Climate modulates grazing effects on primary productivity of arid ecosystems in Argentina

La introducción de fuentes de agua artificiales para uso ganadero en ecosistemas donde el agua ha estado ausente durante su evolución ha provocado cambios en la vegetación. Se ha demostrado que existe una atenuación radial del impacto del pastoreo con la distancia a la fuente de agua, favoreciendo la formación de piósferas. En Argentina, no existen estudios que evalúen cómo actúa el clima como modulador de los efectos del pastoreo en piósferas a lo largo de gradientes ambientales. El objetivo de este estudio fue evaluar y analizar el rol que juega el clima como modulador de los efectos que el pastoreo induce a escala local. Esto permitiría comprender como actúa el clima sobre los efectos del pastoreo. Para ello se seleccionaron 77 piósferas a lo largo de un gradiente climático y se analizó la variabilidad espacial de los valores de NDVI (Índice de Vegetación de la Diferencia Normalizada) en el gradiente de pastoreo y su relación con la precipitación media anual y la temperatura media anual. Se observaron dos patrones diferentes de respuesta del NDVI. El patrón 1 presentó un aumento del NDVI con la distancia al punto de agua, mientras que el patrón 2 manifestó un comportamiento inverso. Las piósferas de los patrones 1 y 2 se ubicaron por encima y por debajo de los 280 mm de precipitación anual y de los 14.5 °C de temperatura media anual, respectivamente. Estos resultados sugieren que existe un comportamiento diferencial de la vegetación frente al pastoreo que estaría modulado por variables climáticas.

Combined Effects of Warming and Grazing on Rangeland Vegetation on the Qinghai-Tibet Plateau

Climate warming has increased grassland productivity on the Qinghai-Tibet Plateau, while intensified grazing has brought increasing direct negative effects. To understand the effects of climate change and make sustainable management decisions, it is crucial to identify the combined effects. Here, we separate the grazing effects with a climate-driven probability model and elaborate scenario comparison, using the Normalized Difference Vegetation Index (NDVI) of the grassland on the Qinghai-Tibet Plateau. We show that grazing has positive effects on NDVI in the beginning and end of the growing season, and negative effects in the middle. Because of the positive effects, studies tend to underestimate and even ignore the grazing pressure under a warming climate. Moreover, the seasonality of grazing effects changes the NDVI-biomass relationship, influencing the assessment of climate change impacts. Therefore, the seasonality of grazing effects should be an important determinant in the response of grassland to warming in sustainability analysis.

Grazing-induced biodiversity loss impairs grassland ecosystem stability at multiple scales

Livestock grazing is a major driver shaping the functioning and stability of grasslands. Although previous studies have documented the effect of grazing on grassland stability, whether this effect is scale-dependent remains unclear. Here, we conducted a sheep-grazing experiment in a temperate grassland to test grazing effects on biomass stability across scales and organizational levels. We found that an increase of grazing intensity increased species stability, but it substantially decreased local ecosystem stability due to reduced asynchronous dynamics among species. Moreover, grazing reduced ecosystem stability at larger spatial scales, but to a lesser extent. By decreasing biodiversity within and across communities, grazing impairs the insurance effects of biodiversity and hence the up-scaling of stability from species to ecosystem and further to larger scales. Our study provides the first evidence for the context-dependence of grazing effects on grassland stability via shaping biodiversity and contributes to bridging fine-scale experiments and broad-scale ecosystem management.

Grazing effects of sea urchin Diadema savignyi on algal abundance and coral recruitment processes

Herbivores control algae and promote coral dominance along coral reefs. However, the majority of previous studies have focused on herbivorous fish. Here we investigated grazing effects of the sea urchin Diadema savignyi on algal abundance and coral recruitment processes. We conducted an in situ cage experiment with three density conditions of D. savignyi (0, 8, 16 indiv. m−2) for three months during the main coral recruitment season in Taiwan. Results demonstrated a strong algal control by D. savignyi. At the end of the experiment, average algal cover was 95% for 0 indiv. m−2, compared to 47% for 8 indiv. m−2 and 16% for 16 indiv. m−2. Average algal biomass at 8 indiv. m−2 declined by one third compared to 0 indiv. m−2 and almost zero at 16 indiv. m−2. On the other hand, a negative grazing effect of D. savignyi was observed on coral recruitment processes. Notably, at 16 indiv. m−2, the density of coral recruits declined and mortality of small coral fragments (proxy of coral juveniles) increased. Our results confirm findings of previous studies and indicate the need to balance both positive (strong algal control) and negative (physical damage) influences of Diadema grazing to facilitate the coral recruitment process.