Invasive exotic species constitute a large and growing threat to biodiversity and ecosystem stability in the Sonoran Desert region. Early in the twentieth century, when fields and open desert surrounded the Desert Laboratory, only three species of exotic plants grew on the grounds. One hundred years later, suburban sprawl has nearly isolated the Desert Laboratory grounds from natural desert, and weeds and cultivated plants have invaded from nearby yards and gardens. Now 52 of the 346 species in the Desert Laboratory flora are exotic. Of these, red brome, an annual grass from the Mediterranean region, and buffelgrass, a perennial grass from southern Africa, are a major fire hazard. Most native Sonoran Desert plants do not survive wildfire. By promoting frequent burning, these exotic grasses might greatly alter the species composition and structure of natural Sonoran Desert communities. Buffelgrass poses an additional threat by depriving native shrubs and trees of soil moisture.
In 1983, Tony Burgess, Raymond Turner, and Janice Bowers surveyed the grounds of the Desert Laboratory in a regular grid, recording all species of exotic plants encountered on the gridlines. The graph above illustrates the distribution of six exotics in 1983. The stippled area on the map for Erodium cicutarium (filaree) shows where this species occurred in 1903. It has since naturalized throughout the grounds. Bromus rubens (red brome), not even present in 1903, was first collected on the Desert Laboratory grounds in 1968, at which time it was not common. The species has since become one of the most abundant winter annuals on the grounds. The Desert Laboratory grounds were fenced in 1907 to exclude domestic livestock. Subsequent disturbances included clearing and digging for a natural gas pipeline and a city sewer line, installation of electric utility lines, excavation of shallow clay quarries, and construction of a landfill. Although localized, these disturbances apparently gave exotics a foothold for invasion, whence they spread into undisturbed patches, perhaps after drought when deaths of native plants left open space.