Invasive Weed Species

Tumble mustard

Weeds are defined as plants that interfere with human activities. Invaders are a type of weed species that has been brought to our country from another part of the world, and have begun to spread into our environment where they interfere with the ecosystem. These plants present a great threat.

Impacts of Invasive Species

Invasive species have many negative impacts on our livelihoods and our natural ecosystems. These include:

• Loss of Biodiversity – When invasive weeds enter an ecosystem they replace the indigenous plants that have historically grown there. This can lead to local extinctions of indigenous plant species. Many bird, insect, and other animal species will not be able to feed or live on invasive weeds, and will be forced to leave the area to find a better ecosystem to live in.
• Ecological Imbalance – When invasive weeds replace indigenous plants they cause an ecological imbalance by changing natural cycles. Many indigenous species, such as the Australian wattles are very flammable. This increases the risk of fires in the area, which may threaten fire-sensitive species.
• Prevention of Access – Some invasive weeds such as the prickly-pear have spines or thorns. When these species take over an area and form thickets that are impenetrable. This can prevent access to water supplies, grazing areas and shade trees.
• Soil Erosion – Invasive species such as the Australian wattles are easily ripped out by strong winds or floods. This will leave the soil exposed and susceptible to soil erosion.
• Reduced Water Resources – Many invasive weeds threaten our water resources. When these species invade ecosystems, they replace indigenous species that have a lower water requirement. Gum trees and wattles have a very high water requirement, and have been shown to be one of the leading causes of streams and rivers drying up. A 2007 student estimated that up to 16% of the water that could be harvested in catchments is lost to invasive species. This is particularly concerning as we are already a water stressed region, and will experience less rainfall in the future due to climate change.
• Changing Natural Soil Composition – Some invasive weed species have the ability to change the composition of the soils they grow in. The needles of pine trees are acidic, causing soil acidification underneath the tree. Amaranth has a very high nitrogen requirement and so can deplete the available nitrogen in the soil faster than other plants, causing nitrogen deficiencies.
• Create Dense Floating Mats – Invasive weeds that grow in water, such as water hyacinth, can form thick mats when they invade a water source. This can block irrigation pumps, reduce the flow of water along canals, and even cause livestock to drown. This is because the livestock see a solid mass that they can walk on, become tangled in the plants, and drown in the water.
• Increase Agricultural Input Costs – Nearly all of the worst agricultural weeds are invasive weed species. Controlling these weeds is expensive and time consuming. Hopefully through this learning programme you will be equipped with the knowledge to control weeds more efficiently and reduce your costs!

Invasive Weeds in Zambia

In Zambia we have the National Strategy on Biological Invasions. The aim of this strategy is to and reduce the negative impacts of invasive species in the country through four objectives:

1.    Prevent the introduction of new species that pose a risk of becoming invasive

2.    Remove invasive weeds where possible

3.    Reduce the spread of invasive weeds

4.    Reduce the negative impacts of existing invasive weed populations

With all management decisions, planning an effective control strategy for invasive weeds requires an understanding of the impacts of the plant we want to control. Not all invasive weeds have the same impact on the environment, and they can also have different impacts in different areas. Blackjack and pompom weed are two common invasive weeds. However blackjack is not as aggressive as pompom weed, and usually only grows in disturbed soil. Pompom weed is a very aggressive invasive weed, and has invaded many of our grasslands and replaced indigenous species. Blackjack is therefore less of a threat to the environment than pompom weed, and this will affect our management strategy. In Zambia we rank weeds into three categories, based on the threat they pose.

Category 1: Declared Invasive Weeds

These invasive weed species pose a great threat to our environment. They must either be completely removed (Category 1a) or controlled to a safe level (Category 1b) by the person on whose land they are growing.

Category 2: Declared Invasive Plants with Commercial Value

These invasive weed species pose a threat, but also have economic value. These include trees that are used for timber, or grasses that are used for feeding livestock. A permit from the government is required to have these species growing on your land, and you must ensure that they do not spread beyond the border of your property.

Category 3: Invasive Ornamentals

These invasive weed species have historically been used in gardens. Although existing plants are allowed to continue growing, the person on whose land they grow must take action to ensure they do not spread. No new plants may be planted.

Common Invasive Weeds

The figures are all of common invasive weed species you will probably see in your area. The captions contain information about each species. You can use the figures to identify these species and take the appropriate actions to remove or control them. For more information about specific invasive weeds you can go to www.invasives.org.za

Summary

Invasive weeds present a great threat to both us and the environment we live in. Although some species have an economic benefit, every effort must be made to remove invasive weeds where there have no economic benefit.

References

1.    Blignaut, J.N., C. Marais, and J. Turpie, Determining a charge for the clearing of invasive alien plant species (IAPs) to augment water supply in South Africa. Water Sa, 2007. 33(1).

2.    Bromilow, C., Problem plants and alien weeds of South Africa. 2010: Briza.

3.    Armstrong, A. and H. Van Hensbergen, Impacts of afforestation with pines on assemblages of native biota in South Africa. South African Forestry Journal, 1996. 175(1): p. 35-42.