An invasive plant species, or any invasive species, is one that arrives in a habitat it had not previously occupied, then establishes a population, and spreads autonomously (Simberloff 2010a).
It easily adapts to the new environment and grows rapidly until it dominates or “invades” an area by outcompeting and hindering or killing other species, usually the native species.
In Southern Togo, Africa, Radji et al. (2010) reported that 15% of the forest fragments under study were very highly invaded and 31% were slightly invaded.
Most dominant among the invaders is the neem tree (Azadirachta indica).
The introduction of invasive plant species, either native or alien, has become a grave threat to biodiversity conservation.
Many invasives (plant and animal species) have in fact led to the extinction of various species (Simberloff 2010a).
It is therefore important that crop selection, as well as the choice of existing plants for special use such as companion crops, shade crops, and hedge crops, should be well thought of.
It should also consider the long-term effects of these plants on various concerns not only on sustainable agriculture but also on the environment and biodiversity.
It is basic in crop farming that the process of selecting crops to be grown in any farm area has to consider adaptation, the existence of and accessibility to the market, and profitability.
In addition, anyone cannot just introduce a lovely plant that will turn out later to be an invasive weed or any organism purported to be a rich source of nutritious food that will in time become a ravaging pest, as in the case of the introduction of the golden snail in the Philippines.
Examples of plants that have become invasive in different parts of the world include the Australian paperbark (Melaleuca quinquenervia), South American water hyacinth (Eichhornia crassipes), African molassesgrass (Melinis minutiflora), American tufted beardgrass (Schizachyrium condensatum), firetree (Morella faya) (Simberloff 2010a), common reed (Phragmites australis), reed canarygrass (Phalaris arundinacea), Elymus athericus, Douglas fir (Pseudotsuga menzeisii), and Virginia pine (Pinus virginiana) (Simberloff 2010b).
In the Philippines, some of the important invasive plant species are the large-leaf mahogany (Swietenia macrophyla), paper mulberry(Broussonetia papyrifera), hagonoy or Siam weed (Chromolaena odorata), coronitas or baho-baho (Lantana camara), ipil-ipil (Leucaena leucocephala), Chinese creeper (Mikania micranth), yemane or gmelina (Gmelina arborea), Acacia mangium, Eucalyptus camaldulensis, Toona ciliata, water fern (Salvinia molesta), and water hyacinth (Sinohin and Cuaterno 2002; Baguinon et al. 2005).
In particular, mahogany is considered a major invasive plant species in the logged-over forests in the Philippines.
Mahogany possesses characteristics that make it successful in invading natural forests.
It has a fruit that contains an average of 62 seeds and one tree can possibly produce 60 fruits.
Upon maturity, the fruit (botanically a capsule) opens and releases the seeds.
Each seed is light and is supplied with a flat, elongated appendage that favors dispersal by wind.
The seed is capable of germination in less than a month even in areas that are relatively shaded.
A mahogany plantation has been likened to a “green desert.” The leaves of the mahogany tree are rarely browsed by animals.
When they fall upon abscission, they form a thick mat on the floor which prevents seeds, including its own, from having contact with the soil.
This hinders seed germination.
The reddish leaves can be rich in tannin and decompose slowly.
In addition, it is possible that mahogany retards the growth of other plants through allelopathy (Baguinon et al. 2005).
Ironically, it appears that the deleterious effects of mahogany on native vegetation and biodiversity have not been properly disseminated.
The planting of mahogany trees is continuously promoted and there is a steady demand for potted seedlings.
At the time that the paper was presented, Veronica O. Sinohin was with the Ecosystems Research and Development Bureau of the Department of Environment and Natural Resources (ERDB-DENR) while Wilma R. Cuaterno was with the Bureau of Plant Industry (BPI).
On the country report under the names of N.T. Baguinon, M.O. Quimado and G.J. Francisco are University of the Philippines, Los Baños and Forest Management Bureau, Department of Environment and Natural Resources.
The water hyacinth (Eichhornia crassipes), commonly called erroneously water lily, is likewise an introduced, invasive plant species that have now become a nuisance in many rivers, ponds, and waterways.
It has impeded the natural flow of water causing spillage or change in the direction of the water current.
As a result, some urban areas are always flooded on rainy days.
The ability of neem trees to dominate the native vegetation was clearly shown by Radji et al. (2010) in their study of the effects of alien species on forest fragments in Southern Togo.
In General Santos City and Sarangani, Philippines, it is obvious that the neem tree is no longer a simple avenue tree and tree plantation species.
It has grown rapidly and has widely spread by natural means.
Contrary to promotional campaigns at the onset of its introduction, its purported multiple uses such as for lumber production and as insect repellant have not been proved.
The fruits of neem tree are eaten by birds and some nocturnal animals, thus seeds are easily dispersed.
The seeds germinate easily and the tree grows rapidly even with scarce water. In addition, ruminant animals including goats do not feed on the leaves and bark.
Where the trees are closely planted or in pastures where there is dense growth, the growth of native grasses and other plants under the canopies has been suppressed.
The neem tree is certainly an invasive plant species.
BAGUINON NT, QUIMADO MO, FRANCISCO GJ. 2005. Country report on forest invasive species in the Philippines. In: The unwelcome guests. Proceedings of the Asia-Pacific forest invasive species conference. Retrieved November 14, 2011, from http://www.fao.org/docrep/008/ae944e/ae944e09.htm.
RADJI R, KLU K, KOKOU, K. 2010. Forest invasion by alien plant species: the case of neem tree (Azadirachta indica A. Juss.) in Southern Togo. Int. J. Biodvers. Conserv. 2(10):300-307. Retrieved December 5, 2011, from http://www.academicjournals.org/ijbc/PDF/pdf%202010/Oct/Radji%20et%20al.pdf.
SIMBERLOFF D. 2010a. Invasive species. In: Navjot SS, Ehrlich PR, editors. 2010. Conservation Biology for All. Great Clarendon Street, Oxford: Oxford University Press. p. 131-152.
SIMBERLOFF D. 2010b. Native invasives (Box 7.1). In: Navjot SS, Ehrlich PR, editors. 2010. Conservation Biology for All. Great Clarendon Street, Oxford: Oxford University Press. p. 131-132.
SINOHIN VO, CUATERNO WR. 2002. Invasive alien species resource directory for the Philippines. Excerpt from the paper presented during the workshop on “The Prevention and Management of Invasive Alien Species: Forging Cooperation through South and Southeast Asia”. August 14-16, 2002, Bangkok, Thailand. Retrieved November 14, 2011 from http://www.arcbc.org.ph/arcbcweb/pdf/vol2no4/30-32_invasive_philippines.pdf.