On the Topic of an Invasive Species

Acacia mearnsii

An invasive species is generally defined as a species that is introduced to an area that it is not native to, spreads rapidly, is harmful to its non-native environment, and is potentially harmful to the economy and human health. Acacia mearnsii, also known as the Black Wattle or the Australian Acacia, is considered one of the worst invasive species in the world.

There are many different species of acacia trees around the world but the Black Wattle tops the list of them for worst invasive species. The Black Wattle tree is an evergreen that varies from 6- 25 meters in height and up to 50cm in diameter. A. mearnsii grows in moderate climates and cannot tolerate extreme heat or cold. A late chill in the rainy season will affect the tree’s survival and growth rates if it is young. However, older trees can survive a mild frost. In addition, A. mearnsii has a high transpiration rate and would not survive a drought. The tree also requires a certain soil composition in order to root and grow. However, the Black Wattle tree is known to thrive on drier, poorer soils, although it favors the moister, richer soils.

With so many limitations, it is difficult to believe that the Black Wattle tree would be such a powerful invasive species. But the fact remains that it is. A. mearnsii has an astounding reproduction rate. The tree is a hermaphrodite and flowers “profusely” in the winter time, although the exact season varies from location to location. It can also flower periodically throughout the year. The acacia tree is sexually mature at about 5 years of age and begins to produce seeds rapidly. The small yellow flowers on the tree are self-fertile, but with bees as the main pollinators, cross-pollination does occur. The seed pods mature in about 14 months and are dispersed simply by gravity or by the seeds drying up and being blown away by the wind as well as spread by rodents and birds. In some areas the seeds are spread to other locations by grazing cattle and other animals. Another interesting fact is that the seeds of the Black Wattle tree can lie dormant in the soil for up to six years and still remain viable. In addition, burning the area clean of the acacia trees only helps the seeds in the soil to germinate buy cracking the harsh outer coating of the seed. This makes controlling and irradiating the tree very difficult.

Along with it’s amazing reproductive rate, what makes A. mearnsii an invasive species is its ability to out-compete all other plants for water, nitrogen, and organic molecules. In a study done in 2004, researchers Dye and Jarmain discovered that a single Black Wattle tree has a water intake of 7 mm of water per day and results in a whopping 185 mm of rainfall lost per year per tree. In areas with little rainfall, such as South Africa, the Black Wattle tree is a huge threat to the surrounding ecosystems.

The Black Wattle tree, native to Australia, can now be found in over 50 countries and across six continents. It was introduced to Hawaii in 2002 and has taken over much of Maui. In all areas that it is found, the Black Wattle tree threatens biodiversity and reproductive rates of surrounding species. It alters the surrounding ecosystems and out-competes all surrounding vegetation for nutrients. This species of tree, if not controlled, is an enormous threat to not only ecosystems in areas with little rainfall, but to all the ecosystems it inhabits across the world.

As a form of control, there are many uses for the Black Wattle tree around the world. It is cultivated worldwide for tannin, a substance that tans animal hides into leather. The Black Wattle tree is said to produce more tannin than most tanniniferous plants. It is also used as lumber for fences, telephone poles, etc, in less industrialized nations. One other good thing about the Black Wattle tree is that, with such a long and intricate root system, the tree helps prevent erosion of the land by holding the soil in place.

With all that can be obtained from the Black Wattle tree, despite it’s destruction to the surrounding ecosystems, the tree is actually very useful for industrialization. If possible, a useful form of control for A. mearnsii is simply to utilize all of its parts. Other methods of control include spraying the acacia saplings with triclopyr, a systemic foliar herbicide, as well as the use of cecidomyiidae, also known as gall midges or gall gnats. Gall midges feed on plant tissues and create what is known as a gall, or an abnormal growth on a plant. There is research being done on whether or not the gall midges inhibit the reproductive rate of the Black Wattle tree. Another bug species that has been identified as a promising control agent for A. mearnsii is in the species Dasineura. The Dasineura sp. prevents fruit formation, but does not affect vegetative growth. This inhibits the Black Wattle’s ability to reproduce. Another insect species that was introduced into South Africa in 1993 is the Melanterius maculates, a seed-eating weevil, also native to Australia. The weevil caused reductions in the Black Wattle population in some areas. The larvae feed on the developing seeds inside the pods and the adults feed on the green pods and pinnules. Also in South Africa, a local fungus that colonized stumps was isolated and added to cut acacia trees to prevent re-growth.

A. mearnsii is a threat to earth’s biodiversity and some countries’ water supplies. Getting this invasive species under control is imperative. The Black Wattle tree is a danger to the surrounding ecosystems and cannot be left unattended. Despite the tree’s uses, it is too much of a threat to be left alone.

Sources

"Acacia mearnsii." AgroForestryTree Database. World Agroforestry Center, n.d. Web. 4 Apr 2011. .

Bauhus, J., P.K. Khanna, and N. Menden. "Aboveground and belowground interactions in mixed plantations of Eukalyptus globulus and Acacia mearnsii." Canadian Journal of Forest Research 30. (2000): 1886-1894. Web. 3 Apr 2011. .

Beck, Sascha, Robert Dunlop, and Annabel Fossey. "Stomatal length and frequency as a measure of ploidy level in black wattle, Acacia mearnsii (de Wild)." Botanical Journal of the Linnean Society 141.2 (2003): 177-181. Web. 9 Apr 2011. .

Brown, AG, and Ho Chin Ko. "Black Wattle and its Utilization." Rural Industries Research and Development Corporation (1997): 1-180. Web. 9 Apr 2011.

Duke, James. "Acacia mearnsii de Wild.." Acacia mearnsii de Wild.. Perdue University, 1983. Web. 20 Apr 2011. .

Dye, Peter, and Caren Jarmain. "Water use by black wattle (Acacia mearnsii): implications for the link between removal of invading trees and catchment streamflow response." South African Journal of Science 100. (2004): 40-44. Web. 5 Apr 2011. .

Maleka, Evelyn. "ACACIA MEARNSII IS A THREAT TO SOUTH AFRICA." Invasion Biology. NISL-Ecological Informatics, 24 Jan 2007. Web. 5 Apr 2011. .