The invasive Opuntia stricta flourishing under the canopy of a native tree in the Kruger National Park. (Photo: SANParks)
Managing alien species in a large protected area such as the Kruger National Park is “daunting”, with one to two new invasive or naturalised species having been recorded annually since 2000, according to two South African National Parks (SANParks) scientists.
“The first records of alien plants are from the 1930s and since then, there has been a consistent increase in the introduction, arrival and detection of new alien plant species,” Khensani Nkuna, a junior scientist at the Kruger, and Llewellyn Foxcroft, a scientist in invasion ecology, wrote in an article published in SANParks’ 2023/2024 research report.
Naturalised species are alien species that form populations “that endure (maintain a reproductive population) for at least 10 years without direct intervention by people”, according to the Centre for Invasion Biology at Stellenbosch University.
In 2017 there were 360 alien plant species reported in the Kruger, with the number gradually increasing to 407 in the most recent survey in 2023, the SANParks scientists wrote. But a large portion are ornamental plants with “unknown impacts” on native biodiversity and 261 of these were reported only in tourist camps and staff residences. A further 146 species have been reported from areas beyond human habitation.
The 2023 study led by Foxcraft found that 21 species are now categorised as invasive, 58 as naturalised and 30 as casuals. Of the invasive species, 12 are globally widespread and five are listed among the 100 worst invasive alien species globally — water hyacinth, West Indian lantana, the sour prickly pear, Triffid weed and the giant sensitive tree (catclaw mimosa), which are listed among 100 of the world’s worst invasive alien species.
“Updating alien species lists is challenging but necessary for ongoing surveillance and monitoring to ensure that current and emerging threats are identified and promptly addressed,” Foxcroft and Nkuna said.
The updated list clears the way for an overview of priorities and associated resource allocation.
Other management information extracted from the updated list includes details on the life history of species. For example, herbaceous plants are the prevailing life form among alien species in Kruger, with 46% already naturalised and 13% invasive. South America is the source of most of Kruger’s naturalised and invasive species.
Such knowledge can provide important information for formulating targeted management efforts and its implementation, the researchers said.
“For instance, conservation managers can focus on mitigating the spread of invasive species with shorter life spans (herbaceous plants) and species from ‘high-risk’ regions.”
Widespread invaders
In another article on how alien plants are reducing native plant diversity in riparian areas in the latest research report, Foxcroft and Nkuna wrote that changes in plant composition can have cascading effects through the ecosystem, affecting everything from soil health to animals that depend on native plants for food.
“For the Kruger National Park, this is concerning, as cocklebur, downy thorn-apple, and parthenium [famine weed] have become widespread. All three species originate from the Americas; downy thorn-apple was introduced to South Africa for ornamental purposes, while parthenium and cocklebur were accidentally brought in via agricultural products.”
They have since become widespread in the country. “These aggressive invaders have also invaded riparian areas along the major rivers in Kruger reaching up to 100% cover in certain areas.”
Cocklebur and downy thorn-apple are mostly confined to sandy substrates in riverbeds and often occur together, while parthenium invades more compact soils of the riparian areas.
A study by a research team from the Institute of Botany, Czech Academy of Sciences and funded by the Czech Science Foundation, in cooperation with Foxcroft and Nkuna, focused on measuring the effects of these invaders.
Conducted along the Letaba, Sabie and Shingwedzi Rivers, it found that cocklebur, downy thornapple and parthenium affect native plant species richness in the Kruger. Invaded areas had lower species richness, diversity and evenness compared with adjacent uninvaded areas.
Differences in species richness were most pronounced in sites dominated by parthenium. In contrast, the most negative effect on species diversity and evenness was detected in sites invaded by downy thorn-apple.
Interestingly, the researchers found that some native plant species, such as shade-loving abutilon, wild cucumber, buffalo grass and veld violet, were more frequent and abundant in the invaded vegetation.
“This may be due to habitats created by the invaders in sandy riverbeds creating conditions that favour their growth. This finding provides a nuanced perspective on the complex dynamics of invasive plants in these ecosystems.”
The SANParks scientists said the survival of native plants in invaded plant communities is important because eradicating cocklebur, downy thorn-apple and parthenium in Kruger is nearly impossible.
