The proliferation of invasive species can have profound effects in nature. Most notably, these invaders are a direct threat to species diversity and endangered flora and fauna. In fact, invasions create a greater loss of biodiversity than any other factors outside of habitat loss and direct exploitation (Barnes, 1998). Our natural areas are continually fighting the threat of invasion and many times it prevails. However, periodically an invader becomes established and its population explodes. Charles S. Elton referred to this in his classic 1958 book The Ecology of Invasions: "It is not just nuclear bombs and wars that threaten us, though these rank very high on the list at the moment: there are other sorts of explosions."
In a time when preservation and conservation are mainstream topics, the understanding of exotic species is paramount. This paper will cover some general background on exotic species and explore one particular invader of south Florida, Melaleuca quinquenervia.
An invasive species is a species that has certain biological characteristics that allow for rapid colonization and population growth. Characteristics of invasive plants include: production of small seeds, early age at reproduction, reproduce by seed and vegetatively, seeds are animal dispersed, no special seed germination requirements, self-fertile, and no close relatives among native species (Randall and Marinelli, 1995).
Not all exotic species become invasive. In fact, very few species become established for reasons such as a lack of chemical defenses and mutualistic relationships. However, those exotic species that do become invasive are particularly troublesome. Their effects in the United States range from coast to coast. Some of the better-known invaders close to Oxford are Amur honeysuckle, kudzu, and zebra mussels. In Florida, one out of every three plants is non-native (Enserink, 1999).
Invasions usually occur as a result of disturbance. This may come in the form of a tornado, hurricane, blowdowns, etc. However, most of the disturbances are man-made (i.e. clearing for development, etc.). Many areas are subject to invasions including both aquatic and terrestrial habitats. Particularly vulnerable are wetlands, which bridge the gap between aquatic and terrestrial habitats.
Wetlands are vital resources. They are hotspots for biodiversity, nurseries for fish and crustaceans, help regulate runoff, mitigate flooding, control erosion, etc. One of the most outstanding wetlands is the Everglades of southern Florida. This amazing area is currently under attack by an exotic invasive tree, Melaleuca quinquenervia.
M. quinquenervia is native to Australia and in the Myrtaceae family, which is the third most diverse plant genus in Australia (Turner et al., 1998). Most Melaleuca spp. are small trees and shrubs. M. quinquenervia is one of 10 large tree species placed in the M. leucadendra species complex and is the only species that inhabits south Florida.
M. quinquenervia was introduced to Florida in the early 20th century. The first trees originated from seeds sent from Sydney, Australia in 1906. The seeds were planted along Biscayne Bay. M. quinquennervia was introduced into the Everglades in 1936 when seeds from Broward Co. were aerially broadcast over the eastern portion of the Everglades (Turner et al., 1998). In 1941, the United States Army Corps of Engineers planted M. quinquenervia on levees south of Lake Okeechobee for erosion control. This is one of the major sites of infestation today along with the eastern Everglades, western Lee Co., and the Big Cypress area in southeastern Collier Co. (Turner et al., 1998). M. quinquenervia infests ~ 202,000 ha (500,000 ac) of wetlands in south Florida including 10,000-20,000 ha (25,000-50,000 ac) of monocultures (Turner et al., 1998).
M. quinquenervia thrives in south Florida due to the similarities with its native habitat in Australia. The climate of south Florida is sub-tropical as is the native habitat in Australia. Both areas are located at ~26º latitude (26ºN in Florida and 26ºS in Australia) and experience wet summers and dry winters with frequent fires. The role of fire is important in the biology of M. quinquenervia. It has several attributes that allow it to thrive in fire-maintained wetlands. The ability of M. quinquenervia to withstand fire allows it to outcompete fire-intolerant species. The thick, spongy bark helps in retarding the effects of fire. The word Melaleuca is Greek for black and white and is said to refer to the white bark that is often charred by fire (Debenham, 1962). Fire rarely kills M. quinquenervia trees and epicormic sprouts often form on the trunk that promote more growth (Turner et al., 1998). Like some pine trees, M. quinquenervia releases seeds when fires occur. The problem in the Everglades is that many of the sawgrass prairies are being converted to M. quinquenervia forests.
