possible advantages, such as the ability to reproduce sexually and asexually, rapid sexual maturity and high phenotypic plasticity. The ability to breed either asexually or sexually is important because asexual reproduction allows for huge amounts of offspring while sexual reproduction allows for a more robust mix of genetic material, leading to greater phenotypic plasticity in the species as a whole.

The data for this mechanism goes from way back to the very beginning of invasive species study to current research now. The majority of studies agree in a general sense on which kind of traits mark an invasive species, but there are differences in how invasive each trait can make a species. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found that invasive species tended to only have a small subset of the invasive traits, and that many of these invasive traits were found in non-invasive species as well. This is one of the great difficulties in invasive species research: while many generalities can be made about invasive species, there are always blatant exceptions to these observations ^4-6.

Superior competition

An obvious trait of invasive species is their ability to out-compete native species. There are often huge differences between how an invasive species interacts with its native ecosystem, and with the ecosystem it is invading. Most often, the invading species simply has a better chance at acquiring resources. Since these resource situations can be modeled as zero-sum games, any gain for the invader is a loss for the native. After all, new resources are not generally being created in these situations.

There are other reasons that an invader might be a superior competitor. One example is that an invasive plant may be inedible to the local predators, allowing it to flourish where the native species are constantly held in check. Combined with a high rate of reproduction, the predators would find themselves competing with each other over increasingly fewer and fewer native plants, while the invader is taking the place of the native species. Eventually the predators would drive the native species out of the ecosystem due to increasing pressure and lack of replacement, and the invader becomes more established in the mean time. After the invader has completely taken over the area, there is little left, given the nature of the food web, and the foundation native plants have within it ^{7, 8}.

Alleopathy

There is a common trait among some plant species called as alleopathy. Also known as chemical competition, this mechanism is a competitive tactic in which a plant will secrete various chemicals to make the surrounding area undesirable to other plant species. The purpose of alleopathy is to essentially give the invader some breathing room in order for it to establish itself. Alleopathy comes in two flavors: direct and indirect. Direct alleopathy consists most commonly as a chemical secreted by the invasive which inhibits growth nearby. Indirect alleopathy is much more subtle. Examples of this include cheatgrass or eucalyptus trees that produce large amounts of dry, flammable materials, which encourage fires that control or eliminate competition.

One example of this is with the knapweed species Centaurea diffusa. This Eastern European weed has spread its way through the western United States. Experiments show that 8-hydroxyquinoline, a chemical produced at the root of C. diffusa, has a negative effect only on plants that haven't co-evolved with C. diffusa. This leads to the conclusion that C. diffusa is only a problem outside of its native habitat. Such a result shows that it can be difficult to predict which species will be invasive just form looking at it in its native habitat ^{9, 10}.

Ecosystem based mechanisms

While the previous section dealt with invaders that had special advantages over native species, this section will look at the invaded ecosystems instead. These ecosystems could all be described as not being in equilibrium. Something is out of place, and this gives an invader an opportunity to establish itself where under normal circumstances it normally would not be able.

Unused resources

Imagine a small child throwing bread crumbs into a lake. After a few minutes, the child is surrounded by ducks, busy eating up the crumbs until they are gone. This mechanism works in a similar manner. When examining an ecosystem, it is important to look at two things: the amount of resources available, and how much of those resources are being used. In a stable ecosystem, all of the available resources are used up, and all of the species there have enough to continue to survive.

Yet what happens if spare resources happen to appear in an ecosystem? For example, lets say a forest fire has taken out all of the grass species in a given area, leaving only hardy brush and established trees. Since these grasses are gone, and the resources used to maintain them are not being utilized, these extra resources can be used by another species, possibly one that is invasive. Sure, these most likely will be used by the native species nearby, but what if these resources require a special trait to acquire or use effectively? Perhaps long taproots are needed. This is where the ducks in my earlier example come in. Such an ecosystem is at risk to invaders, since they can move in and establish themselves with the extra resources. While this many sound like a simple swap, that is not the case. Such exotics are never the same as the species they replace, and can serve to ruin an ecosystem, simply because they have no predators or encroach onto other species.

