The invasion of our trees by the polyphagous shot hole borer beetle has drawn into sharp focus the challenges that our ecosystems face from invasive species.
Last year a report entitled The Status of Biological Invasions and their Management in South Africa was published by the South African National Biodiversity Institute (SANBI). This is the first South African report to attempt to assess the status of biological invasions on a national level and it makes depressing reading.
It is generally accepted that invasive alien species have devastating impacts on native biota i.e. all the animals and plants that live in a certain area, causing decline or even extinctions of native species and negatively affecting ecosystems. One factor is that the alien species do not have the natural predators that they would have in their native habitat. The further success of invasives results from their ability to out-compete local flora and alter the environment to suit themselves.
South Africa is one of a few countries that have legislation specifically aimed at managing the problem of biological invasions. The NEMBA Alien & Invasive Species (AIS) Regulations are undergoing a major update at the moment. The new regulations are expected to be gazetted in the latter half of 2019.
A total of 556 invasive taxa have been listed under the National Environmental Management: Biodiversity Act’s Alien and Invasive Species Regulations. (A taxon is a group to which organisms are assigned according to the principles of taxonomy, including species, genus, family, order, class, and phylum.)
According to the report, the actual number of invasive species is higher, with 775 having been identified to date. Most of these invasive species are terrestrial and freshwater plants (574 species) or terrestrial invertebrates (107 species). (An invertebrate is an animal that does not have a backbone, e.g. an insect or worm.)
The report states that a total of 107 species were considered by experts to be having either major or severe impacts on biodiversity and/or human wellbeing. Interestingly, the vast majority of these (75%) are terrestrial or freshwater plants.
Perhaps we need to be clear on some of the terms that environmentalists use.
Alien species are those that are present in a region outside their natural range as a result of human action that has enabled them to overcome biogeographic barriers. The alien species has been introduced intentionally or accidentally by people.
Introduction: movement of a species, intentionally or accidentally, owing to human activity, from an area where it is native to a region outside that range.
Not all aliens are invasive. Invasive usually means that the species is growing aggressively in an area and stifling the growth of pre-existing plants or fauna.
Scientists see this invasion as a process, what they call the introduction-naturalisation-invasion continuum: a conceptualization of the progression of stages and phases in the status of an alien organism in a new environment which posits that the organism must negotiate a series of barriers. There are four major invasion stages: pre-introduction, incursion, expansion and dominance.
Indigenous or native species: species that are found within their natural range where they have evolved without human intervention (intentional or accidental). The term also includes species that have expanded their range as a result of human modification of the environment that does not directly impact dispersal (e.g. species are still indigenous if they increase their range as a result of watered gardens, but are alien if they increase their range as a result of spread along human-created corridors linking previously separate biogeographic regions).
Naturalised or established: Alien species that sustain self-replacing populations for several life cycles or over a given period of time without direct intervention by people, or despite human intervention.
Weed: a plant that causes negative impacts. Weeds can either be alien or native.
Prohibited species: species that are not native to South Africa listed as prohibited under the National Environmental Management: Biodiversity Act, 2004, Alien and Invasive Species (A&IS) Regulations, 2016.
(These definitions have been adapted from the glossary of terms in the SANBI report.)
The study showed that alien species richness was highest in the Savanna, Grassland, Indian Ocean Coastal Belt and Fynbos biomes.
One of the most alarming effects of the invasives is that on our water supply. Studies estimate the combined impacts of invasive plants on surface water runoff at between 1 450 to 2 450 million m3 per year. If no remedial action is taken, reductions in water resources could rise to between 2 600 and 3 150 million m3 per year, severely impacting water-scarce regions and cities.
Working for Water, which controls invasives countrywide while facilitating job creation, has cleared 2.5 million hectares of invasions since its inception, but this is only a fraction of the invaded area
Invasive species have also been identified as a significant threat to biodiversity throughout the country. Such losses of biodiversity have large negative knock-on effects on the economy and food security, among others.
There are also considerable economic consequences of alien plant invasions in South Africa. Studies have revealed that invasions have reduced the value of fynbos ecosystems by over US$ 11.75 billion and that the cost to clear the alien plant invasions in South Africa is around US$ 1.2 billion. (B. W. van Wilgen, et al)
However, the concept of alien invasion is not entirely clear cut. Climate change is making it harder even to decide who the invaders are. How do you define “native” on a warming planet, when plants and animals are already moving toward the poles or up mountainsides in search of climate conditions they can tolerate? Should we consider them “invasive” in their new homes? Regardless of what we label them, conservationists will be reluctant to remove them from their new environs—to do so would stymie their chances of adapting to the warmer future we’re creating.
Those of us who grew up on the Highveld can remember when the only time we saw grey louries or go-away birds was when we were on holiday in the Lowveld. Nowadays they are common on the Gauteng smallholdings. Their migration was originally due to prolonged drought in the Lowveld a couple of decades ago, but having come here of necessity, they have decided to stay. Yes, they are indigenous but their range has greatly increased.
There are also the non-natives that we actually like. Most domestic crops are exotic in most of the places they’re grown, having been introduced. They not only replace native communities, but also alter the human nutritional environment.
Some environmentalists are arguing that we need a more nuanced approach to invasives.
Even our NEMBA invasive species legislation acknowledges that some introduced species may be cultivated under certain circumstances. A Category 2 listing means that a permit must be obtained to authorise activities involving the listed species. For example, eucalyptus trees can be grown under permit, in order to provide forage for honey bees.
There are many examples of habitats having been destroyed by human activities, where alien species have established themselves, thereby giving sources of food and protection to the surviving native species. Dismantling mixed communities by removing non-natives can be far from straightforward. The ecological and evolutionary impacts of non-native populations that are functionally integrated in new communities mean that incautious eradication might create the risk of unintended, counter-productive outcomes and that there will often be no straightforward way to restore native communities to a pre-invasion state. Should we impose further risk on already endangered natives by severing these relationships?
There are also examples of alien species being endangered in their own ranges. The wattle-necked softshell turtle is an oft-quoted case. They are endangered in their native China, but are considered invasive in Hawaii. So should we eradicate them from Hawaii and run the risk of their becoming extinct? Some conservationists are saying leave them alone. As the human population increases, driving more animals and plants toward extinction, a species’ second home may be the only one it has.
Some conservationists say that often “invasion” is just another word for “change.” Already, the flora and fauna of countries around the world are more homogeneous than they once were, as globalization has, accidentally or intentionally, moved exotic species from one place to another.
A relatively new term has been introduced into the invasive management sphere, and that is conciliation biology, which has been put forward by Dr Scott P. Carroll, an evolutionary biologist and ecologist affiliated with the University of California.
Conciliation Biology is based on these premises:
- The environment has been radically altered by the activities of humans;
- The environment will continue to change in the future;
- It is not feasible to eradicate non-native species;
- The cost of attempting to do so is prohibitive.
Conciliation biology is that part of invasion biology that focuses not on prevention or eradication of invasive species, but instead predicts and manages outcomes of longer-term native–non-native interactions at the levels of individual, population, species, community, and ecosystem.
Conciliation biology recognizes that many non-native species are permanent, that outcomes of native–non-native interactions will vary depending on the scale of assessment and the values assigned to the biotic system, and that many non-native species will perform positive functions in one or more contexts. Managing such mixed and novel systems will require integrated schemes responsive to change.
Sometimes it is impossible to restore the ecosystem of an ecologically degraded area and sometimes “rewilding” is all we can hope for.