Current Summary

In review, the science behind invasive species has a good deal of evidence to support that these invaders can cause problems. However, the belief that exotic species are completely damaging to the new ecosystems they find themselves in is not entirely correct. There is evidence to suggest that some invasive species can be either helpful or at least non-harmful.

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Invasive species, when in a new environment, can cause large problems. They can upset the food chain by having qualities that no species in the current ecosystem can combat (the Cane Toads or the Lionfish). They can spread to cover incredible distances (like green algae) and shove life out of the way, and even out-compete the native animals in their niches (many types of invasive mussels) and cause extinctions.

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However, what many people do not know is that certain invasive species could have some beneficial properties. Some could perform important acts, such as cleaning water (Asian Oysters) or fixing important nutrients (green algae) better than the native species. Others could be put to good use should they be found in an ecosystem, like the creation of biofuels (green algae). They can also simply benefit an ecosystem, like the Red Sea invaders bringing the eastern Mediterranean from an ecological vacuum to a functioning ecosystem.

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But above all, what people do not know is that there are many unknowns in this science. The scientists who were in the Mediterranean and catching mostly invasive species are unsure of whether or not these species will cause mass extinctions in the sea, or whether they will simply fill the empty niches in the east and thrive. Also, Sax et al. does not know if the plants he studied will continue to live side by side, native and naturalized, in a one-to-one relationship, or if they will eventually level off and the native species will begin to decline.

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The evidence is out there for both sides, but the study of the effects of invasive species on the ecosystems they invade is not an exact science and it depends entirely upon the ecosystem involved and the nature of the species present in it and those invading.

General Misconceptions

People have a lot of ideas about invasive species that might not be entirely accurate. For instance, while many can recognize invasive species (such as kudzu, knapweed, etc) because of the problems they cause, their idea of native plants can be slightly skewed. Many of the crops grown in the US today are not native to the Americas. They arrived during the settling of the New World with the Europeans for use in agriculture. While not technically invasive because the spread is mostly controlled by the farmers who grow them, they are non-native.

Another misconception about the science of invasive species is that it is rather sure that these species have negative impacts on the ecosystems they invade. But it is not entirely sure whether this is true or not. In a previous post, I mentioned that there is a one-to-one ratio of invasive and native plants on the islands in the Pacific ocean according to Sax et al.. In that same paper, Sax mentions that it is too early to tell if this pattern of native and invasive species will continue or if it will level off, or possibly the native populations will decline towards extinction. It is the same with the issue in the Mediterranean sea. Although several of the species invading from the Red sea are displacing the native species, the majority of them are filling an ecological 'vacuum' that was present in the eastern part of the sea. This is because even before global warming, this part of the Mediterranean was warmer than typically supports the native species, so the invasive Red sea creatures are filling niches that were previously empty. In this case, it seems like the invasive species from the Red sea were beneficial to the eastern Mediterranean by reviving, in a way, a dead ecosystem. But the point is that the scientists are still unsure as to what will occur in the Mediterranean. Will the invasive species stay separate from the native species, or will the new species take over the jobs of the native Mediterranean aquatic life and kill them off? There is just too much uncertainty to tell for sure at this point.

Finally, as mentioned in the previous post, there is an alternative hypothesis to invasive species being the cause of biodiversity loss. Some scientists believe that the invasion of exotic plants and animals is an effect of the degradation of the ecosystem, not the other way around. Humans cause environmental issues that alter the climate and destroy the landscape, and the invasive species come in to fill in the gaps left by anthropogenic activities. This would make sense in terms of the issue in the Mediterranean at the moment. There was a hole in the ecosystem in the eastern side where native species would not thrive, and so those of the Red sea came in to fill it in. But, again, the certainty is not there.

Positives of Invasives

Before now I have been focusing on the detrimental aspects of invasive species, but what most people do not know is that the effects of invasive species are not always negative. There are some benefits provided by the introduction of a new species.

