After looking at that heading the first thought that may pop into your head are movies like E.T, Men in Black or Star Wars, however it’s a very different type of organism that I am about to chat to you about today.
If you take a living organism such as a plant and put it in a new location, thousands of km/miles away even on a separate continent it will either die due to certain obstacles such as dying on route to its new location, being preyed upon by certain animals, it may not acclimatize to the different weather patterns or even may not get the correct nutrients it needs from the soil. Sometimes however certain types of plant species are able to overcome these barriers and become naturalized. Once this happens these plant species can spread across large areas. Due to the growth of globalisation over the past 500 years the ability for these alien invaders (hence the name because they invade native ecosystems and take over) has spread all over the world. Alien vegetation has been recorded as early as the 19th century. There are alien plant species that do not become invasive, conservationists are mostly concerned with the invasive plant species. The early settlers who colonized the different parts of the world brought with them different species intentionally for a variety of different reasons such as for ornamental use, medicinal reasons, fire wood or to stabilize dunes. Other species were brought accidentally, being carried from seed onto ships by sailor’s boots or equipment. The amount of introduced plant species in the United States, United Kingdom, Australia, South Africa, India, and Brazil range from about 2000 to 50,000 species however these numbers include animals, birds and plants, I will just be dealing with plants in this post.
Factors which are affected
Alien plant invasions are considered a major cause of human environmental change and has become a growing problem in every biome on earth. Biological invasions threaten the world’s biodiversity by changing the structure and function of ecosystems and disrupting important biological interactions. These invasions have also been considered a major cause of recent extinctions and have had a substantial economic impact. The problem with biological invasions is that it is expected to become a larger problem in light of global climate change. Invasive species can displace indigenous species and in some places completely dominate certain areas. Some plant species found within city environments can spread into surrounding natural areas, where they can pose large threats to indigenous species. Almost two-thirds of all known invasive alien plant species have originated from gardens.
Alien species can affect agriculture, forestry and human health. Biological invasions are also widely recognized as being the second-largest global threat (the first being direct habitat destruction) to biodiversity. The biodiversity is lowered due to the alien vegetation which spreads and takes over the indigenous vegetation. When there is no indigenous vegetation the pollinator’s decrease, the birds that eat the pollinators decrease as well as the animals that rely on the indigenous plants for food decrease and thus the predators of those animals decrease. It’s a domino effect. In a number of areas around the world, the most troublesome, time-consuming and costly tasks of conservationists and managers are those relating to controlling alien species, preventing impacts with ongoing repairing of systems damaged by alien species. A number of alien plant species draw in large amounts of water through their roots compared with indigenous plant species which are more water wise. Some examples of water guzzlers are the Pine species, Wattle species and Eucalyptus species. Another way alien plants can affect indigenous ecosystems is by changing fuel loads, which can in turn affect fire behavior and, ultimately, change the fire regime characteristics such as frequency, intensity, extent, type, and seasonality of the fire. An example of this can be seen in the Western Cape of South Africa which is home to the Cape Floristic Region. The vegetation type called fynbos needs fire at certain year intervals. The heat or in some instances the smoke of the fire stimulates the seeds and they start growing (this phenomenon is called serotiny) however with the spread of alien invasive plants, it creates larger fuel loads. This leads to more intense (hot) fires which instead of stimulating the seeds, kills them. A further complication is that the seeds of the alien invasive species are also stimulated by fire and are adapted to these more intense fires. This causes the alien invasive species to spread more rapidly taking all the nutrients from the indigenous plants.
There are three methods which are commonly used around the world to combat the spread of alien invasive plants. These are:
Mechanical clearing is done with a variety of tools such as axes, chainsaws, slashers, loppers (large sheers), handsaws and bow saws, tree pullers and brush cutters. Literally these tools are used to cut trees and bushes down as well as pulling them and their roots out of the ground. Brush cutters are used for removing densely packed infestations.
