Compensation masks trophic cascades in complex food webs

Ecological networks, or food webs, describe the feeding relationships between interacting species within an ecosystem. Understanding how the complexity of these networks influences their response to changing top-down control is a central challenge in ecology. Here, we provide a model-based investigation of trophic cascades ‒ an oft-studied ecological phenomenon that occurs when changes in the biomass of top predators indirectly effect changes in the biomass of primary producers ‒ in complex food webs that are representative of the structure of real ecosystems. Our results reveal that strong cascades occur primarily in low richness and weakly connected food webs, a result in agreement with some prior predictions. The primary mechanism underlying weak or absent cascades was a strong compensatory response; in most webs predators induced large population level cascades that were masked by changes in the opposite direction by other species in the same trophic guild. Thus, the search for a general theory of trophic cascades in food webs should focus on uncovering features of real ecosystems that promote biomass compensation within functional guilds or trophic levels.

Interspecific competition and tolerance to defoliation in four grassland species

Herbivory and competition are known to influence plant performance. Recent investigations showed that tolerance is an important strategy for plant survival under herbivory, but few have examined its interaction with plant competition. We evaluated in a greenhouse experiment if plant tolerance to defoliation is related to species presence in a grazed habitat and how plant tolerance to herbivory changes in a competitive environment. Regrowth capacity of four grassland species, Trifolium repens L., Rumex acetosella L., Vicia sativa L. subsp. nigra (L.) Ehrh., and Senecio jacobaea L., was evaluated as the capacity to compensate for 75% of aboveground biomass removed. Target plants were also grown in competition with Festuca rubra L. subsp. rubra, and different clipping treatments were applied. Plant biomass (above- and below-ground) was measured after 5 months. Rumex acetosella, T. repens, and S. jacobaea were able to compensate fully when grown alone. In competition, only R. acetosella was still able to regrow. In no case did clipping the competitor improve target plant performance (i.e., no beneficial effect from competitor release). These results highlight the importance of considering plant competition when studying plant responses to herbivory.Key words: herbivory, regrowth, competitor release, biomass compensation.