Ecology/Invasive species

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Chapter 6. Ecology of Invasive Species

Everyone may not accept what a particular ecologist or group of ecologists deem as "the way things work". Ecology is a relatively young and complex science. As a result, some ecological concepts are based largely on conceptualizing rather than experimentation. It is not easy to test many ecological hypotheses, because it is nearly impossible to control all variables in nature and to replicate nature in controlled settings. The subject or science of "invasive species" involves the basics of ecological thought and has considerable import to global species diversity. However, this is a subject entangled with controversy.

Kudzu, a Japanese vine species invasive in the southeast United States, growing in Atlanta, Georgia

According to Shea Katriona et al. (2002)[1], the Earth’s ecosystems are being majorly impacted by biological invasions. She suggests that the invasions can be studied by multiple viewpoints including,(1) the characteristics of the invaders,(2) the characteristics of the invaded communities,(3) resources available, and (4) natural enemies. Katriona used the community ecology theory by linking these viewpoints and by applying recent niche concepts with the communities in which they invade, which provides a predictive framework for invasion ecology. The framework presented here, however, shows how core issues of invasion ecology can be introduced into communities and lead to better understanding of the effects of invasion on earth's ecosystems.

Species status and terminologyEdit

The mosquito fish can have both detrimental and beneficial effects.

Even for biologists, establishing the distributional status of a species in a particular location is not always straight-forward. Questions often arise as to what exactly makes a species native as opposed to non-native, because some non-native species have no known negative impacts (Woods and Moriarty, 2001)^ . In fact, sometimes the same species can have both beneficial and detrimental effects. For example, the mosquito fish (Gambusia affinis)has been widely introduced around the world because of its suppression of larval mosquitoes, but also has negative impacts on native species of insects, fishes, and amphibians (Colautti and MacIsaac, 2004)^ . Also, natural biological invasions, generally considered range expansions, and introductions involving human activities are important and could be considered a normal ecological process (Vermeij, 2005)^ .

Non-native and native species may be sometimes considered invasive, with invasions often following human-induced landscape changes. The subsequent "damage" to existing ecosystems is simply a value judgment(Foster and Sandberg 2004)^ . For example, various status classifications exist based solely on the location or the discipline that a biologist is working in (Helmreich, 2005). As a result, many important terms relevant to invasion biology, such as "invasive", "weed", or "transient", include qualities that are "open to subjective interpretation" (Colautti and MacIsaac, 2004)^ . Accordingly, attempts to redefine commonly used terms in invasion ecology have been difficult because many authors and biologists are partial to a favorite definition (Colautti and MacIsaac 2004)^ . Also, the status and identification of any species as an invader, a weed, or an exotic are "conditioned by cultural and political circumstances" (Robbins 2005)^

The large number and current complexity of terms used in invasion ecology makes interpretation of the literature challenging and often intimidating. "Exotic", "alien", "transplanted", "introduced", "nonindigenous", and "invasive" are all words that have been used to describe plants and animals that have been moved beyond their native ranges by humans (Williams and Meffe 2005)^ . "Foreign", "injurious", "aquatic nuisance", "pest", and "non-native", are also used as descriptive terms, each with a particular implication. Even the use of what seem to be simple, basic terms to articulate ecological concepts "can confuse ideological debates and undermine management efforts" (Colautti and MacIsaac 2004)^ .

Table 9-1 illustrates the relationships among the most common terms describing species status. Setting aside the obvious difficulty that it is not always certain whether a species is truly native or not native to a particular location (requiring evidence of its presence prior to arrival of humans at that location), the greatest area of controversy involves the division of introduced species into invasive and non-invasive ("all others" in Table 9-1)because "harm" can be a subjective term based on individual judgement. Although there is a tendency to view "harm" or "pest" largely in economic terms, here we need to focus on the concept in an ecological sense.

Table 9-1. Terms related to introduced and invasive species
INTRODUCED (broad definition)
Established in the wild
(narrow definition)
All others
not listed*

*Not listed in any "official" source as a pest species

Prevalence of Invasive SpeciesEdit

Acacia baileyana or Cootamundra Wattle is an invasive species on the island of Australia

Invasive species are considered a "significant component of global change" as they cause both ecological and genetic changes in the communities that they invade (Sakai et al. 2001)^ . In fact, the number of species that are transferred from one biogeographical area to another each day is thought to be very large. Also, while the geographic trends and prevalence of these invasive species are not easily predicted, islands tend to contain the largest proportion. On average, though, most countries accommodate about 20% of invasive species in their vegetation. On a global scale, it is estimated that invasive species have come to occupy about 3% of the Earth's total ice-free surface over the last 500 years (Mooney and Cleland 2001)^ .

Invasive TheoriesEdit

The concept and implications of invasive species and invasion are explained by several ideas, including: (1)The Biotic Resistance Hypothesis and (2) The Invasive Meltdown Theory.

Biotic Resistance HypothesisEdit

The Biotic Resistance Hypothesis states that species-rich communities are more resistant to invasion because these types of communities are able to use the resources/nutrients more effectively/efficiently than communities with low species-richness.

The native cottontail rabbit among non-native plants like the garlic mustard, mugwort, and burdock

Species diversity, a term related to species richness, is also important to the Biotic Resistance Hypothesis. Species diversity has two components- (1) Species richness represented by s- which is the number of species in a community which is an assemblage of populations, and (2) Species eveness represented by e- which is the number of individuals of each species in a community.

There are several factors as to why invaders succeed: (1) they are highly competitive and which may allow them to reproduce more rapidly (2) proximity to the community is a crucial factor dealing with invasion and is divided into two parts- (a) abundance of the invader and (b) distance from the community that is being invaded upon, (3) dispersal rate of invaders to new habitat (there may be new migration paths that become available), and (4) degree of disturbance.

