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Invasive alien species are plants, animals, pathogens (agents that cause disease) and other organisms that are non-native to an ecosystem, which may cause economic or environmental harm or adversely affect human health. They impact adversely on biodiversity, resulting in the decline or elimination of native species — through competition, destruction, or transmission of pathogens — and the disruption of local ecosystems and ecosystem functions.

Invasive alien species, introduced and/or spread outside their natural habitats, have affected native biodiversity in almost every ecosystem type on earth and are one of the greatest threats to biodiversity.

Since the 17th century, invasive alien species have contributed to nearly 40 percent of all animal extinctions.
The problem continues to grow at the expense of socio-economic, health and ecological issues. Invasive alien species worsen poverty and threaten development through their impact on agriculture, forestry, fisheries and natural systems, which sustain livelihoods in developing countries.

This damage is aggravated by climate change, pollution, habitat loss and human-induced disturbance.

Alien invasive species in Zimbabwe
The following plant species are specified as invasive alien species in Zimbabwe and it’s everyone’s responsibility to clear any growing plant and report to EMA inspectors the emergence of such plants on their properties.

Aquatic Species (Those found in water bodies)
Common Name Botanic Name
Water hyacinth Eichhornia crassipes
Kariba weed Salvinia molesta
Azolla Pistiastratiotes
Water lettuce Azollafiliculoides
Water fern Salvinia auriculata
Terrestrial Species (Those found on land)
Common Name Botanic Name
Jointed cactus OpuntiaAurantiaca
Moonflower cactus Harrisia martini
Cactus rosea Opuntia fulgida
Wild oats Avenafatua
Dodder Cuscutaspp
Cherry pie Lantana camara

Invasive species are introduced through many means. Intentional introductions have often been for agricultural or ornamental purposes.
Once introduced, some of these species escape their enclosures or cultivation and can become established as viable populations.
Accidental introductions are usually the result of contaminated freight or movement of contaminated wood.

Are all exotic species invasive?
No, actually only a small percent of introduced species ever become invasive. However, it is nearly impossible to predict which species will become invasive and new species are introduced every day.

Some species are present for many years before they exhibit invasive characteristics. Many invasive species go through a “lag phase” in which their populations grow slowly until they reach a size large enough for the population to explode and/or become adapted to the local environment and become invasive.

What type of harm does an invasive species do?
Since invasive species are in a new environment, free from natural predators, parasites, or competitors, they often develop large population size rapidly;
These high populations can out-compete, displace or kill native species or can reduce wildlife food and habitat;
Some also have the potential to disrupt vital ecosystem functions, such as water flow, nutrient cycling, or soil decomposition;
Other invasive species cause massive amounts of economic damage to the agricultural business by destroying crops and contaminating produce;
Some invasive species can cause direct harm to humans or domestic animals e.g. lantana camara and opuntia fulgida;
They propagate diseases;
Competition between introduced and native species can lead to extinctions;
They replace indigenous vegetation and reduce the rich vegetation species or biodiversity;
Some of the worst invaders that affect water bodies are: Kariba weed, water hyacinth, dodder and water fern. Many rivers and dams are clogged by these resulting in reduced flow of water in water bodies reducing the amount of water available for irrigation. They also affect aquatic life and disturb recreation activities in water bodies.

Invasion of aquatic weeds are associated with a range of impacts on water quality. Dense mats of these weeds can impede water flow, which increases the rate of siltation in water bodies and inhibit diffusion of air into the water, resulting in lower concentrations of dissolved oxygen. Lower oxygen concentrations combined with the increased amounts of organic waste that collect beneath the floating plants limit light penetration thereby prevent the development of planktonic algal blooms a typical result of eutrophication e.g. invasion of salvinia molesta in lake Kariba.

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