DOI: 10.5061/dryad.djh9w0vzb
title: Environmental stress gradients regulate the relative importance of predator density- and trait-mediated indirect effects in oyster reef communities

description: Predators affect community structure by influencing prey density and
 traits, but the importance of these effects often is difficult to predict.
 We measured the strength of blue crab predator effects on mud crab prey
 consumption of juvenile oysters across a flow gradient that inflicts both
 physical and sensory stress to determine how the relative importance of
 top predator density-mediated indirect effects (DMIEs) and trait-mediated
 indirect effects (TMIEs) change within systems. Overall, TMIEs dominated
 in relatively benign flow conditions where blue crab predator cues
 increased oyster survivorship by reducing mud crab oyster consumption.
 Blue crab DMIEs became more important in high sensory stress conditions,
 which impaired mud crab perception of blue crab chemical cues. At high
 physical stress, the environment benefitted oyster survival by physically
 constraining mud crabs. Thus, factors that structure communities may be
 predicted based on an understanding of how physical and sensory
 performances change across environmental stress gradients.
description: Blue crab predator effect trials were performed in enclosures
 (1.25 m x 1.25 m x 0.3 m; l x w x h) that contained an oyster reef in the
 center as a refuge for mud crabs. Oyster shells were glued together to
 create artificial oyster clusters that were used to manipulate the
 placement of oyster spat within the enclosure. Four oyster spat were
 epoxied to the artificial oyster clusters and 4 clusters were placed
 within the constructed oyster reef and 4 clusters outside the reef. In
 total, each cage contained 32 oyster spat with 16 spat each inside and
 outside the reef.  Fifteen mud crabs were added to the oyster reef in the
 enclosure. One of four blue crab predator treatments
 (control/cull/predation-risk/lethal) were added to enclosures. Control
 treatments contained no blue crab predator. Cull treatments simulated blue
 crab predation without the presence of blue crab cues by removing 5 mud
 crabs at 24 hours. Predation-risk blue crab treatments conatined one blue
 crab that was mobile but with chelipeds clamped shut with heat-shrink
 tubing covered by duct tape and cinched down by a cable tie to prevent
 blue crabs from attacking mud crabs. Lethal blue crab treatments consisted
 of one mobile and unrestrained blue crab.  Enclosures
 were deployed on intertidal mudflats at either Priest Landing or Skidaway
 Narrows during either mean tide or spring tide. The number of mud crabs
 recovered, oysters alive, and oysters dead were counted after 48 hours.
 Blue crab predator effect size on oyster survival was calculated using
 ratio-based indices. The formulas used were as followed:
 Density-mediated indirect effect = (oyster survival with lethal
 blue crab/ oyster survival with predation-risk blue crab) - 1 
     Trait-mediated indirect effect = (oyster survival
 with predation-risk blue crab/ oyster survival with no blue crab) -
 1 Total predator indirect effect = (oyster survival
 with lethal blue crab/ oyster survival with no blue crab) - 1
 The numerator was the number of surviving oysters in a single
 replicate for the stated treatment and the denominator was the average
 number of surviving oysters of the stated treatment for a given trial
 block. Flow measurements during each trial block were estimated using the
 predictive relationship between tidal range and flow conditions
 (regression equations derived by Pruett & Weissburg
 (2018)).