摘要：In southwestern Chubut (Patagonia, Argentina), shrubs provide safe sites for the recruitment of almost all the species of the community. As different shrubs species are subjected to different sheep grazing pressures, the knowledge of their response mechanisms to grazing is crucial to the designing of sustainable systems of grazing management for these ecosystems. This work tested two hypotheses related to the adaptive and plastic responses to grazing for the three dominant shrubs of this community (Adesmia campestris, Mulinum spinosum and Senecio filaginoides). We studied the chemical composition of leaves of these shrubs under two contrasting sheep grazing pressures. We carried out a chemical screening to sequentially extract three groups of different polarity carbon based secondary metabolites (CBSM): oil, phenols and hydrocarbon compounds. We found that (1) the less preferred species (Senecio filaginoides) contained higher levels of the three fractions of CBSM than the other two species (adaptive response), and (2) only in Senecio filaginoides the CBSM content, in particular those of oil and hydrocarbon fractions, increased with grazing. The absence of plastic responses in the other two species could be related to the presence of structural defenses (thorns)
