Species like the Karoo acacia are estimated to have encroached on 10 to 20 million hectares across South Africa’s grassy biomes. Photo: Alexey Yakolevlev, Wikimedia Commons
The nebulous ecology of native invasions
Associate Professor Adam West is an ecologist based in UCT’s Department of Biological Sciences. He is co-author of the paper, titled ‘The Nebulous Ecology of Native Invasions’ (Trends in Ecology and Evolution, 2017), along with Dr Lloyd Nackley (Oregon State University), Professor William Bond (South African Ecological Observation Network) and Andrew Skowno (South African National Biodiversity Institute).
As West explains it, in the past invasive plant species were mostly assumed to be aggressive alien species. “The classic example is something like what the Australian acacias do here in South Africa,” he explains. “They can spread rapidly and change everything from the soil to the biotic interactions to the way water flows through the system.”
But in the Anthropocene it is not just alien species that are spreading beyond their traditional boundaries.
“We’re starting to see indigenous plants exceeding their historical ranges and causing considerable impact. And invasion biology is not quite sure how to deal with that,” says West.
Lloyd Nackley is a former postdoctoral fellow in plant physiology at UCT and current assistant professor at Oregon University in the USA. In the 21st century, says Nackley, the impact of expanding native species is often indistinguishable from invading alien species.
Not only is the phenomenon increasingly prevalent but it is also global: He points out that in the United States 10 to 20% of all invasive plants are native species while in South Africa, tens of millions of hectares are affected by the expansion of native species.
The case of the disappearing grassland
Time-Series Photography from Hillside near Lesseyton in the Eastern Cape Province of South Africa. Left panel from 1925, middle panel from 1993, and right panel from 2011. The photographs clearly show the increasing density of Vachellia karroo trees moving from a mostly open grassy landscape to a heavily wooded thicket. Image: Timm Hoffman
One important example of the phenomenon is the encroachment of trees into grassland. “Trees and shrubs are on the move in Africa and around the world,” explains West. “Contrary to popular assumptions, there has been a noticeable and quantifiable increase of trees into global savannas and grasslands over the last 50 years.”
South Africa is no exception: Species like the Karoo acacia (Vachelia karroo), vaalboom (Terminalia sericea) and mopane (Colophospermum mopane) are estimated to have encroached on 10 to 20 million hectares across South Africa’s grassy biomes, mostly in the north-western regions of the country.
Altering the density of trees, whether they are native or alien species, can have serious repercussions for grassland areas. The encroachment of trees impacts everything from agricultural management to local biodiversity, water catchment areas and levels of carbon and nitrogen cycles.
“In Africa we have seen how the expansion of native trees has reduced grass productivity and species richness,” says West. “This results in decreasing quality of available forage and can impact savanna wildlife.”
According to Nackley, the effects of these changes also go beyond regional or national boundaries to include planetary effects: “Changes in the composition of savannas is of global significance considering that savannas occupy a fifth of the Earth’s land surface, contain some of the most iconic biodiversity, and support livestock grazers as well as wild herbivore populations.
“Alteration of the competitive dominance of trees and grass can also have large Earth–atmosphere ecological feedbacks.”
So why is this happening more frequently? West points to human activity. “Changing temperatures and levels of carbon dioxide, fluctuating rainfall and human transformation of the landscape are all the most likely reasons behind the increase in native species becoming invasive,” he says.
Nackley also points out that this phenomenon is new. “If an invasive species is defined as an organism occurring outside its natural past or present range whose presence and dispersal is due to intentional or unintentional human action – and climate change is recognised as being forced by anthropogenic factors – climate-driven range expansions can be considered as something other than historical biological colonisation.”
Managing native species expansion in the future
“Dealing with alien plant invasions is conceptually straightforward,” says West. “Mostly this involves identifying, locating and removing the non-native species in question.”
The management of native plant invasions on the other hand is complicated both by uncertainty about future environmental conditions as well as what the end objective of such management should be.
“Rather than deciding if a species is desirable based on where it existed in the past, novel ecosystem management aims for a more pragmatic approach that considers how biomes change over time,” he says.
This approach has consequences for present-day environmental policies. West and Nackley agree that such policies need to be broadened from simple historical delineations and towards policies that take the dynamics of ecosystems (how energy flows through the system) into account.
In some cases native invasions may result in a slow and gradual change in the ecosystem. In others the change can be abrupt. To deal with this variability, West suggests that mechanistic models be used. In addition, restoring conditions to those before the invasion will not necessarily result in a reversal to the previous, non-invaded plant communities.
Given these variables, West and Nackley suggest that managing the occurrence of invading native species could range from complete eradication to tolerance and even consideration of the ‘new’ species as an enrichment of local biodiversity.
As an example of how traditional alien plant management can be adapted to include native species, West describes the local invasive plant removal and social development programme Working for Water. This programme has been recognised for its efforts in removing alien species from more than a million hectares of land across South Africa.
“In the case of alien tree species in water-catchments, the response is simple: cut it out. But in the case of expanding native species it is a lot less clear as to what the action should be.”
Yet the importance of creating such adaptable models to deal with native plant expansion cannot be overstated.
“At current-day rates of encroachment, a volume of water equal to the Berg River Dam will be lost to the expansion of alien tree species in the Western Cape by the year 2045. We urgently need to understand the potential impacts of native plant expansions elsewhere in a similar manner,” he says.
Story by Ambre Nicolson