Dynamics of plant community structure in a semiarid Patagonian natural reserve: relationships with guanaco grazing and climate.

Disturbance by native herbivores is generally low due to co-evolution with vegetation. However, it is unclear whether overgrazing can occur at high population densities. In a wildlife reserve of northeastern Patagonia, we predicted that guanaco overgrazing would be evidenced by: 1) guanaco density over the environmental carrying capacity; 2) decrease in the cover of total canopy, perennial grasses, plant bases, biological soil crusts, and litter, and increase in the proportion of bare soil and desert pavement, and 3) temporal trends of plant and soil variables will be negatively associated with guanaco density, while remaining little affected by climatic variation. We annually assessed guanaco density and plant and soil indicators during 15 years in five plant communities. Guanaco density increased during the early years of the study, and then oscillated below or within the estimated carrying capacity range at densities among the highest reported for sedentary guanacos. Meanwhile, the cover of plant bases and biological soil crusts increased over time at most plant communities, while perennial grasses and litter displayed irregular trends. Finally, seasonal water availability significantly explained interannual changes in variables of vegetation structure, while the unique contribution of guanaco density was not significant. Besides, we found a negative relationship between guanaco density and grass cover only in very dry years. Thus, these findings were not consistent with the hypothesized overgrazing scenario, indicating that overgrazing by sedentary guanacos under regular dispersal conditions is unlikely even at high population densities.