CHAPTER 10: INVASIVE ALIEN SPECIES

Lead Authors: Munyaradzi Chenje, Jennifer Mohamed-Katerere

“Invasive alien species are emerging as one of the major threats to sustainable development, on a par with global warming and the destruction of life-support systems. These aliens come in the form of plants, animals and microbes that have been introduced into an area from other parts of the world, and have been able to displace indigenous species.”

PRESTON AND WILLIAMS, WORKING FOR WATER PROGRAMME/SOUTH AFRICA 2003

INTRODUCTION

Alien – that is non-native – species have been introduced both accidentally and intentionally. Intentional introductions are, and have been, motivated by economic, environmental and social considerations. In the forest sector, for example, Pinus, Eucalyptus and Acacia species are important sources of pulp, timber and fuelwood, yet at the same time they have placed tremendous strain on water resources. In Southern and Eastern Africa, these species are the backbone of plantation forestry, bringing in valuable foreign currency, yet at the same time decimating land and water resources. In South Africa, for example, they consume 7 per cent of available water (Preston and Williams 2003). Many introductions, however, are unintentionally coming into countries with other goods and, in the case of marine IAS, in the ballast water of ships.

Box 1: Invasive alien species

IAS are also commonly referred to as invasives, aliens, exotics or nonindigenous species.

IAS are species, native to one area or region, that have been introduced into an area outside their normal distribution, either by accident or on purpose, and which have colonized or invaded their new home, threatening biological diversity, ecosystems and habitats, and human well-being.

The extent to which introduced species may proliferate and spread is affected by the state of the receiving ecosystem. An alien species may find a vacant niche and spread, or it may compete for one already occupied by a native species. Some IAS proliferate because they find no natural enemies in their new habitat.

Although some species have invaded habitats on their own, human activity such as exploration, colonization, trade and tourism has dramatically increased the diversity and scale of invasions by alien species.

Sources: CBD 1992, Shrine and others 2000, ESA 1998

Although only a small percentage of these alien species will become invasive, when they do their impacts are immense, insidious and usually irreversible, and they may be as damaging to native species and ecosystems on a global scale as the loss and degradation of habitats (IUCN/SSG/ISSG 2000). In Africa some important ecosystems are under threat, consequently undermining development and livelihood opportunities, increasing human vulnerability and threatening human well-being. Thus, IAS have a direct bearing on Africa’s ability to meet the Millennium Development Goals (MDGs) and their targets. (These goals and targets, and progress towards meeting them, are set out in Annex 1).

Although not all alien species will become invasive or threaten the environment, this is an area in which a clear policy approach is necessary because of its potentially wide-ranging impacts when they do become invasive, and because of the difficulties, including financial costs, in reversing its impacts. One experience which illustrates this dilemma is the introduction of Lates niloticus (Nile perch) into Eastern Africa. The Nile perch has had immense economic value in the countries where it has been introduced, but it has also wreaked havoc in the ecosystems, resulting in the loss of endemic species and altered ecosystems with knock-on effects for livelihoods. Alien species, such as Bufo marinus (cane toad) and Chromolaena odorata (bitter bush) (IUCN/SSG/ISSG 2004) have been used for biological control and as an ornamental, and then subsequently become invasive.

Increased mobility and human interaction have been key drivers in the spread of IAS. On the one hand, increasing global connectedness – through trade, travel and tourism – has enriched the lives of people all over the world, through increased opportunities for sharing information and knowledge as well as improved access to a range of biodiversity (McNeely and others 2001). Increased access to biodiversity has created new opportunities for forestry, agriculture, aquaculture, horticulture, and biodiversity-based industries including the pharmaceutical sector. However, this increasing interaction has had its costs too:

“It has broken down the natural barriers of oceans, mountains, rivers and deserts which for millennia provided the isolation essential for unique species and ecosystems to evolve. In just a few hundred years these barriers have been rendered ineffective by major global forces that combined to help alien species travel vast distances to new habitats and become alien invasive species” (IUCN/SSG/ ISSG 2000).

The impacts of IAS
are immense,
insidious, and usually
irreversible and they
may be as damaging
to native species and
ecosystems on a
global scale as the
loss and degradation
of habitats.

IUCN/SSG/ISSG 2000

The challenge facing Africa is how to respond – to known IAS and to new introductions of alien species that could potentially become invasive. First, Africa needs to develop systems for evaluating the risks and benefits associated with alien species, and for deciding when to use them and when to prevent their introduction or eradicate them. This entails considering the economic, development, environment and human well-being costs-and- benefits, and recognizing the close relationship between these sectors. Second, Africa faces the challenges of how to translate its policy objectives into effective management practice. When species are identified as a threat, appropriate responses may include establishing systems for their eradication, as well as for controlling and monitoring their introduction. When alien species are used, developing early warning and assessment systems regarding their behaviour as well as effective response systems is essential.