Introduction

The taxon Prosopis (Leguminosae, mesquite) includes some of the most common tree species in the dry tropics (Pasiecznik et al., 2004). Several species, all of American origin, have been intentionally distributed throughout the tropical world, because of their acclaimed roles as fast-growing, drought-tolerant, multipurpose trees. They are valued as a rehabilitation tool for degraded rangelands, shade, fodder (the pods are palatable for livestock and humans), honey, charcoal, timber, fuel, and several other resources (Fagg and Stewart, 1994; Felker and Moss, 1996; Pasiecznik et al., 2001). Large-scale systematic plantings have been made in parts of Africa, several oceanic islands, the Middle East and the Indian subcontinent since the mid 1900s, but ad hoc introductions have been common since the early nineteenth century (Harding, 1988; Fagg and Stewart, 1994; Felker and Moss, 1996; Tewari et al., 1998; Pasiecznik et al., 2001; van Klinken and Campbell, 2001; Mauremootoo, 2006; Ogutu and Mauremootoo, 2006).

Introduced mesquite species have become invasive in many countries, while some species are a nuisance to humans and livestock within their native ranges (DeLoach, 1985; Dussart et al., 1998). As a result, mesquite is now seen to be causing substantial negative economic, environmental, and social impacts over large parts of the world (van Klinken and Campbell, 2001; Mauremootoo, 2006; Ogutu and Mauremootoo, 2006; Zimmermann et al., 2006). However, indications are that the per capita impacts of invasive populations could be greater in their exotic ranges.