Direct and interactive effects of enemies and mutualists on plant performance: a meta-analysis

MORRIS, W. F.; HUFBAUER, R. A.; AGRAWAL, A.; BEVER, J.; BOROWICZ, V. A.; GILBERT, G.; MARON, J.; MITCHELL, C.; PARKER, I.; VÁZQUEZ, D. P.

Ecology

Año: 2007 vol. 88 p. 1021 - 1029

0012-9658

Plants engage in multiple, simultaneous interactions with other species; some(enemies) reduce and others (mutualists) enhance plant performance. Moreover, effects ofdifferent species may not be independent of one another; for example, enemies may compete,reducing their negative impact on a plant. The magnitudes of positive and negative effects, aswell as the frequency of interactive effects and whether they tend to enhance or depress plantperformance, have never been comprehensively assessed across the many published studies onplant–enemy and plant–mutualist interactions. We performed a meta-analysis of experimentsin which two enemies, two mutualists, or an enemy and a mutualist were manipulatedfactorially. Specifically, we performed a factorial meta-analysis using the log response ratio.We found that the magnitude of (negative) enemy effects was greater than that of (positive)mutualist effects in isolation, but in the presence of other species, the two effects were ofcomparable magnitude. Hence studies evaluating single-species effects of mutualists mayunderestimate the true effects found in natural settings, where multiple interactions are thenorm and indirect effects are possible. Enemies did not on average influence the effects onplant performance of other enemies, nor did mutualists influence the effects of mutualists.However, these averages mask significant and large, but positive or negative, interactions inindividual studies. In contrast, mutualists ameliorated the negative effects of enemies in amanner that benefited plants; this overall effect was driven by interactions between pathogensand belowground mutualists (bacteria and mycorrhizal fungi). The high frequency ofsignificant interactive effects suggests a widespread potential for diffuse rather than pairwisecoevolutionary interactions between plants and their enemies and mutualists. Pollinators andmycorrhizal fungi enhanced plant performance more than did bacterial mutualists. In thegreenhouse (but not the field), pathogens reduced plant performance more than didherbivores, pathogens were more damaging to herbaceous than to woody plants, andherbivores were more damaging to crop than to non-crop plants (suggesting evolutionarychange in plants or herbivores following crop domestication). We discuss how observeddifferences in effect size might be confounded with methodological differences among studies.