Species-specific disturbance tolerance, competition and positive interactions along an anthropogenic disturbance gradient

Question As it has been found that stress promotes positive interactions mediated by physical amelioration of the environment, is it possible that interactions may turn positive with increasing chronic anthropogenic disturbance (CAD) intensity? Also, is it possible that species that do not tolerate disturbance may require environmental amelioration by their neighbours in disturbed areas, whereas tolerant species may not? Location The semi-arid grassland in Concepcion Buenavista, Oaxaca, southern Mexico. Methods We assessed interaction intensity and importance through a neighbour removal experiment along a CAD gradient for three species differing in disturbance tolerance. Water potential was monitored on vegetated and bare soil. Results A shift from competitive effects in low CAD sites to positive interactions in degraded sites was found. The disturbance-tolerant species did not respond to CAD, whereas the less tolerant species changed its interactions drastically in terms of growth and reproduction. The species with medium tolerance had an intermediate response. Neighbours promoted germination in all species. Vegetation removal reduced soil humidity. Conclusions Positive interactions seemingly resulted from the amelioration of the abiotic stresses induced by vegetation removal. The dependence on neighbours to germinate, grow, or reproduce suggests that if CAD eliminates the plant cover, vegetation will hardly recover. Irreversible changes are known to occur in communities where positive interactions predominate, but CAD may set the conditions for irreversible shifts even in communities where interactions are normally competitive.