Asymmetrical Interactions between Ungulates and Phytophagous Insects: Being Different Matters

We conducted a 3-yr field experiment that manipulated the presence of ungulates (domestic sheep and Spanish ibex Capra pyrenaica, Bovidae) and the monophagbus beetle Timarcha lugens (Chrysomelidae) to evaluate (1) the effects on beetle abundance and interaction with the woody crucifer Hormathophylla spinosa; (2) the reciprocal effect of this insect on the interaction between ungulates and the plant; (3) their impact on H. spinosa reproductive output; and (4) the abundance of an associated taxon, Ceutorhynchus sp. nov. (Curculionidae). Our experiment showed that ungulate exclusion increased Timarcha abundance, whereas Timarcha had no effect on ungulates. H. spinosa reproductive output increased threefold promptly when ungulates were excluded, although in the third year Timarcha began to have effects on shrub reproduction. Ungulates affected weevils both by exploitative competition and by incidental predation, with the total number of weevils emerging per shrub being more than fourfold higher in shrubs excluded from these mammals than in shrubs open to them. In addition, during the last year, the abundance of weevils was also affected by Timarcha removal. Our study demonstrated that phytophagous insects can compete with other herbivores highly dissimilar in size, ecology, and taxonomy, such as ungulates. These interactions usually produce asymmetrical results, sometimes closer to amensalism or predation than to pure competition.