Invasive Species from an Evolutionary Perspective

Though it may seem fairly innocuous, introducing new species to an ecosystem can have very profound effects. Some new species are introduced intentionally with hopes of changing an ecosystem for the better, while other species are introduced unintentionally or out of ignorance of the potential problems that may result. When an introduced species has an adverse effect on an ecosystem, they are known as invasive species. The fact that introduced species evolve in a different ecosystems with different organisms tends to create a slew of unforeseen consequences.

Invasive species can be introduced in several ways. Many invasive species are introduced with good intentions of improving an ecosystem or combating some sort of ecological problem. An example of a species introduced to combat a problem is the Cane toad (Bufo marinus).

Cane toads are originally native to South and Central America, inhabiting subtropical forests. However, they were introduced to many countries worldwide in hopes that they would help control a type of beetle that decimates sugar cane crops. Unfortunately, the toads didn’t help very much with the beetle problem. Instead, they started eating native species of terrestrial animals and competing with native amphibians for resources and breeding grounds. Cane toads also secrete a venom strong enough to kill many domestic animals and in some cases, humans. Although some efforts have been made to curtail their numbers, populations are still growing ("Ecology of Bufo Marinus" 2006).

Kudzu, a vine originating in Japan, was widely planted in the southeastern United States to remedy erosion problems. The vine worked quite well for erosion control. In fact, it worked too well. Kudzu spreads very quickly taking over everything in its path. It out-competes native plants by growing over them and blocking out light as well as out-competing native plants for nutrients with its massive root system. Control efforts have been moderately successful, but the vine’s range in the U.S. continues to expand (Forseth and Innis 2004).

Sometimes humans will unintentionally introduce an invasive species. Perhaps the best example of this would be organisms found in ballast water of ships. In order to adjust buoyancy as the ship’s load changes, they pump water in and out of their ballasts. When this occurs, organisms can end up in the ballast tanks where they are transported all over the world. The next time ship the docks and discharges its ballast water, the organisms are unintentionally introduced to a new ecosystem. The San Francisco Bay-Delta ecosystem has over 230 non-native species due to ballast water stowaways. Efforts are being made to stop more non-native species from being introduced via ballast water. The US has enacted several laws, such as forcing ships to exchange ballast water in high seas, not at port. Unfortunately, these types of laws are quite difficult to enforce (Buck 2007).

Lastly, invasive species can also be introduced out of sheer ignorance. A perfect example would be an irresponsible pet owner releasing a non-native species in a local ecosystem rather than disposing of the animal the proper way. Snakehead fish are turning up in U.S. waterways is a result of such ignorance and irresponsibility. The snakehead fish (from family Channidae), are highly predatory fish from Asia and Africa. They have evolved the ability to breathe atmospheric air, which enables them to live in polluted and/or stagnant water. This air-breathing ability also allows them to “walk” on land in order to relocate to other bodies of water. Several populations have been found in Maryland and at least one case has been linked to an irresponsible pet owner. Although some populations were exterminated, an active breeding population still exists in the Potomac River (Fahrenthold 2007).

The reason that invasive species can be so devastating to an ecosystem all boils down to evolution. As a species evolves in an ecosystem, other species in that ecosystem evolve with it. A natural balance of predator and prey is established and populations are kept in check. Competition for resources in a native ecosystem is also balanced. When a species is released into an ecosystem different than the one it originally evolved in, two things can happen. The species may die out due to improper environmental conditions, improper food sources, disease, or predation. The other possibilities are that the invasive species introduce new diseases that wipe out other species, or that they have evolved to live very well in that ecosystem, perhaps more so than native species. If the latter is the case and the species is able to reproduce, the results could be devastating.

The Cane toad has evolved to have very toxic venom called bufotenine. In its native range, there is an abundance of natural predators that are not affected by its venom. In the toad’s introduced range, however, predators are affected by the venom, allowing more toads to survive and reproduce. However, recent research has shown that snakes in Australia (an introduced range) are evolving features which help them consume Cane toads (Shine 2005).Other possible solutions scientists have postulated involve taking evolution into their own hands. Some are studying genetically engineering a lethal Cane toad-specific virus or introducing genetically modified Cane toads to invasive populations ("Ecology of Bufo Marinus" 2006).

