Herbivore identity mediates the strength of trophic cascades on individual plants

F J F Van Veen,D Sanders

doi:10.1890/es13-00067.1

Abstract

The strength of top‐down indirect effects of carnivores on plants (trophic cascades) varies greatly and may depend on the identity of the intermediate (herbivore) species. If the effect strength is linked to functional traits of the herbivores then this would allow for more general predictions. Due to the generally sub‐lethal effects of herbivory in terrestrial systems, trophic cascades manifest themselves in the first instance in the fitness of individual plants, affecting both their numerical and genetic contributions to the population.We directly compare the indirect predator effects on growth and reproductive output of individual Vicia faba plants mediated by the presence of two aphid species: Acyrtosiphon pisum is characterised by a boom and bust strategy whereby colonies grow fast and overexploit their host plant individual while Megoura viciae appear to follow a more prudent strategy that avoids over‐exploitation and death of the host plant. Plants in the field were infested with A. pisum, M. viciae or both and half the plants were protected from predators.Exposure to predators had a strong impact on the biomass of individual plants and the strength of this effect differed significantly between the different herbivore treatments. A. pisum had a greater direct impact on plants and this was coupled with a significantly stronger indirect predator effect on plant biomass. Although the direct impact of predators was strongest on M. viciae, this was not transmitted to the plant level, indicating that the predator‐prey interactions strength is not as important as the plant‐herbivore link for the magnitude of the indirect predator impact. At the individual plant level, the indirect predator effect was purely due to consumptive effects on herbivore densities with no evidence for increased herbivore dispersal in response to presence of predators.The nature of plant‐herbivore interactions is the key to terrestrial trophic cascade strength. The two herbivores that we compared were similar in feeding mode and body size but differed their way how they exploit host plants, which was the important trait explaining the strength of the trophic cascade.

Highlights

Communities of interacting species are shaped by the direct interactions among species such as predation and interference competition, and by indirect interactions
We directly compare the indirect predator effects on growth and reproductive output of individual Vicia faba plants mediated by the presence of two aphid species: Acyrtosiphon pisum is characterised by a boom and bust strategy whereby colonies grow fast and overexploit their host plant individual while Megoura viciae appear to follow a more prudent strategy that avoids over-exploitation and death of the host plant
Total plant biomass When protected from predators, mean cumulative population sizes in single-species colonies were significantly larger for M. viciae at 7448 6 908 than for A. pisum at 4548 6 1041 (t1,18 1⁄4À2.1, p 1⁄4 0.05)

Summary

Introduction

Communities of interacting species are shaped by the direct interactions among species such as predation and interference competition, and by indirect interactions. The term is used widely to refer to the effects of carnivore removal on whole producer communities, both in terms of diversity and total biomass, and on subsets of communities and single species populations (Strong 1992, Schmitz et al 2000). The latter case is sometimes termed a species cascade (Polis 1999). Earlier considered to be a feature mostly of aquatic systems (Strong 1992), there is ample evidence for terrestrial trophic cascades (Schmitz et al 2000, Halaj and Wise 2001, Shurin et al 2002, Mooney et al 2010a)

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