Are Patagonia grasslands being overgrazed? A response to Marino et al. (2020)

摘要

1. Based upon primary productivity estimates, Oliva et al. (2019) concluded that, at the end of last century and after long periods of overgrazing, Patagonia's domestic stocks adjusted to regional-scale herbivore carrying capacity. Populations of guanaco, a native camelid, increased thereafter, driving combined grazing pressures once again over carrying capacity in some areas. 2. Marino et al. (2020) argued that grazing is not really at equilibrium because domestic stocks are concentrated in areas that remain overgrazed. They support the ideas that guanaco density is auto-regulated by resource-defence territoriality, and that guanacos are weak competitors with domestic stock, occupying only marginal areas. In their view, Oliva et al. (2019) put guanacos in the role of scapegoats, leaving domestic stocks unchecked. 3. Equilibrium at regional scale does not preclude overgrazing and under-grazing at local scales. By separating areas with and without domestic stocks, Marino et al. (2020) estimated overgrazing at 28% in Chubut Province and 73% in Santa Cruz Province. Our recalculations show 28% and 47% domestic overgrazing, respectively. However, when combined with guanaco densities, these increase to 48% for Chubut and 108% for Santa Cruz. 4. We question the hypothesised lack of competitive value and efficient selfregulating mechanisms that would prevent guanaco populations from overshooting carrying capacity. A dataset of 13 sheep farms showed mean density of 26 ± 3.8 guanacos/km~2 and high combined grazing pressures. This was also observed in a protected area of Chubut that reached 42 guanacos/km~2 and crashed during drought, with 60% mortality. Thereafter, guanacos increased to 70 guanacos/ km~2, with recruitment rates that showed a complex response of density dependence but remained relatively elevated at densities above the estimated carrying capacity. 5. Synthesis and applications. Marino et al. (2020) are right to question the apparent equilibrium of domestic st