There seems to be no evidence that any natural predators are evolving to take care of the Kudzu problem. However, scientists are going back to Japan and China to search for natural predators that evolved along with kudzu. They have found several natural predators, but unfortunately none of them are suitable for introduction to the United States as they would prey on native plants as well (K.O. Britton 2001).

There has been a good deal of research done on the evolution of the snakehead fish. It is highly speculated that the snakehead developed the ability to “walk” over land in response to the yearly dry seasons that occur in their natural habitat. When a pool starts to dry up, the snakehead simply leaves the shrinking body of water and looks for a new one. A study done in 2002 used mitochondrial DNA to track down the origins of snakeheads. The study suggests that modern snakeheads come from a lineage of ancient fish. They first originated in Africa around 120 million years ago when the Indian subcontinent was still a part of Africa. The two genera of snakeheads (Channa and Parachanna) are a result of vicariance that occurred when the Indian subcontinent separated from Africa. Eventually the Indian subcontinent collided with Asia, which established the Asian populations (Li 2006). Other studies show that snakeheads have evolved cellular mechanisms which allow them to live in waters polluted with toxic metal ions (Atif 2008) and they have evolved to produce an antibacterial mucus which protects them in stagnant, bacteria filled waters(Kuppulakshmi 2008). Given the vast amount of time, variety of environmental conditions, and molecular/cellular survival mechanisms snakeheads have developed, it is no surprise that they have become an invasive species which is difficult to control.

The reason invasive species occur is because species evolve in separate ecosystems and later become intermixed due to human actions. In the future, efforts should be made to be more diligent in preventing the introduction of invasive species. While scientists investigate new methods to remedy established invasive species, the general public should be educated on the potentially disastrous ecological effects of mixing separately evolved species. Although this clash of species may be devastating in the present, evolution will go on. Invasive species will continue to evolve in their new ecosystems just as the ecosystem will evolve with them.

Works cited

Atif, F., Kaur, M., Ansari, R. A., Raisuddin, S. "Channa Punctata Brain Metallothionein Is a Potent Scavenger of Superoxide Radicals and Prevents Hydroxyl Radical-Induced in Vitro DNA Damage." Journal of Biochemical and Molecular Toxicology 22.3 (2008): 202-08.

Buck, Eugene H. "Ballast Water Managment to Combat Invasive Species." CRS Report for Congress. Ed. Congressional Reserach Service, 2007.

"Ecology of Bufo Marinus". 2006. IUCN/SSC Invasive Species Specialist Group. 11/25/2008 2008. <http://www.issg.org/database/species/ecology.asp?si=113&fr=1&sts=&lang=EN>.

Fahrenthold, David A. "Potomac Fever." Washington Post 7/8/2007.

Forseth, Irwin N., and Anne F. Innis. "Kudzu (<I>Pueraria Montana</I>): History, Physiology, and Ecology Combine to Make a Major Ecosystem Threat." Taylor & Francis, 2004. 401 - 13. Vol. 23.

K.O. Britton, J. Sun, D. Orr, Z. Yang, C. Ping, T. Mingyi, J. Zi-de. "Usda Interagency Research Forum on Gypsy Moth and Other Invasive Species." Potential biocontrol agents for kudzu from China. Loews Annapolis Hotel, Annapolis, Maryland: 2001 Interagency Research Forum, 2001. 24-27.

Kuppulakshmi, C., Prakash, M., Gunasekaran, G., Manimegalai, G., Sarojini, S. "Antibacterial Properties of Fish Mucus from Channa Punctatus and Cirrhinus Mrigala." European Review for Medical and Pharmacological Sciences 12.3 (2008): 149-53.

Li, X., Musikasinthorn, P., Kumazawa, Y. "Molecular Phylogenetic Analyses of Snakeheads (Perciformes: Channidae) Using Mitochondrial DNA Sequences." Ichthyological Research 53.2 (2006): 148-59.

Shine, Ben L. Phillips* and Richard. "Adapting to an Invasive Species: Toxic Cane Toads Induce Morphological Change in Australian Snakes." Proceedings of the National Academy of Sciences of the United States of America 101.49 (2005): 17150-55.