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.

Introduction

Approximately 1600 species are recognized in the family Cactaceae and, with one possible exception, all are native to the New World (Gibson and Nobel, 1986; Wallace and Gibson, 2002). The exception is an epiphyte, Rhipsalis baccifera (previously cassutha) (J. S. Miller) Stern, which appears to be indigenous in Africa and Madagascar (Wallace and Gibson, 2002).

Most species of the Cactaceae have leafless photosynthetic stems that bear spines on modified axillary buds called areoles. The cacti are predominantly succulent and adapted to survive in extreme xeric habitats. Because of their often bizarre structures and appearances, many species are now cultivated widely around the world mostly as curiosities and ornamentals. Some species of cacti are used as a source of fruit and fodder, and as hedge plants (Casas and Barbera, 2002). The most common and commercially important species in the Cactaceae is Opuntia ficus-indica (Linnaeus) Miller, usually known as either “prickly pear” or “cactus pear”. This species is cultivated in many countries for its fruit and as fodder (Barbera et al., 1995) but also as the main host for production of the carmine cochineal insect, Dactylopius coccus Costa (Homoptera: Dactylopiidae) (Casas and Barbera, 2002), and supports a flourishing dye industry in Peru, Chile, Bolivia, and the Canary Islands (Flores-Flores and Tekelenburg, 1995).

Several species of Cactaceae, introduced either deliberately or accidentally into countries outside the Americas, have become invasive.