PARENT SESSION
Symposium #22: Species Interactions in Desert Communities: Dynamics of Resource Supply and Utilization .

Organized by: M Price and RJ Smith
Wednesday, August 7. 1:00 PM to 4:30 PM. Turquoise Ballroom, TCC.

Buffel grass invasion on Sonoran Desert plant communities.

BURQUEZ, ALBERTO^*,1, ¹ Instituto de Ecologia, Hermosillo, Sonora, México

ABSTRACT- The introduction of exotic species generally does not have major immediate effects on the ecological dynamics of natural ecosystems. Specific environmental requirements prevent many agricultural, forage, and ornamental species from escaping the boundaries where they were planted. Few of those that do become ruderal escape and displace native species. Buffel grass (Pennisetum ciliare), an African grass introduced as forage in the southwestern USA and northern Mexico, is a prime example of invasion and subsequent change in the structure, composition and functioning of natural communities. This grass has been widely planted for the past 3 and 4 decades in Mexico and the USA, respectively. It is now present in almost every frost-free arid region of North America. At the present moment, this species has had the greatest impact on Sonoran Desert communities where buffel grass has followed two invasion trajectories: one where human land clearings and seeding has promoted its absolute dominance, and another which displaces most native species through natural dispersal and colonization. In both cases, natural or anthropogenic disturbance plays a major role in establishment. With time, a grass/fire cycle —previously unknown to desert dynamics, allows populations to permanently establish, spread and successfully eliminate native species. Management policy, economics and societal values determine the speed and extent of invasion, in addition to "purely" ecological factors. The replacement of the arborescent desert by buffel grasslands reduces standing crop biomass almost four fold, reduces alpha diversity by almost one order of magnitude, and changes key soil features. We estimate that about two million hectares of Sonoran Desert had been transformed by 2000. Landscape fragmentation caused by buffel grass invasion changes very local, site-dependent features, as well as large-scale patterns such as hummingbird migrations.

KEY WORDS: Fire, Invasive, Sonoran Desert, Fragmentation