Invasive Meltdown TheoryEdit

Aerial picture of the Soo Locks between Lake Superior and the lower Great Lakes in Sault Ste. Marie, Michigan, USA

The Invasive Meltdown Theory states that as species are added to an ecosystem, each one representing a potential disturbance, the native system is perturbed in such a way that the system reaches a threshold, at which point it cannot resist any further and invasions occur exponentially. Every individual invader is a threat, but it is their collective impact that can cause the greatest damage to a habitat. This phenomenon is termed invasional cartels, and creates the conditions for a meltdown.

One illustration of the meltdown hypothesis is the invasion of the great lakes by zebra mussels. The lakes, once incredibly murky, became crystal clear after 15 years of filtering action by the mussels that were brought in on cargo ships from the Black Sea. While the result was desirable, the mussels caused the natural defenses of the lake to be out of flux and the lakes a very easy target for invasion of non-native species. When the amount of species invading the lake are greater than the environment can support, a collapse or meltdown of the entire environment results.


A male Ring-Necked Pheasant at Deer Park, WA.

The counterargument to the meltdown theory involves naturalized species. A naturalized species is one that has become established without any major impact to the environment. Mitchell argues that many naturalized species get a "bad name" from the invasive species that cause a great deal of trouble, such as the zebra mussels mentioned above.[2] A good example of a naturalized species is the Ring Necked Phesant. Intended to be a colony of game birds, it was brought to the mid-west where it flourished without disturbing the native biota. It co-exists with native game birds like partridge and grouse.

Invasive species do not necessarily always have negative effects. For example, in North Dakota, one invasive species is being used to battle another. Leafy spurge is a weed that grows rampant throughout North Dakota and inhibits all other plant life where it grows. The solution to this has been to bring in another non-native species, the flea beetle which feeds on leafy spurge alone. Notably, it has no recorded negative impacts on the environment. Therefore, although it is technically an invasive species, it is being used to keep the North Dakota prairie ecosystem intact.[3]

An Aphthona flava flea beetle is shown here feeding on leafy spurge.

Effects of InvasionEdit

Invasive species have the capability of drastically changing ecosystems, and meanwhile can also affect the human economy. Agriculture, forestry, recreation, and disease are just some examples of issues that can be positively or negatively influenced by invasive species. Sometimes the invasive species|invasive species can be beneficial, and sometimes humans face the nuisance of a troublesome invader.

With the expanding global economy, increasing trade volume and international trade agreements influencing trade worldwide, the risks of "alien" invasions are increasing. The high disturbance of industrial areas increases the disturbances of natural habitats and allow for invasive species to establish. In the last three decades in China, the increasing biological invasions run hand in hand with the increase in economic development (Wen Lin et al. 2007)[4]. In the last three decades China has had a significantly expanded economically with global trade and has consequently been affected by invasive species.

Economic and human factors have both indirect and direct factors which influence the transportation and redistribution of invasive species|invasive species. The increase in global trade by land air and sea has intensified the redistribution of invasive species from places that would normally be isolated. The study of ecological factors on the influence of invasive species are vary rarely studied, and may lead to improving the understanding, predictions, and management of invasive species.

At times, import of an invasive species|invasive speciesis desired for economical benefit, but that species is potentially harmful to another ecological or economical aspect. Much effort is being put into accurately designing risk assessment technologies to determine the net economic benefit of incorporating a nonnative species to an environment. Keller Keller et al. (2007) analyzed an Australian plant quarantine program (in which plants were used to make ornaments) to develop such a risk assessment program over a wide range of variables. They took into account long-term invasive effects over 10-500 years, and their results indicated that the benefits outweighed the costs in this case. The equation that was developed is: ER=((1-a)0A+a0(1-A))VT-a0(1-A)VI-D, where 0 is the number of new species,VT is the benefit of a single species, a is the rate of invasion, VI is the economic loss caused by an invasive species,A is a proportional accuracy,Dis the cost of administering the risk assessment preparation and execution, and ER is the resulting annual net benefit. This equation seems very straightforward and has accurate and even possibly underestimated results for net benefit. However, the terms can be hard to determine, and in the case studied in this project, the annual cost of risk assessment analysis was estimated to be $300,000. This is accounted for, however, by the term D, and is thus incorporated into the equation.

The Aedes albopictus, also known as the Asian tiger mosquito,has invaded many countries including the U.S. in the last two decades. It is a common carrier of diseases, including the West Nile virus.

Such risk assessments are important to the balance of economic and ecological success for a case in which incorporation of an invasive species would be necessary.

Invasive pests can kill plants and hamper agriculture, but sometimes can affect the health of animals and even humans. For example, the West Nile virus was never introduced to the Western Hemisphere until 1999, when mosquitoes that carried the disease were somehow transferred across the Atlantic Ocean. Mosquitos are the most prominent insect vectors of disease, and Juliano and Lounibos (2005)[5] used this quality to study the dyanamics of invasive species. The introduction, spread, and impacts of an introduced species can be fairly accurately measured in terms of spread of an associated disease. This particular study focused on the Aedes albopictus mosquito. This species was observed under varying conditions, and the effects of competition and predation were measured to determine whether they could slow or prevent invasion. Studies such as this can determine what types of environments (including both biotic and abiotic factors)are susceptible to invasion. This particular project emphasizes that full understanding of such a process must include introduction, spread, and long-term impact of the invasive species. If a study only monitors the current conditions, important dynamics of the system may be overlooked.


Although there exists many terms describing what is usually called species status for a particular location, most terms are not especially controversial and largely reflect that different disciplines (e.g., agriculture, botany, ornithology, entomology) have developed preferred terminologies for describing essentially the same things.


  • [6]Colautti and MacIsaac 2004.
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