Impact of grazing and desertification in the Chihuahuan Desert: Plant communities, granivores and granivory

Livestock effects on plant communities through overgrazing (desertification ) should affect the structure and functioning of semarid rangeland communit ies. We measured plant, granivorous ant and rodent communities and rates of seed removal by rodents and ants in grazed (by livestock) and ungrazed des ert grasslands as well as mesquite and creosotebush shrublands to test hypo theses on the effects of grazing and desertification on ecosystem structure and functioning. In desert grasslands grazing reduced the cover of perenni al grasses, particularly the dominant Bouteloua eriopoda, but the cover of forbs and shrubs did not differ between treatments. One species of perennia l grass, Dasyochloa pulchellum increased in grazed grasslands compared with grassland exclosures. Detrended correspondence analysis showed that grazin g caused desert grasslands to shift in community structure towards the shru blands. There were more seed harvesting ant and rodent species in the creos otebush shrublands than in the grasslands and mesquite shrublands. Grazing had no effect on the diversity of ants or rodents within grasslands, and de trended correspondence analysis revealed no clear trends in granivorous ant community structure in the grazed and unsated grasslands or the mesquite a nd creosotebush shrublands. Ants removed more seeds than did rodents in the grassland sites but rodents removed more seeds than did ants in the creoso tebush sites and seed removal rates by rodents and ants were the same in th e mesquite sites. Our data support the hypothesis that livestock grazing le ads to a shift from grassland to shrubland in the Chihuahuan Desert, with a ssociated changes in the structure and functioning of faunal communities. B ecause grasslands support few species and lo iv densities of rodents, seed harvesting ants are the most important granivores in these desert grassland s. On a larger scale, we therefore hypothesize that the observed dominance of rodents as seed harvesters in the Chihuahuan desert is a function of the desertification of desert grasslands to shrublands by livestock, and that associated feedback effects may complicate the regeneration of degraded com munities.