Primary productivity and animal local density covary in a partially migratory wild guanaco population

Ungulates have physiological and behavioral mechanisms to respond to seasonal variation and resource availability to meet their nutritional needs. To understand the effects of primary productivity in long-term studies, vegetation indices (e.g., enhanced vegetation index, EVI) can be used as indirect indicators of the nitrogen available to herbivores in arid and mountain environments. In this research, we explore the relationship between the variation in guanaco's local density (Lama guanicoe) and the EVI between 2008 and 2017 in a wild partially migratory population of La Payunia Reserve (Mendoza, Argentina) using a weighted linear regression. Our results indicated a significant relation between guanaco local density and EVI (R² = 0.68; p-value = 0.003), suggesting that the interannual variation in guanaco local density can be explained, at least in part, by higher aerial net primary productivity. Possibly, when nutrient availability is high, the local environment can support a higher local animal density than in years in which primary productivity is lower. When nutrients are limited, some individuals may use other areas not considered in this study. Even preliminary, these results bring new insights into the ecophysiology of this extremely plastic ungulate and contribute to understanding its energetic strategy.