DOI: 10.5061/dryad.ghx3ffbv9
title: Functional identity of dominant species in a predator community prevails over functional diversity in shaping the top-down control of herbivores

description: Decline in species richness as well as changes in community evenness or
 functional diversity have been hypothesized to jointly affect ecosystem
 functioning. However, disentangling the relative effects of these changes
 in community structure is hard as these different aspects often covary
 with species richness in real-world ecosystems. In this study, we
 investigated the individual and interactive effects of functional
 diversity and community evenness of predators on the level of control of
 herbivorous prey.  Using a highly-replicated mesocosm experiment,
 we crossed three levels of functional diversity of arthropod predators
 with two levels of community evenness while controlling for the effect of
 species richness. Using this experimental setting, we hypothesized that
 the effect size of functional diversity of predators depends on community
 evenness. We expected a positive effect of functional diversity of
 predators on top-down control at a high level of community evenness while
 we thought that species identity and their associated traits should drive
 most of the effect on top-down control at a low level of community
 evenness.  Our results did not provide any evidence for an
 interaction between functional diversity and community evenness nor any
 beneficial effect of increased functional diversity overall on predation
 rates of herbivorous prey. In addition, our results revealed that species
 and functional identity drives most of the effects of predator community
 composition on top-down control of their prey in our study system.
 Assemblages composed of active hunters with low handling time and no
 starvation ability tended to have the highest impacts on prey biomass. By
 indicating that top-down control of herbivorous prey by arthropod
 predators is mainly driven by species and functional identity and not by
 functional diversity, our study provides insights into the consequences of
 ongoing species loss on ecosystem functioning. Future research should now
 explore the predictability of trophic interactions based on functional
 traits of predator and herbivorous prey to anticipate the consequences of
 changes in species composition on ecosystem functioning.
description: To test how functional
 diversity modulates the effect of evenness in predator communities on
 herbivore consumption rates, we set up a mesocosm experiment that crossed
 three levels of functional diversity of predators with two levels of
 community evenness. We selected the aphid
 <em>Acyrtosiphon</em> <em>pisum</em> and the
 leafhopper <em>Euscelidius</em>
 <em>variegatus</em> as the two main herbivorous species for
 the experiment. Eleven
 predator species that are known to prey on aphids and leafhoppers and that
 are common predators found in grassland or faba bean fields were selected
 to define predator assemblages used in the experiment. In order to
 assemble three-species communities that represent three contrasted levels
 of functional richness, we first characterized these species using six
 functional traits involved in the predation. We collected information
 about body size (mm), circadian activity (diurnal, nocturnal), habitat
 domain (soil, foliage), hunting mode (ambush hunters, active hunters),
 averaged handling time (low, high) and starvation ability (yes, no).
 We selected three
 assemblages among the potential 165 assemblages of three species, each one
 in each of the third quartiles of the functional richness distribution.
 This led to combining <em>A. bipunctata, A. nemoralis
 </em>and<em> X. kochi</em> in the low functional
 richness level; <em>A. bipunctata, C. carnea
 </em>and<em> P. mirabilis</em> in the medium functional
 richness level; and <em>C. carnea, P. opilio
 </em>and<em> P. mirabilis</em> in the high functional
 richness level. A<em>. bipunctata, A. nemoralis
 </em>and<em> C. carnea</em> were bought at Biobest
 Company, while <em>P. mirabilis, P. opilio </em>and<em>
 X. kochi</em> were collected in the fields and maintained in the
 laboratory. The two levels of community evenness were
 designed to have a low evenness modality (Pielou index = 0.79) and a high
 evenness modality (Pielou index = 1) for all levels of functional
 richness. The total number of predators in each mesocosm was six
 individuals so that the low level of evenness consisted of three-species
 communities composed of four individuals of the dominant species and one
 individual of the two remaining species, while the high level of evenness
 consisted of two individuals of each species. 
 In addition, for the low
 evenness modalities, we considered each situation of species dominance
 among assemblages of three species to control for the effect of predator
 identity. This resulted in four different treatments of evenness for a
 given level of functional richness: three low evenness treatments with
 each species dominating, and one high evenness treatment. As we considered
 three levels of functional richness, we ended up with 12 different
 treatments crossing each level of functional richness with each evenness
 treatment (four evenness treatments crossed with three functional richness
 levels). For a given round of the experiment, we replicated each treatment
 at least three times (one round had four replicates) and considered six
 mesocosm controls with herbivores only. One round of the whole experiment
 therefore consisted of 42 to 54 mesocosms. To ensure robustness of the
 results we replicated the whole experiment four times between 2016 and
 2018 leading to a total of 180 mesocosms surveyed.
 In each mesocosm, we
 introduced 150 aphids and 10 leafhoppers 24 hours before the introduction
 of predators based on realistic densities of these phytophagous species in
 fields (Ammann et al., 2022; Bosco et al., 1997; Erb et al., 2010). We
 assessed the number of aphids and leafhoppers after five days of predation
 exposure using a mouth aspirator and careful visual inspections in each
 mesocosm. The number of prey collected was converted to biomass using
 average weights of 0.51 mg and 3.53 mg for
 <em>Acyrtosiphon</em> <em>pisum</em> and
 <em>Euscelidius</em> <em>variegatus</em>
 respectively (Lamb et al., 1987; Purcell & Suslow, 1987).
 Mesocosms were kept in a greenhouse at ambient temperature and water was
 provided to plants every two days. We did not need any permission or
 appropriate license to perform the experiment.
 The data attached correspond
 to the number of collected aphids or leafhoppers at the end of the
 experiment for each mesocosm and their given experimental modalities
 (e.g., functional richness, evenness and identity of dominant species). In
 addition, we attached the traits used to compute the functional richness
 metrics and explore how identity of predator species was affecting prey
 consumption.