MECHANISMS FOR THE KEYSTONE STATUS OF KANGAROO RATS - GRAMINIVORY RATHER THAN GRANIVORY

Graminivory by kangaroo rats (Dipotomys spp.) was investigated as a po tential mechanism fur tile keystone role of these rodents in the dynam ics of desert grasslands. Experiments confirmed that Ord's kangaroo ra ts (Dipodomys ordii) cut and consumed a large proportion of the tiller s of three Chihuahuan Desert tussock-forming grass species. Field obse rvations indicated that the characteristically cut grass tillers were absent from all-rodent and medium-sized kangaroo rat exclosures, but w ere frequent in large-sized kangaroo rat and rabbit exclosures. indica ting that the medium-sized kangaroo rats (D. ordii, D. merriami) were responsible for grass cutting. Tiller waste as: a, percentage of peak standing crop ranged from 7% in grassland habitats to 0.7% in Flourens ia cernua shrubland. OF the 13 species of perennial, tussock-forming g rasses measured, only one, Muhlenbergia porteri, had no tillers cut by kangaroo rats. This study demonstrates that the keystone role of kang aroo rats in Chihuahuan Desert grassland ecosystems is probably the re sult of their graminivory.