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

Abstract

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 stocks that are concentrated in areas that may be still overgrazed. But ground data show that guanaco populations have inefficient density population regulation and can reach densities well over carrying capacity, even in the presence of sheep. This does not mean that the main control should be on growing guanaco populations but it stresses our conclusion that of the Provinces of Chubut and Santa Cruz a guanaco population value as estimated by Bay Gavuzzo et al. (2015; Table 2); as these authors did not provide estimates for the Dry and Humid joint management of the native-domestic herbivore system is urgently needed. Joint management can be effected through local plans, as current guanaco management permits can only be issued in areas that are not overgrazed by sheep. Farm management plans may in this way transform an apparent competitor into a valuable resource, complementary to sheep raising.

Competitive exclusion and herbivore management in a context of livestock‐wildlife conflict

“…If sheep competitively exclude guanacos from preferred zones pushing those to unused areas, it is unlikely that guanaco culling will be the solution for poor sheep condition. Paradoxically, the increase in joint carrying capacity derived from the spatial and trophic niche partitioning between guanacos and sheep, and its potential productive benefits, which is consistent with the 'included niche' idea previously mentioned, has been recognized explicitly in the same studies that claim that guanaco numbers should be reduced (Oliva et al 2019;Oliva et al 2020). A similar benefit was pointed out by Andrew and Lange (1986) who stated that, as sheep are confined by their physiology to graze within a certain distance of water, those parts of the paddock at the limits of and beyond their grazing range are little or not at all used by them, kangaroos (Macropus rufus and M. fuliginosus) are attracted to this undergrazed areas and may complement sheep by utilizing this otherwise unexploited resource.…”

“…2019; Oliva et al . 2020). A similar benefit was pointed out by Andrew and Lange (1986) who stated that, as sheep are confined by their physiology to graze within a certain distance of water, those parts of the paddock at the limits of and beyond their grazing range are little or not at all used by them, kangaroos ( Macropus rufus and M. fuliginosus ) are attracted to this undergrazed areas and may complement sheep by utilizing this otherwise unexploited resource.…”

“…2006; Oliva et al . 2020), disregarding the potential effects of spatial segregation at intra‐ranch scale in terms of grazing impact and/or carrying capacity estimates.…”

“…Several studies have stressed the high potential for competition between guanacos and sheep because of partial diet overlap and body size similarities (Raedeke 1979;Puig et al 2001;Baldi et al 2004) but the nature of their interactions is still unclear and no study has addressed them directly at a spatial scale coherent with the operational units subject to management decisions. Although there has been a long debate over the empirical evidence of guanacos' role on production losses, or the lack of it, current management paradigm and culling initiatives rely on the assumption of both guanacos and livestock having equal access to total forage under a symmetrical competition process (Oliva et al 2006;Oliva et al 2020), disregarding the potential effects of spatial segregation at intra-ranch scale in terms of grazing impact and/or carrying capacity estimates.…”

Resource and environmental situation and countermeasures for high-quality development of Sanmen Bay, China

“…Extensive existing research on Sanmen Bay focuses on its structural density, and biodiversity of macrobenthos and fishery resources, as well as their temporal and spatial distribution pattern [1][2][3]. Some studies focus on its current environmental quality and interannual variation, as well as its relationship with macrobenthic communities [4][5]. Others have studied the response to changes in water nutrients, wetland area and vegetation form to human activities by means of ocean remote sensing and geographic information systems [6][7][8][9].…”