Differential ant exclusion from canopies shows contrasting top-down effects on community structure.

Predators have far-reaching effects on communities by triggering top-down trophic cascades that influence ecosystem functioning. Omnivory and intraguild interactions between predators give rise to reticulate food webs and may either strengthen or dampen trophic cascades depending on context. Disentangling the effects of multiple predator species is therefore crucial for predicting the influence of predators on community structure. We focused on ants as dominant generalist predators in arthropod communities and set up a differential ant exclusion from canopies to examine its effects on assemblage species composition and densities of five arthropod groups (psocopterans, aphids, spiders, heteropterans and beetles). We coupled a glue band with tubes allowing only the ant Lasius grandis to reach the canopies to isolate its effect from the rest of crawling predators (ants, earwigs) and compared it against a full exclusion and a control. L. grandis alone had widespread effects on assemblage species composition, with contrasting species-specific responses within groups, where some species affected by L. grandis presence were not further affected by the presence of the whole crawling predator assemblage, and vice versa. Overall, L. grandis caused two- to threefold decreases of generalist predators and a threefold increase of aphids. However, it lacked further top-down effects on primary consumers, which only emerged when all crawling predators were present. This differential exclusion demonstrates the distinctive and widespread intraguild effects on community structure of a single ant species that contrast with the top-down effects exerted by the whole crawling predator assemblage.