Pollination interactions reveal direct costs and indirect benefits of plant–plant facilitation for ecosystem engineers

Aims Ecosystem engineers substantially modify the environment via their impact on abiotic conditions and the biota, resulting in facilitation of associated species that would not otherwise grow. Yet, reciprocal effects are poorly understood as studies of plant–plant interactions usually estimate only benefits for associated species, while how another trophic level may mediate direct and indirect feedback effects for ecosystem engineers is hardly considered. Methods We ran a field experiment with two ecosystem engineers (Arenaria tetraquetra and Hormathophylla spinosa) blooming either alone or with associated plants to decompose net effects and to test the hypothesis that pollinator-mediated interactions provide benefits that balance costs of facilitation by ecosystem engineers. Important Findings We found that net costs of facilitation are accompanied by pollinator-mediated benefits. Despite ecosystem engineers producing fewer flowers per plant, they were visited by more and more diverse pollinators per flower when blooming with associated plants than when blooming alone. Although seed production per plant was higher when ecosystem engineers bloomed alone, fruit set and seed set varied between species. In one case (A. tetraquetra), fruit and seed sets were negatively affected by the presence of associated plants, whereas, in another case (H. spinosa), fruit set and seed set were higher and unaffected when ecosystem engineers bloomed with associated plants, respectively. Our findings suggest that besides experiencing direct costs, ecosystem engineers can also benefit from facilitating other species via increasing their own visibility to pollinators. Thus, we highlight that pollination interactions can compensate for costs of facilitation depending on ecosystem engineer species. This study illuminates how the outcome of direct plant–plant interactions might be mediated by indirect interactions including third players.