An exotic species is a species that occurs beyond its native range, most often because humans have moved it, whether intentionally or not. Most exotic species do not establish viable populations in the new areas to which they have been moved because the new environments may not meet their needs, because native species may outcompete them or otherwise displace them, or because there may not be a sufficient number of individuals to become established. However, a small number of exotic species go on to become invasive species—exotic species that rapidly spread and increase in abundance at the expense of native species and ecosystems. While there is no definitive list of qualities that predict which exotic species can become invasive, many invasive species have the following in common: (1) they begin to reproduce at an early age; (2) they can reproduce rapidly; (3) they lack sufficient predators in their introduced range; (4) they disperse easily; (5) and they are generalist species, able to survive in a variety of ecosystems.
Invasive species displace native species through competition, predation, and habitat alterations. They often thrive in environments disturbed by human activities.
Spread of invasive species
As stated above, invasive species spread and invade new areas because human activities move them there. Some of the most prominent means by which human activities facilitate the spread of invasive species include:
- Agriculture: Large industries exist to grow agricultural plants for crop production, ornamental plants for gardens, grasses for pastures, and livestock for food. Many of these organisms later escape from cultivation and captivity and go on to invade and harm local ecosystems. Other species spread when industry workers accidentally harvest the seeds of weedy plants along with commercial seeds, and then sow those seeds in new localities, while microbes, parasitic organisms, and insects may be transported with plant leaves and roots, in potting soil, or even attached to transported animals. One of Africa’s worst plant invaders, the triffid weed (Chromolaena odorata), continues to spread because of its sustained use to boost soil fertility on agricultural lands (Uyi et al., 2014).
- Accidental transport: Many invasive species spread to new areas because people transported them unintentionally. Domestic rats (Rattus spp.) and mice (Mus spp.) are classic examples: they have spread around the world as stowaways aboard ships (Box 7.3). A great number of invasive plants that currently occur in South Africa’s Cape Floristic Province arrived by accident by clinging onto the luggage and hiking gear of tourists (Anderson et al., 2015). The ballast tanks of ships are also common hiding places for invasive aquatic species: DNA analyses have shown how a German ship carried tiny snails in this way along the entire length of Africa’s Atlantic coast, providing invasion opportunities across this entire shipping route (Ardura et al., 2015).
- Biological control: Environmental and agricultural organizations sometimes use biological control (Section 4.2.7) to manage the spread of, and harm caused by, invasive species. While this approach can be very effective, in rare cases the biocontrol agent can become invasive and harm native species, rather than its intended target. For example, domestic cats that were introduced to Marion Island off Africa’s south coast feasted on native seabirds—in some cases, even causing seabird extirpations—instead of the rats and mice they were meant to control (Bloomer and Bester, 1992). For this reason, very careful research is necessary to test the appropriateness of a biocontrol agent before being released.
- Deliberate introductions: Soon after their arrival, colonists released hundreds of European birds and mammals into countries like South Africa and Kenya to make the African countryside feel more familiar. Other species, especially fish (e.g. trout, bass, and carp), were released to provide food and recreational opportunities. Many of these species have subsequently become so successful that they harm native species. For example, mesquite (Prosopis juliflora), introduced to Ethiopia from Mexico to reduce soil salinity, proved such a successful invader that it completely displaced local plants; the subsequent encroachment even threatens the viability of Ethiopia’s few remaining Grevy’s zebras (Equus grevyi, EN) (Kebede and Coppock, 2015). The introduction of the Nile perch (Lates niloticus, LC) to the Rift Valley to boost local fisheries likely led to the extinction of hundreds of fish species endemic in Lake Victoria (Pringle, 2005).
- Captive escapees: Many invasive species were originally kept as pets or ornamental plants but have escaped from captivity to establish feral populations that harm local wildlife. Some of the most problematic aquatic invasive species are common pets that people dumped in streams, lakes, or storm drains because they could not care for them anymore. Finding these escapees before they establish should thus be a priority. For example, a recent survey found that 258 alien ornamental plant species growing in South Africa’s Kruger National Park are at risk of becoming invasive—most of these plants were subsequently removed from the park (Foxcroft et al., 2008).
FitzPatrick Institute of African Ornithology, DST-NRF Centre of Excellence,
University of Cape Town, South Africa.
Invasive species are one of the main threats to biodiversity. Island ecosystems are particularly vulnerable; of the 156 bird species that have gone extinct in the last 500 years, more than 90% lived on oceanic islands (IUCN, 2019). This vulnerability is mainly due to species on oceanic islands evolving in the absence of competing species or predators, and thus lacking adequate defences against introduced species (including humans).
