SMALL-MAMMAL REGULATION OF VEGETATION STRUCTURE IN A TEMPERATE SAVANNA

Explanations for documented increases in woody plant dominance in gras slands and savannas of North America include atmospheric CO2 enrichmen t and changes in climate, livestock grazing, and fire regimes. However , tree/shrub encroachment has also coincided with the eradication of a once widespread native herbivore, the black-tailed prairie dog (Cynom ys ludovicianus). We used field experiments and repeat aerial photogra phy to demonstrate that prairie dogs, and the herbivores and granivore s associated with their colonies, probably maintained grassland and sa vanna by preventing woody species such as Prosopis glandulosa (honey m esquite) from establishing or attaining dominance. Prosopis seed and p od disappearance was 3-99 times greater within prairie dog colonies. A nts were the primary agent of seed removal, whereas prairie dogs and a ssociated vertebrates were the primary agents of pod removal. Survival of Prosopis seedlings protected from vertebrate herbivory was similar on and off prairie dog colonies (approximate to 60%), whereas surviva l of unprotected seedlings was 3 times greater off- than on-colony. On -colony, prairie dogs and associated herbivores girdled and destroyed all Prosopis saplings within 2 d of planting; survival of 1-yr-old see dlings was reduced by 50% after 3 mo of exposure to on-colony herbivor es. Despite high levels of woody plant seed disappearance and seedling herbivory, on-colony ''seedling'' reserves were substantial (950 plan ts/ha). Thus, prairie dogs and the fauna that occur on their colonies suppressed rather than eliminated Prosopis from the colony site. Remov al of prairie dogs led to rapid development of Prosopis stands. Repeat aerial photography showed that Prosopis canopy cover on a colony erad icated in 1950 (27%) increased to a level (61%) comparable to that of off-colony Prosopis stands (65%) within 23 yr. These data illustrate h ow transitions from grassland to woodland vegetation can be mediated b y a rodent herbivore. They further demonstrate how purposeful or inadv ertent removal of native herbivores can have unforeseen effects on pla nt species composition and landscape physiognomy. Investigations of en vironmental constraints on vegetation distribution and abundance shoul d take into account the historical role of herbivores in shaping the p resent system. Inconsistencies among historic accounts of woody plant distribution and abundance in semiarid western North America may be re solved by considering population dynamics of prairie dogs. Widespread eradication of this formerly abundant rodent has eliminated a signific ant constraint to woody plant establishment on many semiarid grassland and savanna landscapes and has thereby facilitated transitions to shr ubland and woodland states. Past land management designed to remove on e perceived impediment to livestock production appears to have contrib uted significantly to development of another management problem that i s now a major detriment to sustainable livestock production.