Net neutral effects of a generalist vertebrate predator on seed production result from simultaneous suppression of plant antagonists and mutualists

Abstract

Trophic cascade theory predicts that predators may confer a positive effect on plants through suppressing plant antagonists or a negative effect through limiting plant mutualists, but these positive and negative cascading predator effects have seldom been examined in combination. In a marshy Tibetan alpine meadow, we conducted a factorial experiment (presence vs. absence of both predators and herbivores) using replicate field enclosures over a growing season to determine the effects of the generalist predator, the plateau frog (Rana kukunoris), on the density of herbivorous grasshoppers (Chorthippus fallax) and the visitation rate of insect mutualists (pollinators), as well as corresponding levels of leaf damage and seed production in a Tibetan lotus (Saussurea nigrescens). Frogs reduced the capitulum visitation rate of insect pollinators. Frogs also reduced grasshopper density and the corresponding leaf damage in the plants. Consequently, simultaneous suppression of plant antagonists and mutualists resulted in net neutral effects of frogs on seed set ratio and seed number per capitulum in S. nigrescens. Our study suggests that simultaneously examining the effects of predators on plant mutualists and antagonists is necessary to fully understand the roles of generalist predators in natural food webs.