Structural anti-herbivore defense reduction of two Patagonian spiny shrubs in response to long time exclusion of large herbivores

•M. spinosum has higher spinescence than A. volckmannii.•M. spinosum and A. volckmannii showed an increase of spinescence induced by herbivory.•These responses may be the result from a spatial rearrangement of thorns.•These responses may be evidences of co-evolution between shrubs and large browsers.

This work focuses on the structural defenses of the two dominant spiny shrub species of the Patagonian shrub-grass steppe. We compared the amount of structural defenses (spines and thorns) of Mulinum spinosum (Cav.) Pers. (Apiaceae) and Adesmia volckmannii Phil. (Fabaceae) between grazed and long term non-grazed plots. M. spinosum showed a higher spinescence than A. volckmannii. Moreover, both species plants located in grazed plots showed higher spinescence than those located in non-grazed ones. Therefore, we can conclude that (1) M. spinosum has higher spinescence than A. volckmannii, probably because sheep prefer M. spinosum flowers than A. volckmannii leaves; and (2) both M. spinosum and A. volckmannii showed an increase of spinescence induced by herbivory. These responses may be evidences of co-evolution between these shrub species and large wild browsers that guarantee an adequate capacity to tolerate herbivory disturbance. Such capacity would be important to maintain community functioning, because shrubs provide secure sites for seed germination and, in some cases, are able to set symbiotic interactions with N-fixing bacteria.