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The introduction and naturalization of alien species represents a serious threat to many natural protected areas. One such case of worldwide concern is the impact of feral livestock on arid ecosystems. Damage suffered by Echinopsis (=Trichocereus) terscheckii dominating the landscape of rocky slopes was surveyed in seven locations within the Ischigualasto-Talampaya World Heritage Site (Argentina) by measuring the frequency, position on the plant and extent of damage. At the same time we employed transects to estimate the abundance of autochtonous and feral large herbivores (Lama guanicoe, Bos taurus, Equus asinus) from their dung. Our results show relatively high damage levels (40-77% of individuals damaged, more than 5 dm(3) removed by plant in some sites), particularly within 0.50-1.75 m above the ground, showing herbivores to be the main responsible for them. We also found significant differences between sites in variables measuring damage level and in the intensity of use by the two feral livestock species but not by guanacos. The frequency of damaged cacti below 1.75 m (but not above) was significantly positively correlated among locations with the frequencies of cattle and donkey dung, and the damage suffered by individual cacti was also correlated with donkey and cattle dung in their surroundings after correcting for spatial effects. However, all correlations were non-significant in the case of guanacos. We conclude that the continued presence of feral livestock, particularly donkeys, leads to damages to columnar cacti with potential effects on their populations and the physiognomy of this protected landscape. (C) 2011 Elsevier Masson SAS. All rights reserved.