DOI: 10.5061/dryad.931zcrjkz
title: Indirect effects shape epifauna habitat

description: We tested the response of algal epifauna to the direct effects of
 predation and the indirect consequences of habitat change due to grazing
 and nutrient supply through upwelling using an abundant intertidal
 rhodophyte, Gelidium pristoides. We ran a mid-shore field experiment at
 four sites (two upwelling sites interspersed with two non-upwelling sites)
 along 450 km of the south coast of South Africa. The experiment was
 started in June 2014 and ran until June 2015. Four treatments (predator
 exclusion, grazer exclusion, control, and procedural control) set out in a
 block design (n = 5) were monitored monthly for algal cover for the first
 6 months and every 2 months for the last 6 months. Epifaunal abundance,
 species composition, algal cover, and algal architectural complexity
 (measured using fractal geometry) were assessed after 12 months. Predation
 had no significant effect on epifaunal abundances, while upwelling
 interacted with treatment. Grazing reduced the architectural complexity of
 algae, with increased fractal dimensions in the absence of grazers, and
 also reduced algal cover at all sites, though the latter effect was only
 significant for upwelling sites. Epifaunal community composition was not
 significantly affected by the presence of herbivores or predators but
 differed amongst sites independently of upwelling; sites were more similar
 to nearby sites than those farther away. In contrast, total epifaunal
 abundance was significantly affected by grazing, when normalized to algal
 cover. Grazing reduced the cover of algae, thus epifaunal abundances were
 not affected by the direct top-down effects of predation but did respond
 to the indirect effects of grazing on habitat availability and quality.
 Our results indicate that epifaunal communities can be strongly influenced
 by the indirect consequences of biotic interactions.
description: The
 dataset was collected this study were used to distinguish upwelling and
 non-upwelling sites. Data for algal cover were used to assess the
 influence of treatment (fixed, 4 levels), upwelling (fixed, 2 levels) and
 site (nested in upwelling, random, four levels) on percentage algal cover.
 This was done at the start of the experiment (no significant effects of
 any factor or interaction) and again towards the end of the experiment.
 Analyses of the epifauna were based on data collected at the end of the
 experiment when all plots were destructively sampled. We used the same
 nested design to test the effects of treatment, upwelling and site on:
 epifaunal abundance (i.e., total number of epifaunal individuals) in each
 plot and density of epifauna per unit algal cover (number of epifauna per
 cm² of algal cover) in each plot. Analyses were performed using Statistica
 12 (StatSoft), with alpha = 0.05.
description: The readme file contains an explanation of each of the variables
 in the dataset and its measurement units. Information on how the
 measurements were done can be found in the associated manuscript
 referenced above. Funding This work was supported by the South African
 Research Chairs Initiative of the Department of Science and Technology and
 the National Research Foundation (grant number 64801) to CDM.