Invasive species pose many threats to island biodiversity. Newly arrived mammalian predators have exacted the greatest toll. Seemingly unable to appreciate the danger posed by these strange new arrivals, the ecologically-naïve adult birds simply remain on their nests to be eaten rather than fleeing. Introduced herbivores can have devastating impacts, because many island plants lack defences like tough leaves or thorns. Most devastating are domestic goats and rabbits, introduced by early island explorers to provide a source of food in the case of shipwreck, that have grazed many once-lush islands down to the ground. Introduced plants can also outcompete native plants. For example, the Mexican thorn (also called mesquite, Prosopis juliflora) has formed dense thickets on the once sparsely vegetated lowlands of Ascensión Island, making those areas unsuitable for both nesting seabirds and sea turtles.
Some of Africa’s least-transformed islands are the sub-Antarctic Prince Edward Islands, 2,000 km southeast of Cape Agulhas, and Gough Island, 2,800 km west of Cape Town. Their small size, isolation, and lack of sheltered harbours prevented human settlement. Nevertheless, their large seal and seabird populations were frequently exploited for oil and skins in the 19th and early 20th centuries. In the 19th century, sealing parties accidentally introduced house mice (Mus musculus) to Marion Island, the larger of the two Prince Edward Islands, and Gough Island. The mice flourished by eating native invertebrates and plants, probably causing the local extinction of one flightless moth on Marion Island. A few domestic cats were brought in to control the mice at Marion Island’s weather station, established in 1948. Instead, the cats targeted the island’s birds, which were easier prey. By the 1970s, some 2,000 cats were killing an estimated 450,000 seabirds each year, greatly reducing the island’s burrow-nesting petrels and even driving some species to local extinction (Bloomer and Bester, 1992). The events on Marion contrasted with nearby predator-free Prince Edward Island that continued to support vast breeding populations of burrowing petrels.
A pioneering initiative eradicated Marion Island’s cats in 1991, using a combination of introduced cat influenza, hunting, trapping, and poisoning (Bloomer and Bester, 1992). Researchers hoped Marion’s seabird populations would recover within a decade but had not considered the impact of mice once the cats were removed. The precedent was set on Gough Island; in 2001, introduced mice were discovered to predate on large numbers of seabird chicks, including Tristan albatross (Diomedea dabbenena, CR) chicks more than 100 times larger than themselves (Davies et al., 2015; Dilley et al., 2015a). It was hypothesised that mice are more likely to attack seabirds when they are the sole introduced predators on an island. Sure enough, the first attacks on Marion’s albatross chicks were recorded in 2003 (Figure 7.D); by 2015, the attacks had increased dramatically (Dilley et al., 2015b).
Fortunately, it is possible to eradicate invasive species from islands. In 2014, Australia removed mice, rats, and rabbits from sub-Antarctic Macquarie Island (Parks and Wildlife Service, 2014), which is almost twice the size of Gough Island. Plans are now also in place to eradicate Gough’s mice in 2019. The island’s isolation facilitates this effort—damage to other species can be minimized, and possible spread of toxins or diseases will be confined. To access areas inaccessible on foot, helicopters will be used to spread poison bait from specially designed hoppers slung under the aircraft. Some poisoning of non-target native individuals is inevitable, but this is a small price to pay compared to extinctions of those species. If adequate measures are put in place to prevent subsequent reintroductions, there is hope of restoring at least part of the island’s natural balance.
Impact of invasive species
Invasive species have many negative consequences for native biodiversity: they displace native species through competition, alter the structure and composition of natural communities, and sometimes also hybridise with native species. These impacts may also translate to financial losses, as invasive species compromise ecosystem services (Figure 7.12), damage infrastructure, and spread infectious diseases.
Invasive species often become pervasive because they outcompete and displace native species. One such example is the Mediterranean mussel (Mytilus galloprovincialis), which was accidentally introduced to South Africa in the mid-1970s via European ships. A superior competitor, the exotic mussel soon started displacing native mussels and limpets, especially in the inter-tidal zone of South Africa’s west coast (Branch and Steffani, 2004). Considered South Africa’s most successful marine invasive, recent evidence suggests that the Mediterranean mussel is continuing to spread north into Namibia and along South Africa’s east coast towards Mozambique.
Another superior competitor is the water hyacinth. A native to South America’s Amazon forest, this species was intentionally introduced as a showy ornamental plant to dams, ponds, and lakes across Africa in the early 20th century. The plant established well, but then started reproducing and spreading at such rapid rates that water bodies across the region were soon covered by a dense mat of leaves. With little surface exposure and water movement, eutrophication and suffocation followed, leading to the deaths of countless fish and other aquatic organisms (Villamagna and Murphy, 2010). A biological control program targeting hyacinth showed promise during the 1990s; however, eutrophication from fertiliser overuse (which stimulate growth of hyacinth and other invasive aquatic plants) may be contributing to this species’ recent resurgence (Coetzee and Hill, 2012; Bownes et al., 2013).
A single gum or pine tree can transpire as much as 50,000 litres of water per year, while plantations of these trees can reduce water resources in an area by as much as 70%.
