Remotely sensed primary productivity shows that domestic and native herbivores combined are overgrazing Patagonia.
Abstract
Carrying capacity is the maximum animal density an area can sustain without deterioration of its resources. Overgrazing has degraded Patagonia, but sheep stocks decreased and gave way to mixed systems with cattle, goats and guanacos (native wild camelids). The objective of this paper was to develop a method to estimate the carrying capacity based on the remotely sensed data, and to assess wild and domestic herbivore numbers in order to establish if grazing stocks have evolved to balance with carrying capacity. Net Primary Productivity (NPP) MOD17/A3 images and field Aerial Net Primary Productivity (ANPP) data of 66 sites were linearly regressed (R2=0.83, p < 0.01), and the slope 0.236 used to convert MOD17/A3 NPP to ANPP. Harvest index (proportion of ANPP that may be sustainably consumed) was estimated as a function of ANPP and carrying capacity as a consumable forage/estimated annual consumption, set at 500 (sheep and goats), 3,200 (cattle) and 750 kg Dry Matter head-1 year-1 (guanacos). Regional ANPP ± SD (2000-2015) was 758 ± 52 kg Dry Matter ha-1 year-1 and Harvest index was 13.7 ± 0.6%. Regional carrying capacity was 14.8 ± 1.6 M sheep or goats, 2.3 ± 0.3 M cattle or 9.9 ± 1.2 M guanacos. Domestic stock was high from 1920 to 1980, but declined thereafter and remained mostly within 1 SD of mean 2000-2015 carrying capacity. In this century, annual provincial stocks and carrying capacity correlated well (R2=0.94, p < 0.01) with a slope close to 1. Guanacos increased from 0.5 to 2 M between 2000 and 2015, driving linearly combined grazing pressures 36% and 62% above carrying capacity in southern Patagonia provinces in 2015. Synthesis and applications. From the year 2000, after decades of sheep overstocking, domestic stock has shown a regional trend towards a grazing equilibrium, but growth of guanacos might have upset that trend. Participation of guanacos as a critical excess in total grazing pressure is debatable, but management of these populations is necessary and may be increasingly attractive if combined production systems are developed to incorporate wild meat and fibre. Our method, MOD17/A3 enables a carrying capacity evaluation and stock adjustment in these unique mixed grazing systems, preventing further rangeland degradation and loss of ecosystem services.