Alien species threaten biodiversity in South Africa

Researchers compared existing studies to identify alien species that threaten South African indigenous biodiversity the most.

South Africa is one of the most biodiverse regions in the world. At least 107 alien species, 75% being plants, are suspected to have major negative impacts on the country’s biodiversity. These suspected impacts of these species are relatively unknown due to a lack of studies that can agree about what exactly these impacts are.

Alien species can reduce biodiversity in many different ways. One way is known as  hybridisation, where they genetically mix with local species. Alien species can also hunt native species into extinction. Alien species can also bring new diseases and cause environmental damage, such as trees that use too much groundwater.

This study collated all available information about the impacts of alien species on biodiversity in South Africa.

The researchers looked at how specific species of aliens have changed local ecosystems and what indigenous species they threaten. They reviewed studies that used standardised impact scores, and they spoke to experts for their opinions.

They also looked at native species that are currently at risk of becoming extinct and used these “red-lists” to see if alien species are involved in their decline.

The study found that many alien gastropods (snails and slugs), fish, mammals, and plants were implicated in threats to native species.

For example, the Tarebia granifera snail is one of four invasive gastropods in South Africa. It has invaded several rivers, lakes, wetlands and estuaries in the eastern and northern parts of the country, where it has dominated over local snail species.

The Largemouth Bass is one of five invasive fish species in South Africa with known impacts. It has decimated native fish species to near-extinction in several South African water bodies. 

For mammals, the study found that the invasive Black Rat (Rattus rattus) is one of several alien rodent species that have caused the extinction of  some birds, insects, bats and other rodent species through hunting them, eating their food or spreading disease.

Although the impact of these alien species on biodiversity is still relatively low, researchers expect their negative impact to worsen significantly if South Africa doesnt stop their growth.

This study is a first step towards standardising how the impacts of alien species on South African biodiversity can be measured. 

While the study was able to assess how much damage has been done by invasive species in South Africa, it was unable to calculate the damage caused where more than one alien species exists in a local ecosystem. Without this kind of research, governments and other regulatory bodies lack information to protect local biodiversity.

For now, the researchers recommend that regulations in South Africa should focus on the priority alien species listed in this paper to limit damage to local biodiversity.

Abstract

Studies of the impact of alien species on the environment are increasingly being carried out, and there has been an ongoing debate about how to standardise the description of these impacts. This chapter evaluates the state of knowledge on the impacts of alien species on biodiversity in South Africa based on different assessment methods. Despite South Africa being one of the most biologically diverse countries in the world, there have been very few studies that formally document the impacts of alien species on biodiversity. Most of what is known is based on expert opinion, and consequently the level of confidence in the estimates of the magnitude of these impacts is low. However, it is clear that a significant number of alien species cause major negative impacts, and that there is cause for serious concern. There is a growing global effort to assess all alien species with standardised protocols to alleviate the problem of comparing impacts measured using different approaches. Formal assessments have been done for a few alien species in South Africa, but most naturalised and invasive species have not been evaluated, and, we suspect, for most alien species there has been no attempt, as yet, to document their impacts. However, red-listing processes found that alien species were frequently included as a significant extinction risk for several native species of fish, amphibians, and plants. There are very few studies that cover the combined impacts of co-occurring alien species in particular areas, and these studies could provide the rationale for regulation and management, which is often absent. While reductions due to alien species in the value of ecosystem services, the productivity of rangelands, and biodiversity intactness are relatively low at present these impacts are expected to grow rapidly as more invasive species enter a stage of exponential growth.

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