Background
The author writes that "several studies in the last five years have warned against the potential impact of promoting biofuel crops that are known to be invasive or to have potentially invasive characteristics," citing the studies of Raghu et al. (2006), Barney and DiTomaso (2008), Howard and Ziller (2008) and Buddenhagen et al. (2009), while noting that "a large number of proposed biofuel crops share the same traits as known invasive plant species," and indicating that many of them "are already present in Africa."

What was done
In light of these observations, Witt assesses the impacts of several species of the invasive Prosopis genus in Kenya and South Africa, where the spiny trees and shrubs have invaded over four million hectares of crop and pasture land.

What was learned
The Kenyan researcher reports that "communities in Kenya and elsewhere are becoming increasingly concerned about the displacement of other species important for local livelihoods, especially fodder species for livestock." They are also concerned, as he continues, about their encroachment onto "paths, dwellings, water sources, farms and pastureland," as well as their "negative impacts on animal and human health with injuries due to thorns resulting in some human fatalities," as noted by Mwangi and Swallow (2005) and Maundu et al. (2009). In addition, he notes that the plants' tendency to invade riparian zones, dry river beds, and lowlands, where they "tap into underground water sources," means that they also "interfere with drainage, blocking watercourses and exacerbating the effects of flooding." And he states that the displacement of native plants by Prosopis species is especially serious, noting that "the World Health Organization estimates that up to 80% of the world's rural populations depend on [native] plants for their primary health care."

What it means
Witt concludes that the importation and growing of all nonnative species that are known to be invasive elsewhere, and have been deemed to be a high-risk species, "should not be introduced and cultivated," because "the costs associated with invasive species, even those that are deemed to be beneficial, in most cases, outweigh the benefits that accrue from their use," while ending with the solemn warning that "no widespread invasive plant species has been controlled through utilization alone in any part of the world."

References
Barney, J.N. and DiTomaso, J.M. 2008. Nonnative species and bioenergy: are we cultivating the next invader? BioScience 58: 64-70.

Buddenhagen, C.E., Chimera, C. and Clifford, P. 2009. Assessing biofuel crop invasiveness: a case study. PLoS One 4: 1-6.

Howard, G. and Ziller, S. 2008. Alien alert -- plants for biofuel may be invasive. Bioenergy Business July/August: 14-16.

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 10: 33-50.

Mwangi, E. and Swallow, B. 2005. Invasion of Prosopis juliflora and Local Livelihoods: Case Study from the Lake Baringo Area of Kenya. ICRAF Working Paper No. 3. World Agroforestry Center, Nairobi, Kenya.

Raghu, S., Anderson, R.C., Daehler, C.C., Davis, A.S., Wiedenmann, R.N., Simberloff, D. and Mack, R.N. 2006. Adding biofuels to the invasive species fire? Science 313: 1742.