whether interactions between wildlife and livestock are competitive or facilitative is context dependent. intermediary factors that explain how context (seasonal or regional characteristics of the ecological community) affects these interactions are rarely reported. we compared activity time and density in vicuñas (vicugna vicugna) introduced into the chimborazo faunal production reserve (cfpr), ecuador, to describe how they interact with livestock. we compared vicuña density in wetlands and uplands (two landscape structures) with and without livestock (two conditions) using an isodar approach. we measured, over two seasons, vicuña forage abundance, composition, preference and accessibility, time vicuñas spent vigilant, and their flight distances on approach. we tested optimal foraging theory relating to the hypothesis that time mediates behavior, and found that vicuñas were no less frequently vigilant, nor were flight distances greater, during a wet season or in habitats of greater forage abundance and accessibility. we also found no evidence that vicuña behavior was density dependent; instead, we found that more time was spent vigilant by vicuñas when they foraged near livestock in rainy regions during the dry season. although forage abundance was similar throughout cfpr during a dry season, better forage quality in areas occupied by livestock may constitute an effect of their facilitating vicuñas. a puzzling finding, because it was not explained by any of the other variables we measured, was that at low densities vicuñas selected habitat irrespective of livestock, and where their density was higher, it was doubly so adjacent to livestock. we conclude that in the cfpr, spatial heterogeneity in habitat quality determines the interactions between livestock and vicuñas. to support recommendations that minimize competition between wildlife and livestock, and to expand on descriptions of the contexts that determine the direction of species interactions, future study may require a wider sampling of the densities of sympatric large herbivores in general, and, in the cfpr, a closer resolution of spatial heterogeneity in forage plant quality.