Troubled Waters: Biological invasion of the Kafue River | WWF Zambia

Troubled Waters: Biological invasion of the Kafue River

Posted on
31 October 2017
By Sharon T. George

For decades, alien invading plants and animals have enjoyed unrestricted access to new environments within Zambia. Some of these have spread and multiplied to the point where they have detrimental, and often irreversible, ecological and socio-economic impacts. Awareness of the impacts of invasive species in Zambia has varied along the years, but the cost is currently underappreciated and therefore under-addressed, resulting in widespread invasion by some of the worst invasive species on the planet.

In many regions of the world, humans have introduced species to areas outside their normal range of occurrence. If the conditions of new areas are suitable, they can establish and reproduce locally, sometimes occupying the entirety of an ecological niche and outcompeting local native species. When this happens, they are referred to as Invasive Alien Species. However, not all invasive species are foreign or alien, in some instances native species have been known to adapt qualities of invasive species, including excessive reproductive capacity and aggressive tendencies.

Common ways in which invasive alien species are introduced:
 
INTENTIONAL INTRODUCTIONS ACCIDENTAL INTRODUCTIONS
  • Plants cultivated for agricultural, forestry or ornamental purposes
  • Mammals or birds released for hunting purposes or as food
  • Exotic pets released, or escaping, into the wild
  • Aquarium trade
  • Escapes from aquaculture
  • Releases intended to ‘enrich’ the native flora and fauna
  • Escapes from research facilities, botanical gardens, zoos
  • Biological control agents
  • Contaminants such as seeds, invertebrates and other organisms hidden in imports such as agricultural produce, nursery plants, timber imports, machinery, equipment and vehicles
  • Parasites and pathogens of farmed animals or fish
  • ‘Hitchhikers’ in, or on, packaging material, cargo, automobiles or aeroplanes and via ship ballast water on ships
  • ‘Hitchhikers’ on tourists and their clothing, footwear, luggage, or equipment


Invasion of the Kafue Flats
The Kafue Flats finds itself in Zambia’s hottest river catchment area which harbors the highest degree of multi-sectoral water-users - from industrial, commercial, agricultural, domestic and environmental resource needs.  
 
The crucial role that the Kafue ecosystem plays in the Zambian economy is apparent. The Kafue Flats provide 50 per cent of the country’s total hydropower output; 44 per cent of all water for Lusaka, a city bordering 2 million people; livelihoods for almost 1 million people that directly depend on this area; and produces approximately 90 per cent of all sugar in the country, a key agricultural commodity. The region is also home to the highest concentration of cattle in the country (with an estimated 20 per cent of the national herd grazing on the flats in the dry season) and one of Zambia’s most productive wild fisheries, supplying both urban and rural markets.
 
In addition to the social and economic provisions gained from the river, it also supports and sustains vital ecological systems like wetlands, game reserves and bird sanctuaries. There are two national parks (Blue Lagoon and Lochinvar) within the area, both of which are Ramsar sites, in addition to the Kafue Flats Game Management Area which host species, such as the endemic and endangered Kafue Lechwe and the highly endangered Wattled Crane. This demonstrates the richness and uniqueness of the Kafue Flats as well as its crucial role in Zambia’s growing economy.

One of the main threats to this significant ecosystem is the introduction and spread of invasive species. The Kafue Flats has been plagued by five main invasive species of animals and plants. These include three aquatic species, the Water Hyacinth (Eichhornia crassipes), the Nile Tilapia (Oreochromis niloticus), and the Red Claw Crayfish (Cherax quadricarinatus), and two terrestrial plant species, the Giant Sensitive Tree (also commonly known as "mimosa") (Mimosa pigra) and the Sickle Bush (Dichrostachys cinerea).

Aquatic Invasion
The Water Hyacinth, classified among the worst 100 invasive plants in the world, is originally from South America and is now found in more than 50 countries on five continents. Water Hyacinth is very fast-growing, with populations known to double in as little as 12 days. Infestations of this weed block waterways, limiting boat traffic, swimming and fishing. Water Hyacinth crowds native aquatic plants and prevents sunlight and oxygen from reaching the water column and submerged plants, dramatically reducing biological diversity in aquatic ecosystems. The Water Hyacinth is also known to increase evapotranspiration, often over three times above “open pan” evaporation, thus causing significant water loss to wetlands, dams, and reservoirs.

