Invasion biology is a rapidly growing field of ecology. The discipline is filled with terminology used to describe different concepts and components. Unfortunately, some of these terms are used uncritically, leading to misunderstanding and confusion. The information provided below on terminology used in this field is drawn mainly from the 2004 paper by Petr Pyšek and others in the journal Taxon.
It is worth noting that the definitions given here are not exclusive, for example a species can be both alien and naturalised.
Invasion biology is the study of both the species that become invasive in a system and their impacts on the system they have invaded, as well as the remediation of such invasions. Through studies in invasion biology scientists and practitioners hope to better understand, manage and mitigate the impacts of invasive species, and to prevent further invasions from occurring. This research helps scientists, conservation managers, and government agencies to understand and manage the impacts of invasive species.
Alien species (also called introduced, exotic, non-native, or non-indigenous species) are species of animals or plants, or any other types of organism that are introduced to a new area through the accidental or deliberate actions of humans. The important fact that needs to be highlighted is the involvement of humans in the introduction of such species. This definition excludes the natural migration of native species to new areas due to environmental changes or influences. For example, some plants introduced as livestock feed have become invasive in other areas; some species have arrived as ‘hitchhikers’ with imported nursery plants or seeds and both examples would be regarded as alien species.
The term alien also includes all non-indigenous species being farmed either for subsistence or commercially.
Casual alien species survive, and may even flourish for a period, but do not form lasting populations. This means that although there may be populations of these species that may survive and reproduce, they eventually die out. These species rely on repeated introductions into an area for their persistence.
Naturalised species are alien species that form populations that endure (maintain a reproductive population) for at least 10 years without direct intervention by people. Naturalised species do not necessarily spread over large areas.
Feral species are alien species which were originally domesticated for farming or for pets and have subsequently formed populations outside of their domestic situations. Some feral species become invasive.
Invasive species are a sub-category of naturalised species. Invasive species produce reproductive off-spring, often in very large numbers, and thus have the potential to occupy large areas.
Pyšek et al. (2004) suggest that for plants, species that spread over 100m in less than 50 years warrant classification as invasive species. For plant species spreading by roots, rhizomes, stolons or creeping stems, it is suggested that spreading more than six meters within a period of three years would constitute an invasive species. Therefore it can be said that how quickly and how far a species spreads defines its invasive status.
Certainly not. It is necessary to distinguish between alien species that do no harm, and are either benign or useful, and those that are harmful. Useful alien species include most of our non-invasive crops, livestock, garden plants and pets, and there is no argument that these species are highly beneficial. On the other hand, there are alien species that are both clearly invasive and harmful, and have little or no use – again there is no controversy when these pests and weeds are targeted for control or eradication. But there is another category of invasive species that simultaneously provide benefits, and do harm, and trout is one of these species where there is a conflict of interests. Stakeholders hold different perspectives regarding such species, and conflicts can easily arise (see “Why is the control of alien species sometimes contentious?” below).
No. In fact, only a small percentage of introduced species become naturalized or invasive. Many alien species are highly beneficial to humans. Importantly, though, many alien species may not yet be invasive, but may well have the capacity to become invasive over time, for example when they reach habitats that are more suitable, or once conditions change to enhance their reproductive rate and/or ability to spread.
Yes. Many South African plants and even animals have been taken to other countries for their commercial and/or ornamental value. However, once in these countries some of the species have become invasive. Examples of these are the Arum lily (Zantedeschia aethiopica) and several species of Watsonias which are invasive in Australia. The common platannna (Xenopus laevis) is indigenous to South Africa, but was exported pan-globally and is now invasive on four continents.
There are many invasive alien species that simultaneously bring benefits and cause harm. For example some Australian acacias (such as black wattle – Acacia mearnsii) are invasive but also have a commercial value attached to their wood and bark. Pine trees are used in commercial forestry, and provide shade at picnic sites, but are highly invasive and reduce water run-off from catchment areas, and they also displace the native fynbos. Opuntia (prickly pear) species are invasive and also have an economic value in that they provide fruit and fodder for stock farmers. Many species are imported into different countries for their commercial value. However, these species manage to “escape” from their original locations, spread and become invasive. Many alien freshwater fishes are also invasive, but they provide food and opportunities for recreational fishing. Depending on your point of view, you could be for or against such species, and finding ways of dealing with these conflicts is one of the big challenges of invasion biology.
South Africa is a country rich in indigenous biodiversity and is home to three biodiversity hotspots. Invasive alien species that proliferate and spread, exclude species that should occur in the region naturally, impacting negatively on the natural species interactions of that area. This often causes ecosystem functioning to break down, leading to further invasions by alien species. This chain of events can lead to the extinction of indigenous species of a specific area.
The uncontrolled movement of plant and animal species threatens biodiversity in many countries, and may cause devastating disease outbreaks when micro-organisms travel with their plant or animal hosts and take up separate lives in their new range.
Climate change does influence the spread of invasive species. For example, climate change may create more suitable conditions for alien species, previously not invasive, to increase the rate of independent reproduction and spread. Such species may start to spread after having a stable range for a long time. It may be that a species was introduced into the country and did not immediately become invasive. However, as environmental conditions change (e.g. more water and warmer temperatures), the species may overcome environmental constraints and become invasive and out-compete indigenous species for natural resources.
The most important thing is to be aware of the impact of invasive species and to know which species are alien and which are invasive. Should you become aware of an invasive species in your area, it is important for you to inform your local conservation agency. Increasing awareness of the impacts of invasive species is crucial both in South Africa and the rest of the world.
First, try to take a photograph of the organism, so that you can check the species identification in a book or on the internet. South Africa has many field guides that show our indigenous and exotic plants and animals. For plants, an essential reference is Lesley Henderson’s Alien weeds and invasive plants (Plant Protection Research Institute Handbook No.12, Agricultural Research Centre, 2001). There are a number of organisations who can be contacted to determine the status of a particular plant or animal species. These include the Working for Water Programme, Invasive Species South Africa, the Invasive Species Specialist Group and the South African Institute for Aquatic Biodiversity.
One of the best ways is to visit our website regularly. We have started a highlighted paper series which focuses on important research papers. Short news items describing results from C·I·B projects are regularly placed on the site. Our annual reports, with details of our work, are published for public viewing on our website. Find us on Facebook where you are welcome to post your comments. Or visit the links to the National Research Foundation and the Department of Science and Innovation.