Managing exotic grasses and conserving declining species

California's southern San Joaquin Valley, as with much of western North Ame rica, has been invaded by exotic plant species during the past 100-200 year s. The herbaceous cover of these introduced grasses and forbs often creates an impenetrable thicket for small ground-dwelling vertebrates. Contrary to some earlier descriptions of upland habitat of the southern and western Sa n Joaquin Valley as perennial grasslands, recent evidence suggests that mos t of this area was a desert vegetated by saltbush scrub with sparse cover o f native annual grasses and forbs. Many of the small vertebrates that evolv ed in these habitats, some of which are listed as threatened or endangered, are desert-adapted. These species evolved in sparsely vegetated habitats a nd rely on open ground to forage and avoid predation. Preliminary research indicates that populations of giant kangaroo rats (Dipodomys ingens), San J oaquin kangaroo rats (D. nitratoides), San Joaquin antelope squirrels (Ammo spermophilus nelsoni), and blunt-nosed leopard lizards (Gambelia sila), all listed as threatened or endangered, are affected negatively by thick herba ceous cover. This cover also may adversely affect several listed plant spec ies. Removing anthropogenic disturbances does not reduce or eliminate these exotic plants. Fire is effective in reducing herbaceous cover but kills na tive saltbush and often is costly to implement or control. Although livesto ck may have contributed originally to habitat destruction and introduction of exotic plants, we believe that in some years, moderate to heavy grazing by livestock is the best way to decrease the dense cover created by these e xotics. Recent decisions to decrease or eliminate livestock grazing on cons ervation lands without definitive studies of grazing in these habitats may lead to further declines of native species and possible local extinction of some listed plants and animals.