Natural communities are at particular risk in cases where invasive species change ecosystem structure and functioning so much that native species can no longer survive. Such is the case across many parts of Africa, where invasive Australian gum (Eucalyptus spp.) and pine (Pinus spp.) trees (both widely planted for timber) transpire so much water through their leaves (as much as 50,000 litres of water per tree/year; Dzikiti et al., 2016) that they can reduce the availability of surface- and groundwater in an area by as much as 70% (le Maitre et al., 2016). In addition to creating drought conditions, the closed canopies created by these invasive trees reduce the amount of solar radiation reaching the ground, greatly limiting thermoregulatory opportunities for taxa such as reptiles (Schreuder and Clusella-Trullas, 2016). Habitat degradation caused by invasions of the cinnamon tree (Cinnamomum verum), originally from Sri Lanka, has already caused at least ten invertebrate extinctions in the Seychelles (IUCN, 2019).
While many of the invasive species mentioned earlier originated from outside Africa, it is important to note that African species can also become invasive in other parts of Africa when they are moved outside of their native ranges. When invading nearby areas, non-native species can come in close contact with closely-related species, creating a high risk of genetic mixing—also called genetic pollution or genetic swamping—which describes the hybridisation of invasive species with native species. For example, hybridisation with the widespread banded tilapia (Tilapia sparmanii, LC) threatens the survival of Namibia’s Otjikoto tilapia (Tilapia guinasana, CR), globally restricted to the < 1 km2 Lake Guineas (Bills 2007). The Cape platanna (Xenopus gilli, EN), an endemic to South Africa’s Cape Floristic Region, is similarly threatened by hybridisation with the widespread African clawed frog (Xenopus laevis, LC) (Fogell et al., 2013).
Genetically modified organisms
A topic of conflict among conservation biologists is the increased popularity of genetically modified organisms (GMO). A GMO is an organism whose genetic material has been altered to provide useful or improved products and services. To do this, scientists typically use genome editing technologies to transfer genes from a “source” organism into the DNA of the target organism. For example, scientists can transfer a bacterial gene that produces an insect toxin into a crop, such as maize, to obtain a GMO that can resist insect herbivory. Farmers using this GMO maize would then be able to increase production and reduce pesticide use (Gewin, 2003). While GMOs are usually associated with the development of pest-resistant and drought-resistant crops, uses are highly varied. For example, in Senegal, GMO technologies are used to produce tilapia that are better adapted to local ecosystems (Eknath et al., 2007). GMO technologies are being used to develop new and cheaper medicines (Concha et al., 2017), and to combat important diseases: trials in Burkina Faso shows that fungi genetically engineered to produce spider toxins caused a 99% collapse in malaria-carrying Anopheles mosquito populations within 45 days (Lovett et al., 2019). GMOs can even be used for conservation purposes, like developing new methods to combat invasive species (Esvelt et al., 2014), creating more effective bioenergy sources (Beer et al., 2009), and making vulnerable species more resistant to climate change (Piaggio et al., 2017). Some scientists even hope to combat plastic pollution by creating a genetically modified bacterium able to consume plastic waste (Austin et al., 2018).
The use of GMOs is not a new phenomenon. Selective breeding, hybridisation, and other forms of artificial selection—techniques that have been used for much of human history—all result in different forms of genetically modified crops and animal species. However, technological advances in genetic engineering have enabled scientists to transfer genes from and between different taxa that have not previously been used in selective breeding programs (i.e. viruses, bacteria, insects, fungi, and shellfish). GMO technologies that transfer genetic material between wholly disparate taxa has led to concern about the unknown and unintended consequences of such “crossovers”. Some people are also concerned that GMOs that escape from captivity or cultivation (e.g. Gilbert, 2010) could hybridise with closely-related wild species, endangering native wildlife while resulting in new, aggressive weeds and virulent diseases. Additionally, the use of GMO crops could potentially harm non-crop species (e.g. insects, birds, and soil organisms) that live in, on, or near the GMO crops. Concerns have also been raised about the potential effects to people eating GMO foods, leading some governments to regulate GMO research and commercial applications differently than traditional agriculture. However, after decades of research, it appears that GMO food is safe to eat (Blancke, 2015).
GMOs offer benefits such as increased production and reduced pesticide use, but there are concerns about unknown and unintended consequences like hybridisation and invasiveness.
Clearly, GMOs offer a wide range of opportunities which could directly and indirectly benefit biodiversity conservation. However, the benefits of GMOs must be examined and weighed against the potential risks on a case by case basis. It is probably wise to proceed cautiously, to study GMOs thoroughly, and to monitor their impacts on ecosystems and human health in the areas where they are used. These investigations could involve workshops where experts come together to perform environmental impact assessments (EIA) (Section 12.2.2). The potential but unknown impacts of GMOs can also be mitigated by limiting the ability of these organisms to spread or reproduce (Muir and Howard, 2004).