Sometimes, if cutting the entire alien tree down is expensive or you would still like the birds to nest in the tree or maybe just for ornamental purposes, ring barking is a good option. Ring barking is a process whereby an axe or slasher is used to cut off the outer bark layer of the tree 500-700mm wide right around the trunk. This will kill the tree slowly. Some trees species may start to regrow at the base or on the old branches, herbicide can be applied to alter this.
Herbicides can be costly attempted on a large scale but at the same time very effective. There are many different types of herbicide available on the market. There are two types of ways to apply herbicide, the first is applying the active ingredient to the soil which will then be absorbed by the plants roots or applying it directly to the plant itself either with the plant intake such as for grasses. Woody shrubs and trees are commonly cut at ankle height with the herbicide being applied within thirty seconds otherwise then the plant tissue starts to heal and close thus the herbicide will become ineffective.
When the alien invasive species become naturalized they are free from any of their natural enemies from their place of origin. Biological control involves deliberately bringing in predators from the plants place of origin that eat these invasive plants. One may think that this is just bringing in more alien species into the country however copious amounts of research, screening and quarantine periods are conducted before just letting them loose. The bio control agent is tested vigorously to make sure that it is host specific and that it won’t start eating the indigenous vegetation. Some example of bio control agents are weevils, moths, beetles nematodes and flies. It has been very successful in different parts of the world such as North America, Australia and South Africa. While bio control may be very successful it is not a replacement for chemical or mechanical clearing and is usually used in conjunction with each other.
- All staff should be trained correctly before using dangerous equipment such as chainsaws.
- Proper PPE (personal protective equipment) should be used while clearing is being conducted. This includes chainsaw jacket and pants, goggles, gloves and masks when applying herbicide.
- Very careful research and quarantine periods must be done for bio control agents.
Some examples of alien invasive plant species around the world
Port Jackson (Acacia saligna)
Pine (Pinus pinaster)
Weeping Willow (Salix babylonica)
Alligator weed (Alternanthera philoxeroides)
Bitou bush (Chrysanthemoides monilifera)
Scottish Thistle (Onopordum acanthium)
Norway maple (Acer platanoides)
Giant reed (Arundo donax)
Mayweed (Anthemis cotula)
Common milkweed (Asclepias syriaca)
Eastern baccharis (Baccharis halimifolia)
Fanwort (Cabomba caroliniana)
Water hyacinth (Eichhornia crassipes)
Yellow skunk cabbage (Lysichiton americanus)
Policeman’s helmet (Impatiens glandulifera)
wattle (Acacia farnesiana)
common lantana (Lantana camara)
common cordgrass (Spartina anglica)
Once the alien invasive trees or shrubs have been removed it is important to rehabilitate the affected area. Rehabilitation needs to be done after an alien invasive species has completely taken over a particular ecosystem or site, so if there are just one or two individuals, rehabilitation is not needed. Conservation efforts have always concentrated on protecting areas where the land cover has not been largely altered by humans and such programmes must remain a priority. Due to the extent of land use and land cover change by humans all over the world, conservation efforts have widely focused on natural recovery and active restoration of degraded ecosystems in order to restore both ecosystem services and biodiversity. These restoration efforts range from removing anthropogenic (which means it was caused by humans) such as fire, grazing and alien vegetation in order to allow for natural or unassisted recovery, “passive restoration” to humans actively intervening in an effort to speed up and influence the successional direction of recovery (“active restoration”). Given that natural recovery in many ecosystems can take decades, there is often considerable social pressure to intervene to speed up this process, particularly in built up areas where degraded areas are highly visible. Active restoration programmes are becoming increasingly common, often at a cost of substantial amounts of time and labour. In these programmes, land managers intervene in a range of ways to help recovery, including restoring pre disturbance topography (in terrestrial and wetland systems) or river channel patterns; reintroducing propagules of plants or animals; and actively manipulating disturbance regimes such as fire. Restoring previously infested areas is important because if left unattended it will create a new area for re infestation by either the same alien invasive plant or a totally new species.
