Patterns of plant species diversity following local extinction of the plains vizcacha in semi-arid scrub

We examined changes in the diversity of herbaceous plant species at colony sites of the plains vizcacha (Lagostomus maximus) following local extinctions of this species in semi-arid scrub of central Argentina. These large rodents live in social groups, share a communal burrow system, and forage in a common home range around the burrow system. Through intense herbivory, vizcachas create large open patches in the understory of a landscape dominated by dense bunch grass and shrubs. In heavily grazed areas at the centers of vizcacha colonies, species richness of forbs was significantly higher than species richness of grasses. In lightly grazed areas at colony edges, both plant groups were equally common. Following local extinction of vizcachas, the number of forb species declined on colony sites as the cover of grasses increased, and within a year grass species were more numerous than forbs at the edges of colonies. Species richness of grasses increased slowly in the areas that had been heavily grazed by vizcachas. Five years after the population crash of vizcachas, species richness of grasses in centers of extinct colonies was less than 50% of the species richness in areas that had been lightly grazed at colony edges. In contrast, grass cover in areas that had been heavily grazed increased to levels similar to the ungrazed matrix within 2-3 years after extinction. Thus, the major transition in physical structure of the plant community occurred much more rapidly than the recovery of plant species richness. (C) 1999 Academic Press.