Herbivory, hunting, and long-term vegetation change in degraded savanna

Large ungulate populations are associated with the degradation of many forest plant communities, but it is unclear if these population sizes are strictly a contemporary phenomenon. Human exploitation models predict they are not, with ungulate numbers varying with long-term fluctuations in hunting pressure. Alternatively, human disturbance models predict that abiotic limitations normally restrict herbivores, with contemporary increases reflecting increased productivity associated with agriculture and forestry. Both can explain ungulate abundance, but may have different implications for plant conservation because they predict different levels of prior evolutionary exposure to herbivory. Here, I review historical records and stand structure studies from degraded oak savanna of western North America to examine whether current ungulate levels are strictly a contemporary phenomenon. Although it was impossible to quantify pre-European herd sizes, all evidence indicates a strong relationship between hunting pressure and ungulate abundance. Historical accounts repeatedly describe large herds of deer and elk at first European contact, followed by sharp declines immediately after colonization, and then rapid recovery beginning in the early 1900s as subsistence hunting waned. Stand structure data for oak woodland appear to support this model. Present-day oak woodlands mostly derive from mass recruitment from 1850 to 1910, coinciding with the near elimination of ungulates by hunting. Although these results suggest that large ungulate herds are not strictly a contemporary phenomenon, browsing intensity appears to be unprecedented given limited hunting, predator extirpation, and savanna fragmentation within productive pasture and early successional forest. Hunting pressure thus continues to be important, in that it is now largely absent.