Some of these invasive alien species have been around for many years and thus have become historical land marks or heritage sites. A large amount of people depend on these invasive trees or shrubs as a source of income or for recreational use. An example of this is in Cape Town, where a pine forest has become a great place for walking, cycling, picnics and markets. When the city made a proposal to cut the forest of alien trees down the local suburbs were outraged and put up a lot of resistance. The benefits and negative impacts of alien species differ largely in type and enormity and are dependent on the species, the plants invasive potential, the amount to which they have invaded as well as the nature of socio-economic contexts. Value-based conflicts are very difficult to resolve because management authorities have to coordinate the needs of a variety of different stakeholders while still conserving the natural ecosystems. A beneficial management plan is where parties with different value systems agree on a win-win solution where invasive species can still offer benefits, but unfavorable impacts to the environment are reduced. This is potentially possible through open conversation among stakeholders, trade-offs and compromises.
What you can do
Where ever you are living in the world, try and plant indigenous plants in your garden.
Volunteer with your local environmental organisation when they have alien clearing functions.
Remove any alien invasive trees that are illegally grown in your garden.
Take Home Message
Alien invasive plants are scattered in many different parts of the world. They threaten the lives of indigenous vegetation and all the other organisms that depend on the indigenous vegetation. It is a very big concern because now not only do animals and plants (the indigenous ones) have to fight for survival against habitat destruction and urban expansion but they also have to compete with foreign invaders that steal their homes and food.
Genovesi, P., Carnevali, L., Scalera, R. 2015.The impact of invasive alien species on Native threatened species in Europe. ISPRA ISSG, Rome. Technical report for the European Commission.p18.
Bromilow, C. 2010.Problem Plants and Alien Weeds of South Africa. Pretoria: Briza Publications.
Brooks, M.L., Antonio, C.M.D., Richardson, D.M., Grace, J.B., Keeley, J.E., Tomaso, J.M.D.I., Hobbs, R.J., Pellant, M. & Pyke, D. 2018. Effects of Invasive Alien Plants on Fire Regimes. Bio Science, 54(7) :677 – 688.
Coetzee, K.2005. Caring for Natural Rangelands. Pietermaritzburg: University of KwaZulu Natal Press.
Zengeya, T., Ivey, P., Woodford, D.J., Weyl,O., Novoa, A., Shackleton, R., Richardson, D., van Wilgen, B. 2017. Managing conflict-generating invasive species in South Africa : Challenges and trade-offs. Bothalia, 47 (2) : 1–11.
Gaertner, M., Irlich, U., Visser, V. & Walker, G. 2015. Cities invaded.
Holl, K.D. & Aide, T.M. 2011. Forest when and where to actively restore ecosystems ? Forest Ecology and Management, 261(10): 1558–1563.
Horvitz, N., Wang, R., Wan, F. & Nathan, R. 2017. Pervasive human-mediated large-scale invasion : analysis of spread patterns and their underlying mechanisms in 17 of Wilgen China’s worst invasive plants. Journal of Ecology, 105, 85–94.
Kraaij, T. & B.W. van Wilgen. 2014. Drivers, ecology , and management of fire in fynbos. Fynbos: Ecology, Evolution, and Conservation of a Megadiverse Region: 47–72.
Pimentel, D., Mcnair, S., Janecka, J., Wightman, J., Simmonds, C., Connell, C.O., Wong, E., Russel, L., Zern, J., Aquino, T. & Tsomondo, T. 2001. Economic and environmental threats of alien plant, animal, and microbe invasions. Agriculture, Ecosystems and Environment, 84: 1–20.
Richardson, D.M. & van Wilgen, B.W. 2004. Invasive alien plants in South Africa : how well do we understand the ecological impacts ? South African Journal of Science, 45–52.
Tainton, N.M. 1999.Veld Management in South Africa. Pietermaritzburg: University of Natal Press
Xu, H., Qiang, S., Genoves, I. P., Ding, H., Wu, J., Meng, L., Han, Z., Miao, J., Hu, B., Guo, J., Sun, H., Huang, C., Lei, J.,Le, Z., Zhang, X., He, S., Wu, Y., Zheng, Z., Chen, L., Jarošík, V., Pyšek, P. 2012. An inventory of invasive alien species in China. NeoBiota 15: 1–26.