M. quinquenervia exhibits many of the traits common to invasive species. The tree sometimes flowers in its first year and is in flower virtually year-round. The seed capsules are very small and each contains some 200-300 seeds. A mature tree can hold as many as 20 million seeds. The seeds are very light (~34,000 seeds/g) and thus can be dispersed rather easily (Turner et al., 1998). The seeds seem to germinate well under most conditions (wet, dry, etc.). Furthermore, seeds contained within the capsule may stay viable for up to 10 years (Turner et al., 1998). Though the seeds are hardy, the seedlings are more vulnerable to environmental conditions. It appears that this may be a key time to implement management techniques.
The management and control of M. quinquenervia in south Florida has become a large issue. There have been several techniques employed to control M. quinquenervia. These come in the form of chemical, physical, mechanical and biological control procedures.
The major chemical control method is the use of the herbicide Arsenal (a.k.a. Chopper) (Randall and Marinelli, 1995). The active ingredient is imazapyr, which can possibly damage non-target species in the area also. The herbicide is applied to the individual tree after girdling and thus is labor intensive. Other herbicides including Velpar - L, Spike 40P, Garlon 3A, and Rodeo are applied aerially on monocultures (Turner et al., 1998).
Mechanical and physical techniques such as cutting and removing, prescribed burns and flooding have shown some success. Timing is important particularly with the fire treatment. It is most effective on the seedlings before they have developed the spongy bark, which suppresses fire.
One area that is growing in terms of control is that of biological control agents. This measure aims primarily to suppress reproduction. Furthermore, the biological control agent may weaken M. quinquenervia and make it more susceptible to fire. The most promising biological control agent is Oxyops vitiosa (Coleoptera: Curculionidae), a leaf weevil who feeds on young leaves and retards growth. There are several other insects currently being tested in quarantined studies both in the U.S. and in Australia. Some of these insects are Boreioglycaspis melaleuca, Eucerocoris suspectus, Lophytotoma zonalis, Pomponatius typicus, Fergusonina sp., Lophodiplosis indentata, and Poliopaschia lithochlora. Due to the adaptability of M. quinquenervia, it is beneficial to employ several different control techniques.
It is extremely important that M. quinquenervia is controlled so that it doesn't decimate the entire Everglades. M. quinquenervia is a threat to the Gulf Coast of Louisiana and eastern Texas due to the habitat in these areas. In order to halt the spread of this tree scientists, conservationists, other natural resources personnel, and citizens must work together.
In years past, the information regarding invasive species has had two sides. Many of the invasive species are aesthetically pleasing (i.e. Japanese barberry, English ivy, butterfly bush) and thus sold at nurseries and greenhouses. Consequently, wildlife managers, conservationists, and other professionals are trying to persuade people to utilize native species in their gardening and landscaping endeavors. We are just beginning to realize the detrimental effects that an invader can have on natural areas. Education is a key to controlling this problem in the future.
Barnes, T.G. 1998. An introduction to the issues surrounding exotic organisms. Natural Resources Newsletter. University of Kentucky Cooperative Extension Service. Lexington, KY.
Debenham, C.N. 1962. The genus Melaleuca. Australian Plants. 1: 23-29.
Elton, C.S. 1958. The Ecology of Invasions by Animals and Plants. Chapman and Hall. London.
Enserink, M. 1999. Predicting Invasions: Biological Invaders Sweep In. Science Magazine 285 (5435): 1834.
Randall, J.M., and J. Marinelli. 1995. Invasive Plants: Weeds of the Global Garden. Brooklyn Botanic Garden, Inc.
Turner, C.E., T.D. Center, D.W. Burrows, and G.R. Buckingham. 1998. Ecology and management of Melaleuca quinquenervia, an invader of wetlands in Florida, U.S.A. Wetlands Ecology and Management 5: 165-178.
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