DOI: 10.5061/dryad.58p0t
title: Data from: Indirect effects of predators control herbivore richness and abundance in a benthic eelgrass (Zostera marina) mesograzer community

description: 1. Herbivore communities can be sensitive to changes in predator pressure
 (top-down effects) and resource availability (bottom-up effects) in a wide
 range of systems. However, it remains unclear whether such top-down and
 bottom-up effects reflect direct impacts of predators and/or resources on
 herbivores, or are indirect, reflecting altered interactions among
 herbivore species. 2. We quantified direct and indirect effects of
 bottom-up and top-down processes on an eelgrass (Zostera marina) herbivore
 assemblage. In a field experiment, we factorially manipulated water column
 nutrients (with Osmocote™ slow-release fertilizer) and predation pressure
 (with predator exclusion cages) and measured the effects on herbivore
 abundance, richness and beta diversity. We examined likely mechanisms of
 community responses by statistically exploring the response of individual
 herbivore species to trophic manipulations. 3. Predators increased
 herbivore richness and total abundance, in both cases through indirect
 shifts in community composition. Increases in richness occurred through
 predator suppression of common gammarid amphipod species (Monocorophium
 acherusicum and Photis brevipes), permitting the inclusion of rarer
 gammarid species (Aoroides columbiae and Pontogeneia rostrata). Increased
 total herbivore abundance reflected increased abundance of a caprellid
 amphipod species (Caprella sp.), concurrent with declines in the abundance
 of other common species. Furthermore, predators decreased beta diversity
 by decreasing variability in Caprella sp. abundance among habitat patches.
 4. Osmocote™ fertilization increased nutrient concentrations locally, but
 nutrients dissipated to background levels within 3 m of the fertilizer.
 Nutrient addition weakly affected the herbivore assemblage, not affecting
 richness and increasing total abundance by increasing one herbivore
 species (Caprella sp.). Nutrient addition did not affect beta diversity.
 5. We demonstrated that assemblage-level effects of trophic manipulations
 on community structure are the result of distinct and often indirect
 responses of herbivore species. These results underscore the importance of
 understanding herbivore–herbivore interactions in a system commonly
 subjected to both eutrophication and overfishing.
description: Abundances of individual
 eelgrass mesograzer species on Artificial Seagrass Units
 (ASUs)To examine the relative
 importance of bottom-up and top-down control of the eelgrass (Zostera
 marina) mesograzer community, nutrients (2 levels) and predators (3
 levels) were manipulated in a full factorial experimental design (n = 10
 replicates per treatment combination) for seven weeks (June 17 – July 28,
 2011). 60 artificial seagrass units (ASUs) were randomly arranged in three
 rows, parallel to the shore in the subtidal Z. marina zone, and
 distributed such that ASUs were separated by 3 m within and between rows.
 ASUs were made of frayed rope (length = 35 cm) to mimic seagrass or
 branching algal habitat, and were used to sample mesograzers on a standard
 surface area. This data file gives the abundances of the eight mesograzers
 on ASUs for each treatment. Cage.Trt: the predator exclusion cage
 treatment (full = full cage/predator exclusion; partial = partial
 cage/predators have access/cage control; none = no cage/predator have
 access/control). Nut.Trt: the nutrient addition treatment (0 = no
 nutrients added; 1 = nutrients added (30 g Osmocote slow release
 fertilizer)).data.grazers.csvWater column nutrient concentrations in response fertilizer additionWe experimentally tested the effectiveness of slow release fertilizer (Osmocote) for increasing nutrient concentrations at the scale of our replicates and not in adjacent replicates 3 m away. Plot: plot ID. nutrient trtm: nutrient treatment (Y = fertilizer satchel added; N3 is 3m away from fertilizer satchel, N7 = 7m away from satchel to measure ambient nutrient concentrations). dist.to.satchel.m = distance from fertilizer satchel in meters. init.nut.weight is dry mass of fertilizer pellets in g before placing it in the field, fin.nut.weight is final dry mass after three days at the study site. PO4.uM, SiO2.uM, and NOx.uM are nutrient concentrations measured in uM.Nutrient.experiment.data.csv