Intentionally Planted Exotic Species

The introduction and intentional sowing of African grasses in the Sonoran bioregion has not only affected the biotic composition of semidesert grasslands, but has profoundly changed vegetation structure, fire intensity and frequencies, and migratory wildlife corridors within several subregions of the Sonoran Desert proper. Of particular concern to ecologists are the extensive plantings of lovegrasses (Eragrostis spp.), buffelgrass (Pennisetum ciliare) and fountaingrass (Pennisetum sp.) into desert areas where they are capable of dispersing to adjacent areas and outcompeting natives. The "grasslandification" of the Sonoran Desert, adjacent thornscrub and subtropical savanna by buffelgrass (Miller et al. in review) has already occurred on some 600,000 hectares of Sonora, and is becoming notably dominant at lower elevations of Baja California. Cox (1991) recently claimed that buffelgrass is now the dominant herbaceous perennial on eight to ten thousand hectares in the Southwestern USA and Northern Mexico. Mexican agricultural officials claim that they would like to see the area in Sonora planted with buffelgrass grow from 600,000 to more than 2.0 million hectares in the coming decade. The ecological consequences of buffelgrass and higher elevation lovegrass plantings and invasion affect native biota in several ways (Bock and Bock 1992). First, buffelgrass typically invades native desertscrub communities by establishing under the canopies of the dominant legumes there, usually ironwood and mesquite (Miller et al. in review). Because 165 other natives use the canopy of ironwood and mesquite as nurseries (Nabhan and Carr 1994), this disrupts the regeneration of many herbs, cacti and shrubs. Paired sampling of neighboring plots in buffelgrass and in desertscrub have shown that plant richness decreases fourfold, and diversity decreases tenfold (Miller et al. in review). Changes in fire regimes then occur, and in the summer of 1996 alone, there were hundreds buffelgrass-spread wildfires in Sonora below 1000 meters in elevation where historically, fires seldom occurred. Fire-intolerant cacti, woody legumes, small mammals, butterfly larvae and groundand twig-nesting bees are negatively impacted. With lovegrasses numerous native birds decline while only a few thrive (Bock and Bock 1992).

There are also profound changes in standing biomass, nutrient cycling and availability, and in surface water flows. Búrquez and Martínez-Yrízar (unpublished) have found that conversion of arboreal desertscrub vegetation to buffelgrass pasture causes a three- to fourfold reduction in aboveground standing crop biomass. Nitrogen is more easily volatilized in buffelgrass pastures, while phosphorous and potassium are quickly lost after fires. Buffel pastures can choke out or lead to the burning-away of xeroriparian vegetation, including chuparosa bushes which serve as critically important seasonal floral resources for migrant hummingbirds. Calder (pers. comm.) has recently recorded dramatic decreases in Rufous Hummingbirds migrating through desertscrub, which may potentially be attributable to reduced springtime floral resources along their "nectar corridor" through Sonora. Such potential consequences of buffelgrass dominance need to be evaluated for a variety of ecological interactions and consequences.

From:
Nabhan, Gary Paul and Andrew R. Holdsworth. 1998. State of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats and the Adequacy of Protection in the Sonoran Bioregion. p.36-38. Tucson, Ariz.: The Wildlands Project.