Grass-to-grass protection from grazing in a semi-arid steppe. Facilitation, competition, and mass effect

MARTÍN OESTERHELD; MARIANO OYARZABAL

OIKOS

WILEY-BLACKWELL PUBLISHING, INC

Año: 2004 p. 576 - 582

0030-1299

Plants of low palatability often serve as biotic refuges from grazing to palatable plants.Evidence for this facilitation comes from cases where the interacting species havedifferent life form, which may minimize competition. Protected plants act as remnantseed sources that may maintain the palatable populations locally viable through masseffects. Here, we assess (1) the spatial association between a highly palatablePatagonian grass (Bromus pictus ) and less preferred tussock grasses, (2) the roleplayed by seed sources in maintaining the population in the face of heavy grazing bysheep, and (3) the facilitative and competitive components of the interaction. Wequantified B. pictus density and its distance to nearest tussocks inside and outside agrazing exclosure. We also considered different distances from the exclosure, bothleeward and windward, because strong westerly winds may be critical for dispersal.Additionally, we quantified several attributes of protected and unprotected B. pictusplants with and without grazing. Density of B. pictus was about 20 times greater insidethe exclosure than outside. However, this difference was less pronounced in the leewardvicinity of the exclosure than in the windward one, which suggests a mass effect. B.pictus was significantly associated to less palatable tussocks, and the associationbecame stronger under grazing and as distance from the exclosure edge increased.Protection under grazing was associated with a significant increase in plant biomass,height, tiller number, and panicle number, whereas protection in the absence of grazing,which could evidence competition, resulted in reductions of tiller number and paniclenumber, and an increase of height. These results suggest that in areas under grazingpressure on palatable grasses, other less palatable grasses may provide a protectionfrom grazing that outweighs competitive effects. Such protection may generate smallscalemass effects that maintain the population at relatively high density.