Invasive alien plants threaten South African rangelands 

Researchers say up to 10% of invasive alien plant species across South Africa’s grasslands, Karoo, and savannah areas may pose a serious threat.  They say invasive plants could disrupt livestock farming and water supply in some areas, which ultimately threatens the country’s economy.

They reviewed (meaning they summarised and contextualised) all the information available on alien (non-native) plants in South African rangelands, which covers over 70% of South Africa’s land.

Native rangeland plants provide food, medicines, shelter for animals, and water services, all of which might be threatened by alien plant species.

In their review, the researchers discuss various positive and negative effects that invasive plants have on the environment and the economy in South Africa. 

The rangelands include parts of the grasslands, Karoo, savannah and others, but are not found in Fynbos, forest or desert areas. 

According to their review, alien plants might start growing because different animals have different grazing (feeding) patterns, and because of human activity. For instance,  overgrazing can be a problem. 

Fire is also used in these areas to control the plants and grass, so when fires don’t burn for a long time, alien plants may start to grow. And, when people put fences up in these areas, birds may sit on them and spread seeds of alien plants. 

The researchers said there are many alien species in South African rangelands, but less than 10% are serious threats. They noticed that the wet parts of grasslands and savannah had a high number of alien species. In all these areas, the most common alien plants were trees and shrubs. 

Most alien plants were originally introduced for a benefit such as shelter and food for animals, soil protection, fuels and medicines.

They said that although the alien plants could serve as food for the animals, it’s only good if the animals prefer these new plants. If not, the livestock die because they are not eating enough. Alien trees prevent grass from growing and if the trees have thorns, animals cannot get food. They said that when these alien plants cause a decrease in livestock, it affects trade markets and the economy.

Sometimes the alien plants in water sources also disrupt services and products that people rely on, which impacts the economy as well. However, the researchers said alien plants do have benefits when they are managed well and exist in low numbers. 

They said that well-managed areas can be mostly free from invasive plants. Good management includes regular burning of fires, letting depleted grazing fields grow properly again before allowing animals to graze, and keeping an eye on the types of animals that are grazing. 

Since few previous studies explored how alien plants affect rangelands in South Africa, this review adds much-needed context by bringing together the known information on the topic.

The authors, who were from South Africa, hope that their review will assist the country to better manage invasive plants in rangelands.

Abstract

Rangeland covers >70% of the land surface of South Africa, and includes grassland, savanna, thicket, and karroid shrubland vegetation. These rangelands support domestic livestock and wildlife whose economic value is around ZAR 30 billion annually. They are invaded by hundreds of alien plant species, of which 71 have been identified as being of special importance in South African rangelands. These species are able to proliferate in response to disturbances, of which grazing and fire are the two most important for South African rangelands; changes to fire and grazing regimes can therefore promote invasion, especially by alien trees. These trees replace palatable grasses and are generally unpalatable themselves. At a national scale, invasive alien plants are estimated to reduce the value of livestock production by ZAR 340 million annually, but this is expected to increase dramatically as plant invasions spread, and as additional alien species become invasive. Invasive species that have increased their range dramatically by up to 671% between 2006 and 2016 include Campuloclinium macrocephalum (Pompom Weed), Opuntia engelmannii (Small Round-leaved Prickly Pear),Opuntia humifusa (Large-Flowered Prickly Pear), Parthenium hysterophorus (Parthenium Weed), Trichocereus spachianus (Torch Cactus), and Verbena bonariensis (Wild Verbena). Studies that document the impacts of individual species on rangeland composition and structure cover only a few species, including Prosopis species (Mesquite), Acacia mearnsii (Black Wattle) and Parthenium hysterophorus, all of which can dramatically reduce grass cover and the capacity to support livestock, especially at high densities. Invasive plants in rangelands can also be beneficial as sources of firewood, fodder, shade and medicinal products. Some species may offer considerable value when at low abundance but their detrimental impact far outweighs advantages as their abundance increases, resulting in a net negative value. An escalating threat of alien plants to rangelands demands innovative responses in addition to biological control and clearing programmes.

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