title: Does plant diversity benefit agroecosystems? A synthetic review.
abstract: Predictive theory on how plant diversity promotes herbivore suppression through movement patterns, host associations, and predation promises a potential alternative to pesticide-intensive monoculture crop production. We used meta-analysis on 552 experiments in 45 articles published over the last 10 years to test if plant diversification schemes reduce herbivores and/or increase the natural enemies of herbivores as predicted by associational resistance hypotheses, the enemies hypothesis, and attraction and repellency model applications in agriculture. We found extensive support for these models with intercropping schemes, inclusion of flowering plants, and use of plants that repel herbivores or attract them away from the crop. Overall, herbivore suppression, enemy enhancement, and crop damage suppression effects were significantly stronger on diversified crops than on crops with none or fewer associated plant species. However, a relatively small, but significantly negative, mean effect size for crop yield indicated that pest-suppressive diversification schemes interfered with production, in part because of reducing densities of the main crop by replacing it with intercrops or non-crop plants. This first use of meta-analysis to evaluate the effects of diversification schemes, a potentially more powerful tool than tallies of significant positive and negative outcomes (vote-counting), revealed stronger overall effects on all parameters measured compared to previous reviews. Our analysis of the same articles used in a recent review facilitates comparisons of vote-counting and meta-analysis, and shows that pronounced results of the meta-analysis are not well explained by a reduction in articles that met its stricter criteria. Rather, compared to outcome counts, effect sizes were rarely neutral (equal to zero), and a mean effect size value for mixed outcomes could be calculated. Problematic statistical properties of vote-counting were avoided with meta-analysis, thus providing a more precise test of the hypotheses. The unambiguous and encouraging results from this meta-analysis of previous research should motivate ecologists to conduct more mechanistic experiments to improve the odds of designing effective crop diversification schemes for improved pest regulation and enhanced crop yield.
authors.affiliation: Environmental Studies Department, 1156 High Street, University of California, Santa Cruz, California 95064, USA. dletour@ucsc.edu
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Letourneau
authors.forename: Inge
authors.initials: I
authors.lastname: Armbrecht
authors.forename: Beatriz Salguero
authors.initials: BS
authors.lastname: Rivera
authors.forename: James Montoya
authors.initials: JM
authors.lastname: Lerma
authors.forename: Elizabeth Jiménez
authors.initials: EJ
authors.lastname: Carmona
authors.forename: Martha Constanza
authors.initials: MC
authors.lastname: Daza
authors.forename: Selene
authors.initials: S
authors.lastname: Escobar
authors.forename: Victor
authors.initials: V
authors.lastname: Galindo
authors.forename: Catalina
authors.initials: C
authors.lastname: Gutiérrez
authors.forename: Sebastián Duque
authors.initials: SD
authors.lastname: López
authors.forename: Jessica López
authors.initials: JL
authors.lastname: Mejía
authors.forename: Aleyda Maritza Acosta
authors.initials: AM
authors.lastname: Rangel
authors.forename: Janine Herrera
authors.initials: JH
authors.lastname: Rangel
authors.forename: Leonardo
authors.initials: L
authors.lastname: Rivera
authors.forename: Carlos Arturo
authors.initials: CA
authors.lastname: Saavedra
authors.forename: Alba Marina
authors.initials: AM
authors.lastname: Torres
authors.forename: Aldemar Reyes
authors.initials: AR
authors.lastname: Trujillo
mesh_terms: D000383:Agriculture; D044822:Biodiversity; D017753:Ecosystem; D010944:Plants / Q000145:classification*