Invasive species

Photo: European and Mediterranean Plant Protection Organization (EPPO).

The complex combination of different kinds of living organisms creates a synergistic interaction that effectively sustains the stability and proper functioning of the biosphere1. Almost 30 years ago, the UN emphasized the rationale to conserve biological diversity as it presents an intrinsic sociocultural, economic and ecological value to humans. However, the global distribution of species is increasingly becoming more homogeneous owing to the degradation and decline of native species by the introduction of invasive alien species (IAS) outside their native ranges2.

IAS are non-native species that are introduced outside their native range where they establish, spread and pose adverse effects3. The reasons for their introduction are often perceived as beneficial, such as soil stabilizers, sources of fuelwood, climate regulators and ornamental plants. Consequently, at their initial stages of invasion, they are often praised for having achieved their intended purpose of introduction. However, with an increasing cover, they encroach on native species as their net negative impact becomes irrefutably evident. According to IUCN , the impacts of alien invasive species are insidious, immense and usually irreversible. They are a leading contributor to species endangerment and extinction after habitat loss and contribution to worldwide economic losses exceeding $300 billion annually4.

The negative impacts of invasive tree species that aggravate biodiversity degradation include:

Despite the widely acknowledged and documented adverse impacts of IAS, their management remains a social dilemma as trade-offs occur between collective and individual interests as well as immediate benefits and long-term adverse impacts. One of the key challenges to the successful management of IAS is the conflicting perceptions of the true definition of invasive species.

Arid areas in Kenya

IMARA project team in a field mission in Samburu. Photo: Lawrence Nzuve / SEI Africa.

What qualifies species to be invasive and at what stage should they be considered as such?

Often, invasive species establish themselves in a cunning manner: initially presenting themselves as beneficial, only to pose a net negative impact with their increasing densities. While the immediate benefits blur their beneficiaries from the potential future adverse impacts, their increasing densities are linearly and positively correlated with the cost of management. A study5 confirms that at advanced stages of invasion, a unit cost of clearing Prosopis juliflora – one of the most invasive tree species in East and Southern Africa – far outweighs the actual cost of the land cleared. Furthermore, the invasion process is complex as IAS have no jurisdictional boundaries. This advances their rapid spread over large areas within a very short time frame. As such, it is logical to engage community members through a collective landscape management approach to curtail the spread of such invasive species before they advance to unmanageable levels.

Arid areas plants

Plants in arid setup: Invasive species establish themselves in a cunning manner: initially presenting themselves as beneficial. Photo: Lawrence Nzuve / SEI Africa.

The Integrated Management of Natural Resources for Resilience in Arid and Semi-Arid Lands (IMARA) programme implemented by SEI in collaboration with partners including World Vision (lead partner), Northern Rangelands Trust, Safer World and Maasai Mara Wildlife Conservancies Association has worked on community-led land restoration and regeneration since 2018. Dealing with scientific evidence and community-backed insights on invasive species has led to both the mechanical and biological removal of Prosopis juliflora in Laikipia County, Kenya. In the renewed phase of the programme currently in Turkana, West Pokot, Elgeyo Marakwet and Narok Counties, the dilemma strikes as the perceived benefits outweigh the need for removal requiring an integrated and inclusive approach by the programme and the communities.

IMARA meeting

Dignitaries arrive at the launch of Phase 2 of the IMARA project in Narok County, Kenya.
Photo: Lawrence Nzuve / SEI Africa.

Achieving a desirable landscape outcome is an aggregate effect of individual actions with a common landscape management goal. The prevailing disconnects concerning the need to manage invasive species limit the chance of timely action and effective management. In order to sustainably manage invasive tree species, there is a need for community members to have a common understanding of their potential future impacts and the urgency in maintaining a healthy balance of biodiversity, as is the case in the ongoing IMARA programme. Such discussions can only be possible if informed by proven scientific information, as well as knowledge and experience sharing from land users with first-hand experience on the adverse impacts of invasive species.


  1. Hobbs, R. J., Higgs, E. and J. A. Harris (2009). Novel ecosystems: implications for conservation and restoration . Trends Ecol. Evol., vol. 24, no. 11, pp. 599–605, 2009.
  2. World Resources Institute (2005). Ecosystems and Human Well-being: Synthesis . In Millennium Ecosystem Assessment, 2005. Island Press, Washington, DC.
  3. Maundu, P., Kibet, S., Morimoto, Y., Imbumi, M. and Adeka, R. (2009). Impact of prosopis juliflora on Kenya’s semi-arid and arid ecosystems and local livelihoods . Biodiversity, vol. 10, no. 2–3, pp. 33–50, 2009.
  4. Luque G. M. et al. (2014). The 100th of the world’s worst invasive alien species . Biol. Invasions, vol. 16, no. 5, pp. 981–985, 2014. .
  5. Wise, R., van Wilgen, B. and Le Maitre D. (2012). Costs, benefits and management options for an invasive alien tree species: The case of mesquite in the Northern Cape, South Africa . J. Arid Environ., vol. 84, no. 3, 2012.