IPEEC - CENPAT   25619
INSTITUTO PATAGONICO PARA EL ESTUDIO DE LOS ECOSISTEMAS CONTINENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Response of vegetation to the increase in guanaco density after sheep removal in north-eastern Patagonian rangelands, Argentina
Autor/es:
RODRÍGUEZ VICTORIA; LARREGUY CECILIA; MARINO ANDREA; PAZOS GUSTAVO
Lugar:
Saskatoon
Reunión:
Congreso; International Rangeland Congress; 2016
Institución organizadora:
International Rangeland Congress
Resumen:
IntroductionIn arid Patagonia, unsustainable practices in sheep production have typically led to vegetation and land degradation. Guanaco (Lama guanicoe) is the dominant native herbivore inhabiting these ecosystems. Lastly, ranchers and some government agents perceive this herbivore as a threat to livestock production and alerted on a hypothetical risk of vegetation deterioration by guanaco overgrazing because of the growth of some of their populations. In this work, we annually monitored during seven years the cover and composition of four plant communities in a wildlife reserve that was previously a typical sheep ranch. After sheep removal at the time of the reserve creation, a rapid increase of guanaco density was registered (Marino et al., 2015).Materials and Methods This study was conducted in the wildlife reserve San Pablo de Valdés (SPV), located in Península Valdés (PV) (42°36? S; 64°15? W), Argentina. SPV was created to protect representative environments of PV and their biotic interactions. Before sheep removal in 2005, SPV bred 51 sheep.km-2 and showed signs of overgrazing (Codesido et al., 2005). Guanaco density within SPV rose from 3.95 (SE 1.05) in 2006 to 26.3 (SE 6.82) guanacos.km-2 in 2012, and then stabilized around 26.7 during the 2012-2016 period. This last value results in a stocking rate similar to that of sheep before the reserve creation (assuming an equivalence of 1 guanaco = 2 sheep). Thus, under the hypothesis of guanaco overgrazing, this sudden rising of guanaco density would have led to degradation of plant communities in SPV. We annually monitored four plant communities at fixed sampling sites since 2009: two shrub-grass (VC1 and VC2) and two perennial-grass dominated steppes (VC3 and VC4). VC1 and VC2 showed lower guanaco densities than VC3 and VC4. Moreover since 2013, VC3 showed a male-group proportion tenfold higher than the other communities (Marino et al., 2015). Total canopy, bare soil and perennial-grass cover were annually assessed in two 50-m linear transects at each sampling site using the point-quadrat method (250 points per transect). We drew control charts (Manly, 2009) for each variable at each plant community and calculated the 95% and 99% confidence intervals to identify significant differences from the mean for the entire period. We used Principal Component Analysis (PCA) in order to unveil correlations among cover variables, annual precipitation and guanaco density (estimated through the group encounter rate). Results and DiscussionIn VC2, VC3 and VC4 all variables showed temporal oscillations with significant both negative and positive deviations from the mean (at both significant levels evaluated), while VC1 did not show significant oscillations (Fig. 1a). Canopy and perennial-grass cover showed similar temporal trends in all VC (only canopy shown). VC3 was the only one showing clear temporal trends, but only for the last two years: negative for total canopy and perennial-grass cover and positive for bare soil (Fig. 1a).In VC1, VC2 and VC4, annual precipitation correlated positively to plant covers (canopy and grass) and negatively to bare soil cover, while guanaco density was unrelated to them (Fig. 1b). Thus, for these three plant communities we did not find evidence of vegetation deterioration due to guanaco overgrazing. However, in VC3 guanaco density correlated positively to bare soil and negatively to both plant cover variables (Fig. 1b). We argue that this may be due to more intense use of forage resources by the unterritorial male groups established in this plant community since 2013 (beginning of vegetation changes). Territoriality has been proposed as a mechanism for keeping guanaco population density under environmental-carrying capacity (Marino et al., 2015). Figure 1: (a) Mean values (symbols joined by solid lines) (n=2) for total canopy and bare soil at each plant community. Dotted and dashed lines indicate 95% and 99% confidence intervals, respectively. Values falling outside them indicate significant deviations from the mean. (b) PCA for each plant community. VC1 and VC2: shrub-grass steppes; VC3 and VC4: perennial-grass dominated steppes.Conclusions and implicationsPlant communities supporting territorial family groups of guanacos did not show signs of overgrazing. Unterritorial male groups deserves further attention because the observed negative effect on plant cover could be counterbalanced by high potential for medium-term displacement at landscape scale.