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Domestic and wild native herbivores combined are still overgrazing Patagonia rangelands: A response to Marino et al. (2019)

Citation

Oliva, Gabriel Esteban et al. (2020), Domestic and wild native herbivores combined are still overgrazing Patagonia rangelands: A response to Marino et al. (2019), Dryad, Dataset, https://doi.org/10.5061/dryad.fxpnvx0nw

Abstract

1.    Oliva et al. (2019) based upon primary productivity estimates concluded that, after long periods of overgrazing, Patagonia´s domestic stocks adjusted to regional-scale herbivore carrying capacity at the end of last century. Guanaco populations, a native camelid, increased thereafter driving combined grazing pressures once again over carrying capacity in some areas.

2.    Marino, Rodriguez and Schroeder (2019) argued that domestic grazing is not really at equilibrium because domestic stocks are concentrated in areas that remain overgrazed. They support the idea that guanacos auto-regulate their density by resource-defence territoriality and are weak competitors with domestic herbivores, occupying 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 over and under-grazing at smaller (local) scales. By separating areas with and without domestic stocks Marino et al. (2019) estimated 28% and 73% overgrazing in the provinces of Chubut and Santa Cruz, respectively. We recalculated these estimates and found overgrazing of 28% and 47% for Chubut and Santa Cruz, respectively. But when combined with guanaco densities overgrazing increases to 48 and 108% for Chubut and Santa Cruz, respectively.

4.    We question the hypothesised lack of competitive value and effective self-regulating mechanisms in guanacos. A data set of 13 sheep farms show densities of 12-61 (mean 27) guanacos.km-2 with a combined grazing pressure above carrying capacity. Populations in a protected area in Chubut reached 42 guanacos.km-2, crashed during drought with 60% mortality, and increased thereafter to 70 guanacos.km-2, but even at peak numbers recruitment population rates remained extremely high.  

5.    Synthesis and applications.  Marino et al 2019 are right to question the apparent equilibrium of domestic stocks with carrying capacity, as they are concentrated in part of the territory that may be still overgrazed. But ground assessments show that guanaco populations can reach densities well over carrying capacity with or without sheep. This only stresses our conclusion that joint management of the native-domestic herbivore system is urgently needed. Farm management plans may transform an apparent competitor into a valuable natural complementary resource to sheep raising

Methods

This data set of Guanaco (Lama guanicoe) presents  the results of 323 km of ground surveys performed in southern Patagonia. They were subdivided in  361 transects from 0.8 to 10 km in length designed to evaluate guanaco density in 13 farms with a total area of  4,740 km2. Guanaco groups were spotted along the transects using Bushnell 10x25 1000 laser telemeter. Data is presented in an excel .xls file that includes the variables: Farm (name); Farm area (km2); Date of survey; Transect n°; Length of transect (km); Latitude and  Longitude in degrees, minutes, seconds (Datum WGS84); Latitude (X) and Longitude (Y) in Transverse Mercator coordinates (Central meridian -69, False northing 10001965.73 m) of the vehicle position, Azimuth of the road (angle of deviation of the road direction in degrees, where north=0); Angle (deviation of the guanaco group in relation to the main road direction in degrees); Distance to the guanaco group (measured by laser distanciometer, meters), Number of guanacos, Category (Adult, Juvenile or Undetermined). Each row represents a guanaco group spotted. Transects that did not spot any guanacos are presented without coordinates and show 0 guanacos. This data was processed using Distance Sampling 7.0 (Buckland et al. 2005) software for density analysis. It was stratified by date using global density with a Half-normal detectability function and cosine expansion series in order to produce density estimations included in the Table 2 of the paper. The .klm file includes cadastral outlines of these farms, tracks of guanaco survey transects and position of the guanaco groups.  

Usage Notes

There are missing values in the transect names in the Los Machos farm rows 1174-1250

Funding

Instituto Nacional de Tecnología Agropecuaria, Award: 2019-PD-E2-I038-002