What is an Invasive Alien Plant?

Invasive alien plants (or weeds) are non-indigenous plant species that have been introduced and spread outside their natural distribution range and are having negative environmental and/or economic impacts.

A species introduction is usually vectored by human transportation and trade. If a species’ new habitat is similar enough to its native range, it may survive and reproduce as its natural predators and competitors are not present. For a species to become invasive, it must successfully out-compete native organisms, spread through its new environment, increase in population density and harm ecosystems in its introduced range. The problem of invasive alien species is not restricted to the plant kingdom. Many species of introduced fauna species have invaded many environments globally post human introduction, and impacted very significantly in various comparable ways.

What is the problem?

Typically only a small percentage of introduced alien species become invasive. Nonetheless, their impacts are immense, insidious and usually irreversible. They have the ability to transform the structure and species composition of ecosystems by replacing or excluding indigenous species through out-competing them for resources. Invasive alien species can change the functioning of ecosystems. For example alien weeds can alter the fire regime, nutrient cycling and hydrology in native ecosystems. Thus, it is no wonder that invasive alien species are regarded as the second greatest threat to biological diversity globally, after habitat fragmentation. They also pose a huge threat to water security. Several invasive alien plant species are known to utilize far more water than indigenous species. Floating aquatic alien weeds facilitate more water loss through transpiration than would normally be lost through surface evaporation as is the case with water hyacinth in Lake Victoria. In South Africa, invading alien plants are currently using an additional 3 300 million cubic meters of water, and the South African government recognizes that clearing invading alien plants is a far better alternative than dam construction in certain catchments. Invasive alien species have invaded and affected native biota in almost every ecosystem type on Earth, and have affected all major taxonomic groups. In economic terms, the costs of invasive alien species are significant. Globally, the cost of damage caused by invasive species has been estimated to be £1.5 trillion per year, which is close to 5% of global GDP (source). This does not include valuation of species extinctions, losses in biodiversity, ecosystem services and aesthetics. In developing countries, where agriculture accounts for a higher proportion of GDP, the negative impact of invasive species on food security as well as on economic performance can be even greater. Globalisation through increased trade, transport, travel and tourism will inevitably increase the intentional or accidental introduction of organisms to new environments, and it is widely predicted that climate change will further increase the threat posed by invasive species.

The harsh reality concerning the problem of invasive alien species is that it becomes far more expense to control invasions the longer they are left, and the chances of controlling them effectively diminishes simultaneously over time. Already, there are numerous cases around the world, such as in New Zealand and the Island of St Helena off Africa, where the infestations of alien species have caused irreversible damage and it is no longer economically and practically feasible to restore the situation.

Many invasive alien plant species occur in Tanzania. The majority of these grow along rivers, near settlements and in towns and villages where they were used as ornamental plants in gardens or used to create hedges. Several highly invasive species have been observed in formally protected areas, including even the Serengeti National Park, Ngorongoro Conservation Area, Selous Game Reserve (World Heritage Sites). These include species such as Black Wattle Acacia mearnsii, Giant Sensitive Plant Mimosa pigra, Lantana Lantana camara, Seringa Melia azedarach, Pricky Pear Opuntia sp, Pine Tree Pinus sp., Gum Tree Eucayltus sp., and the aquatic weeds Water Lettuce Pistia stratiotes and Red Water Fern Azolla filiculoides. These species have caused extensive problems in other countries because of their invasiveness, ability to spread, and economic and environmental impacts. A few of these species, such as Mimosa pigra and Lantana camara, are listed on the global list of the 100 worst Invasive Alien Species!

The good news, however, is that it is not too late for Tanzania. Invasive species tend to follow what is referred to as a ‘’long fuse big bang’’ population growth curve. They persist at low numbers for many years, slowly increasing and then suddenly their population literally explodes. Available data and record suggest that the invasive alien plant numbers in Tanzania are still at a relatively low level.

What is the PAMS Foundation doing?

The PAMS Foundation implemented the 'Stop the Spread' invasive alien species control programme in southern Tanzania. It focuses on education and awareness to encourage villages to remove invasive alien species, and provides non-invasive replacement plants (seeds and seedlings) in return. The control programme is implemented through a bottom-up approach, in that communities and Districts take ownership of the invasive alien plant problem, and put the necessary measures in place to ensure long-term management of invasive alien plants in their areas. To date, the progress and support from the communities has been outstanding compared with similar projects and approaches implemented elsewhere in Africa.

The PAMS Foundation has also been involved in updating the Invasive Alien Plant Strategic Managament Strategy for Ngorongoro Conservation Area.

The PAMS Foundation has been involved in monitoring & mapping the highly invasive alien weed known as Parthenium hysterophorus in Tanzania for the past 2 years (2010 and 2011). P. hysterophorus is considered to be one of the world’s most serious invasive alien plants, because of its potential for spread, and economic and environmental impacts. It has invaded parts of Australia where it is classed as a Weed of National Significance, Asia and Africa. It has already become invasive in South Africa and Ethiopia, and appears to be rapidly spreading in Uganda and Kenya and more recently Tanzania. The work undertaken is thanks to funds from USAID & Virginia State University. We have now recently joined forces with TAWIRI & Tanzania Conservation Resource Centre. Thanks to Howard Frederick, the first step has been to set up a map where we and other contributors are able to upload any Parthenium sightings. So if you spot Parthenium weed anywhere in Tanzania, please report your sighting on the below website. THANK YOU!

https://partheniumafrica.crowdmap.com/