The data shows that nitrogen and phosphorus are often the limiting factors for a situation such as this ¹¹. However, this is not as dire of a situation as it sounds. For an invasion to occur, there have to be members of the correct invasive species ready and waiting for this flux of resources to occur in its favor. If either of these two parts are not present, the invasive species will have to find another way to establish itself.

Unfilled niches

Basic ecology tells us that every species has a role to play in its native ecosystem; some are general while others are highly specialized. These roles are known as niches. This mechanism describes a situation where the invaded ecosystem in question has unfilled niches, which are promptly filled by an invader. While there are cases where the invader can stay within its niche, these species often bring about other traits in them, which cause harm for the rest of the ecosystem, such as lack of suitable predators or alleopathic traits.

While this mechanism sounds reasonable to the casual reader, the data itself is much more mixed. Various experiments have shown positive, negative and neutral correlations between ecosystems with high diversity and invasiveness. From this I would argue that while this mechanism may make sense in theory, it does not pan out that well in the literature. Perhaps its effect is simply too weak, and is over powered by other mechanisms, or perhaps the concept of a missing niche is too broad in its application, and needs to be more strictly defined, or better yet, combined into one of the other ecosystem based models ¹².

Ecosystem instability

Imagine a situation involving a group of people who have mastered a game, and are able to beat anyone outside of their group. They know the rules inside and out, and are constantly perfecting their strategies to win. Few, if any, have a chance against this group. Then suddenly the rules suddenly change, and this group is at a loss, since their strategies no longer work the way they used to, and other players with vastly different strategies are able to come in, and win where they could not before. This is how the familiar ecosystem mechanism works.

The concept of a successful invasive species seems to be a bit odd. After all, wouldn't species, which have co-evolved together for years, be able to out compete an exotic species? There has been much time for them to evolve and become as efficient possible. This mechanism describes a situation where the ecosystem in question has suffered a disturbance of some sort, which changes the fundamental nature of the ecosystem ¹³. Examples of this type of disturbance can range from the loss of an important predator to the eutrophication of an aquatic environment. In situations such as these, the specialized evolution of the native plant species becomes wasted, and non-native species can gain a foothold.

Human Impact

Ironically enough, it is humans which have the greatest impact on the situation of invasive species. The vast majority of ecosystem disturbance comes various forms of development. Such development can be organized into four categories: conservation, utilization, replacement and removal ¹⁴. These levels describe how we use the land, everything from wildlife preserves to parks to agriculture all the way to urbanization. It is within these land decisions that many of the mechanisms of invasiveness come into play. Take the earlier example of the cheatgrass: it was at first unable to establish itself in the soil, because it was much too hard. Then, when cattle ranchers introduced their cattle to the area, these heavy animals broke up the soil, disturbing it enough to allow cheatgrass a foothold on the ecosystem.

More general scenarios to consider:

• Horticulture and Agriculture; from seed contamination to outright cultivation of exotic species
• Development directly causes a keystone species to go extinct, allowing an invader a chance at establishment
• Pollution runoff can add nutrients into the environment, opening up a niche in the ecosystem
• Genetically engineered plants could escape

Conclusion

The issue of invasive species is a serious problem that has the potential to destroy ecosystems all over the world, taking with it the local economies and important natural resources ². To help with this situation, there are many things we must do. First of all, we must continue the research that has already been started into the causes of invasive species. The more we know, the more we can predict the effects of such species, and be able to do something out.

Also, we need to thing long and hard about importing exotics. We need to ask, "What would happen if this species broke free?". We should understand its lifecycle, and its place in the native ecosystem. Lastly, we need to be careful about the effects of development and other projects that disturb the environment. Disturbance is an easy way to introduce an invasive species into an ecosystem. Something as simply as building a house could open the local ecosystem up to exotics from all over, especially when exotics are part of the development in the first place ¹⁴.

Lastly, we need to understand that the problem of invasive plants species is not a problem that is going to solve itself and go away. Every moment that passes means that another ecosystem has been invaded, and native species within are in danger of facing competition never seen before, and disappearing forever. When this happens, the ecosystem becomes nothing more than the invaders, losing the biodiversity that made it valuable in the first place.

See also

• Introduced species
• Invasive species
• List of invasive species

References

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