Facilitation, competition, and net effects in grass-to-grass interactions: The effect of grazing in Patagonian semiarid steppes

GRAFF, PAMELA; AGUIAR, MARTIN R.; CHANETON, E.J.

Montreal

Congreso; 90th Annual Meeting of the Ecological Society of America (ESA) en conjunto con la INTECOL IX 05 Congress; 2005

Competition and facilitation among plant species often operate simultaneously, and their effects are predicted to change along gradients of consumer pressure and abiotic stress. We tested in a two year field experiment, the hypothesis that grazing modifies the balance between positive and negative interactions in a manner consistent with the prediction of Bertness and Callaway (1994), i.e. the importance of positive vs. negative effects increases as consumer pressure increases. The objectives were (1) to assess the relative importance of competition and facilitation in absence of grazing and (2) to evaluate their dynamics, and the net effect, under increasing grazing intensity. We transplanted plants of two palatable grasses (Poa ligularis and Bromus pictus) far and close from plants of a less palatable grass (Stipa speciosa ). We manipulated belowground competition to reduce its intensity. This setup was repeated in three grazing intensities: exclosure, moderate and heavily grazed fields. Our results show a shift in the net balance of the interaction between palatable and unpalatable species, from clearly negative in absence of herbivores, to positive under increasing grazing conditions. In absence of sheep grazing, belowground competition was the most intense interaction, with significantly overall negative effects on palatable total plant biomass, especially on root biomass. Under grazing, positive interactions outweighed negative competition. The closest vicinity to S. speciosa increased total palatable plant biomass, regardless of the competition treatment, and harbored palatable transplants by decreasing the herbivore impacts on them: Plants growing near S. speciosa had significantly less herbivore bites than those growing far from the unpalatable tussock. Our results demonstrate that selective grazing can drastically change the interaction balance between palatable and unpalatable species, decreasing competitive effects and increasing the intensity of positive interactions associated with anti-herbivore protection.