Over the last decade this weed has become a major problem in the Kafue Flats. It blocks water supply systems, waterways, and inflow pipes at Lolanda water pump station, and hydropower turbines at the Kafue Gorge power plant. Local people struggle with access to the water for domestic use, transportation and fishing activities, regardless of whether they use dugout or powered boats.

Red Claw Crayfish and Nile Tilapia are both believed to have escaped from aquaculture in surrounding areas. When invasive, Red Claw Crayfish is a powerful ecosystem engineer, disrupting the food chain at multiple levels. It destroys aquatic plants, competes with and preys on native invertebrates, devours fish and amphibian eggs, and disturbs fish breeding habitats. It is also known to harm artisanal fisheries by feeding on fish caught in gill nets, damaging the catch and reducing its value. 

The rapid spread of Nile Tilapia is likely to have a similar impact on the Kafue system. To make matters worse, it is now promoted for aquaculture in the Upper Zambezi catchment due to its fast reproductive rate. It outcompetes or hybridizes with native fish species, such as the Three Spotted Tilapia (Oreochromis andersonii). Long-term, hybridization may leave no genetically pure Three Spotted Tilapia in the wild. If nothing is done, Nile Tilapia will continue to spread throughout the range of the Three Spotted Tilapia, leading to its extinction. 
 
Terrestrial Invasion
The spread of Giant Sensitive Tree and Sickle Bush have drastically altered the landscape of the Kafue Flats. The invasion has in turn compromised the occurrence of palatable aquatic plant species for wildlife and cattle, such as millet (Echinochloa spp.), Hippo Grass (Vossia cuspidata), and Nile Grass (Acroceras macrum).

Mimosa pigra, native to South America, is classified among the worse 100 invasives of the world. It has expanded from 2 ha in the 1980s to currently over 3,000 ha, and now occupies a significant proportion of the floodplain. It limits fish reproduction in the wet season, eliminates feeding grounds for water birds including the Wattled Crane (Bugeranus carunculatus), and destroys grazing land for wildlife and livestock alike in the dry season. The open, productive floodplain ecosystem of the Kafue Flats has been transformed into a semi-enclosed shrub system, as is the example around Lochinvar National Park. According to the Department of National Parks and Wildlife, Mimosa has also compromised the tourism potential of the area. It has altered the landscape, blocking the waterfront, and has had a negative impact on the endemic Kafue Lechwe, a major tourist attraction, by reducing their access to food and water, eliminating some breeding (lekking) sites, and displacing them into unprotected areas where poachers kill them.

“A lot of new vegetation has grown a lot in and near the rivers” – Resident of Chunga Harbour

The Sickle Bush, native to southern and central Africa, now occupies a significant proportion of the floodplain and termitaria grasslands in Lochinvar National Park. The encroachment has significant negative impacts on the habitat and breeding cycles of some mammal species, on tourism, and on floodplain use by wildlife, birdlife, livestock and fish.

Resisting Invasion
Little can be done to eradicate invasive species completely once they establish themselves. There may be long-term prospects for limited control using carefully selected biological agents, but in the short to medium term control strategies should focus on preventing and limiting their spread. 

Unfortunately, like many societies, authorities in Zambia have adopted an exclusively reactive approach when dealing with the problem of invasive species. Invasive species cannot and must not be treated like other forms of pollution: they simply don’t stop spreading when release stops, but instead spread in an accelerating manner. The old medical adage “prevention is better than cure" is frequently invoked in relation to alien species’ invasions — and with good reason. We continue to pay a heavy price for the ecological, economic and social damage brought about by alien invasions. Year after year, time and money is spent towards efforts aimed at limiting the destructive impacts of invasions that might have been averted. We are slowly learning, to our great cost, that prevention would have been, and is, far better than cure.
 
WWF Zambia is working with its partners to raise awareness about the seriousness of the problem, to lobby government and businesses to invest in control measures and monitoring practices, to lobby for tougher legislation governing the introduction of invasive species in the country, and to dissuade further introductions to unaffected areas.


 

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