One such benefit is outlined in a paper by Sax et al. (in Links: Species invasions and extinctions: The future of native biodiversity on islands). In it, Sax et al. claim that though invasive animals almost always lead to extinctions of other species, invasive plants can be beneficial on islands. They go on to include data from the International Union for Conservation of Nature's databases, showing the significant difference between vertebrate, invertebrate, and plant extinction patterns over the past 500 years. Finally, they analyze the ratio of native and naturalized plant species (invasive species that have been in the ecosystem for a good deal of time) on oceanic islands. While the level of biodiversity of plants is high on land-bridge islands, it is found to be greatest on oceanic islands. Sax et al. showed that there is a one-to-one ratio of native and naturalized plant species and that over the years, the number of naturalized species has increased on six islands near Australia and New Zealand over the past 160 years. Basically, what this paper is showing, is that with more invasive species on islands, you only get more plant biodiversity. It appears as if the new species may be filling slightly different niches than the native plants and that they are able to live cooperatively successfully. Beyond that, increased biodiversity is very desirable for other reasons, such as maintaining natural processes, air quality, water quality, etc. For a list of reasons for the importance of biodiversity, see the corresponding link in Links.

Invasive species may have certain economic benefits as well, for some perform jobs more effectively than native species. For example, the Asian oyster has been considered for introduction into the Chesapeake bay to help improve the water quality. Asian oyster are more adept filters than the native oysters, and they also have thicker shells and are less prone to acquiring disease. Another potential benefit that is being looked into is the use of algae for biofuel. Some scientists and corporations believe that algae is a much better alternative to biofuel produced by corn. Further research is being done and there is little conclusive evidence, but if this ends up being the case, then explosive populations of invasive algal species such as the green algae in the Mediterranean could be used as a primary fuel source in lieu of oil and natural gas.

Finally, several scientists are coming to believe that rather than a direct causality between native species loss and exotic species increase, invasive species are merely taking the available space left over after the ecosystem has been altered. Didham et al. specifically, in his paper (in Links: Are invasive species the drivers for ecological change) asks whether the invasive species are drive changes in an ecosystem or if they are merely 'passengers on the ride'. In his paper he states that rather than invasive species causing the loss of biodiversity it is more anthropogenic changes that alter ecosystems. Didham claims that human industries cause other problems, such as carbon dioxide increases in the atmosphere, altered landscapes, and changing the biogeochemical cycles, and these drive global warming, which is the main cause of biodiversity loss. Invasive species are just Nature's way of filling in the holes to ensure all the niches are filled.

Uprooting the Food Web

Another problem attributed to invasive species is that of upsetting the food web of an ecosystem. When a new animal is added to an system, it needs to have something to eat and it needs a place on a trophic level. This can cause a large reshuffling in which population sizes can change, or possibly become extinct.

One classic example of an invasive species which caused problems is the Cane Toad (Bufo marinus) in Australia. The Cane Toad is a species of amphibian native to Central and South America. It was introduced several years ago in Australia as a way to eradicate Cane Grubs, which were small insects eating the roots of sugar cane being grown in Australia. But rather than getting rid of the bug infection, the Cane Toad ate anything else, and has now spread over about one-fourth of the country. This wide distribution is due to the fact that the toad is incredibly poisonous and it has nothing that can eat it where it is located. Several high-level predators have been found dead with dead Cane Toads in their mouths. So without a population control, their numbers exploded, and with more toads, Australia began to see a decline in the populations of animals that the toads were eating. This one species rearranged the whole ecosystem simply because nothing can eat them.

Another species that is slowly becoming invasive is the Lionfish (Pterois volitans), off the eastern coast of the United States and in warmer tropical waters. Like the Cane Toad, Lionfish are poisonous, so they do not possess any natural predators in the ecosystems they invade. It also preys upon native species in the Atlantic ocean, upsetting the natural sizes of populations. They mainly cause the same problems the Cane Toads cause in Australia, increasing in numbers with the only limiting factor being the amount of food available to them.

In the article on the Mediterranean sea, they examine a type of algae that has for the most part pushed all life to the fringes of their sprawling meadows. They spread rapidly and, as if sheer numbers was not enough, they secrete toxins that kill mollusks, fish, and sea urchins. They effectively take over a large area of water and once they spread, it becomes very difficult to get rid of it.

In short, invasive species can rearrange the food web of an entire ecosystem and alter it completely. Isn't that enough to deem it a problem?

