Biology and Impacts of Pacific Island Invasive Species. 6. Prosopis pallida and Prosopis juliflora (Algarroba, Mesquite, Kiawe) (Fabaceae)

Prosopis pallida and P. juliflora (commonly referred to as algarroba, mesquite, or kiawe) were introduced from South America to areas in Oceania, Asia, and Africa during the early nineteenth century. In many cases, they naturalized and became widespread. In some places, alien Prosopis species are highly valued for the products and services that they can provide such as shade, cattle fodder, wood for fuel and fence posts, and nectar for honey production. In Australia, four Prosopis species including P. pallida, P. juliflora, P. glandulosa, P. velutina, and their hybrids are considered invasive and are subject to control efforts. After its introduction to Hawai‘i in 1828, P. pallida became a dominant tree in arid areas of the main Hawaiian Islands, replacing the native lowland dry forest species that had been decimated by human activity, particularly by the introductions of goats and cattle. Prosopis pallida also has become an important economic species in Hawai‘i. Prosopis juliflora, a more recent introduction to Hawai‘i, is now spreading and is considered to be a noxious weed. Competition between Prosopis and native species as well as negative impacts of Prosopis on soil and local hydrology have been reported; however in some cases Prosopis species are characterized as midsuccessional species that rehabilitate degraded soils, eventually facilitating later-successional woodland. This provides a potential opportunity to use these species in reforestation efforts. Management decisions regarding these species should include a consideration of both their positive and negative ecological roles. If control or eradication is desired, a number of methods have been employed with various degrees of success. Prosopis L. is a genus of medium-sized trees and shrubs in the family Fabaceae (Leguminosae), subfamily Mimosoideae, with species occurring naturally in Africa, Asia, and in North and South America. The most recent authoritative work on the taxonomy of the genus was conducted by Burkart (1976), who recognized 44 species in five sections. Phylogeny within the genus has been revisited recently, and changes to subgeneric relationships have been proposed (Hunziker et al. 1986, Bessega et al. 2006, Burghardt and Espert 2007). Well-known species of Prosopis include P. glandulosa Torr. (honey mesquite) and P. velutina Woot. (velvet mesquite), which are both native to areas of the southwestern United States and Mexico, and P. pallida (Humboldt & Bonpland ex Willd.) Kunth and P. juliflora (Sw.) DC. from western South America. Prosopis pallida and P. juliflora are among the most notable invasive species in several arid and semiarid areas of the world. Although these two species are morphologically similar and have in the past been treated together as a complex, these apparent similarities may mask important ecological differences (Burkart 1976, Pasiecznik et al. 2001, Burghardt and Espert 2007).