Competition

One side of the argument is that invasive species pose a problem by out-competing native species filled in the same niche. It is widely believed that if an invasive species enters an ecosystem and the job it performs is already filled by a native species, the invasive will typically out-do the native and drive it to the brink of extinction.

One such analytical experiment was performed by Callaway and Aschehoug (in Links: Invasive Plants Versus Their New and Old Neighbors). In it they looked at the effects that Centaurea diffusa, or Diffuse Knapweed, on both the other plants in it's natural habitat and the ones in it's invaded habitat. They showed that this plant had a much greater negative effect on the other weeds in North America (the invaded habitat) than it did on those in its natural area. This is due primarily to the Knapweeds roots, which make it better able to compete for resources. This ties into the idea of evolution, because in the ecosystem where the Diffuse Knapwood evolved, the other species evolved alongside it in order to keep up with its advancements. The plants in North America do not have that advantage, so they have not evolved ways to keep up with the invasive Knapweed. This is generally a basis for the idea that invasive species are problematic; because they did not evolve in the ecosystem which they have invaded, the native species are not equipped to compete with it.

This is explored in a paper by Carol Eunmi Lee (in Links: Evolutionary genetics of invasive species). She claims that the evolutionary genetics of invasive species has not been explored extensively, but it could open up new doors in the science. Recent studies, however, have suggested that the success of some invasions of species has more to do with their ability to withstand natural selection rather than direct competition with native species. Further exploration in this area could demonstrate the importance of evolution in the effects of an invasive species. If the species is completely foreign and the native plants have not had time to adapt to it, then of course the invasive will be in competition with the native species. However, is it possible that if an invasive stayed in an ecosystem long enough, the entire system would evolve with it and create a completely new system?

In the article cited in the first post, we see that there has been a huge influx of tropical species into the Mediterranean sea due to warming of the waters. It is at a peculiar stage at the moment, where the waters are warm enough for the tropical species to thrive but the cold water species are still there. This causes a huge influx of biodiversity, but many scientists believe that this is only because there has not been enough time for the invasive species to exterminate the native ones. In some case studies, this has already happened, with the Red sea species taking the place of the Mediterranean counterparts. One specific example is the rabbitfish, which feeds on algae. So far it has out-competed similar grazing species from the Mediterranean off the coasts of Lebanon, Israel, and Libya, further marginalizing the populations of the native grazers.

Evidently, there is a problem here.

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http://www.nytimes.com/gwire/2009/09/22/22greenwire-biodiversity-a-bitter-pill-in-tropical-mediter-89434.html

The above link (which can also be found in the Links box) deals with a prominent environmental issue facing ecosystems today: the re-assortment of species. Invasive species is a problem that is being seen much too frequently today, especially now that global warming is making certain ecosystems more inhabitable for other species.

In the article, we see that global warming and anthropogenic (man-made) changes to the Suez canal and the eastern part of the Mediterranean sea has caused a huge influx of foreign animals and plants to the Mediterranean from the Red sea. The increased temperature of the oceans has made it so that the Mediterranean can support the life from the Indian ocean, and the Suez canal has been opened up enough to the point that the species can cross freely.

This begs the question of whether invasive species is something that we should fix. Is it truly a problem? There is a lot of information out there about their dangers. Some include the fact that invasive species may rearrange an ecosystems food web because typically these new species have no natural predators in ecosystems where they have not evolved. Also, new species might be better equipped to perform the jobs of native species, and therefore they might out-compete them and drive the native species extinct.

However, there is also information out there to suggest the opposite idea. For instance, in the article above it is shown that though 80% of the fish caught by the ship in the Mediterranean sea were invasive species, there was not much being harmed because the new species were focused mainly in the warmer eastern part of the sea, which had previously been considered an "ecological vacuum". Basically the invasive species were taking up niches that were previously unoccupied. Also, there are some experiments conducted that show that, specifically with plants, increased invasive species only increases species diversity, because each plant has a slightly different niche and so all can live in the same ecosystem without serious conflict.

On this blog, I am going to show all sides of this issue, with supporting evidence to back up each side, in order to unmask the misconceptions of the science and to display the uncertainties.