DOI: 10.5061/dryad.3xsj3txn7
title: Facilitation strength across environmental and beneficiary trait gradients in stream communities

description: Ecosystem engineers modify habitats in ways that facilitate other
 community members by ameliorating harsh conditions. The strength of such
 facilitation is predicted to be influenced by both beneficiary traits and
 abiotic context. One key trait of animals that could control the strength
 of facilitation is beneficiary body size because it should determine how
 beneficiaries fit within and exploit stress ameliorating habitat
 modifications. However, few studies have measured how beneficiary body
 size relates to facilitation strength along environmental
 gradients. We examined how the strength of facilitation by
 net-spinning caddisflies on invertebrate communities in streams varied
 along an elevation gradient and based on traits of the invertebrate
 beneficiaries. We measured whether use of silk retreats as habitat
 concentrated invertebrate density and biomass compared to surrounding
 rock-surface habitat and whether the use of retreat habitat varied across
 body sizes of community members along the gradient. We found that retreats
 substantially concentrated the densities of a diversity of taxa
 including eight different Orders, and this effect was greatest at
 high elevations. Caddisfly retreats also concentrated invertebrate biomass
 more as elevation increased. Body size of invertebrates inhabiting
 retreats was lower than that of surrounding rock habitats at low elevation
 sites, however body size between retreats and rocks converged at higher
 elevation sites. Additionally, the body size of invertebrates found in
 retreats varied within and across taxa. Specifically, caddisfly retreats
 functioned as a potential nursery for taxa with large maximal body sizes.
 However, the patterns of this taxa-specific nursery effect were not
 influenced by elevation unlike the patterns observed based on community
 level body size.Collectively, our results indicate that invertebrates use
 retreats in earlier life stages or when they are smaller in body size
 independent of life stage. Furthermore, our analysis suggests that
 facilitation strength intensifies as elevation increases within stream
 invertebrate communities. Further consideration of how trait variation and
 environmental gradients interact to determine the strength and direction
 of biotic interactions will be important as species ranges and
 environmental conditions continue to shift. 
description: We collected net-spinning caddisfly retreat structures at five
 riffles along an elevation gradient encompassing 985 m and 70 km of river
 length beginning in the headwaters of the East River and ending in the
 mainstem of the Gunnison River Colorado, USA, in August of 2016 and 2017. 
 Retreats and any associated beneficiary taxa were collected by hand and
 placed in vials. We quantified and identified density, biomass and body
 size of invertebrates associated with caddisfly retreat structures
 (hereafter retreat) and compared these measurements to those from
 invertebrates sampled from the nearby rock surface (hereafter rock) of the
 same rock from which a retreat was sampled.  In the
 laboratory, invertebrates from retreat and rock samples were counted,
 measured in body length to the nearest mm, and identified to genus. Total
 invertebrate density in each sample was estimated as no.
 m<sup>-2</sup> scaled to the surface area of the corresponding
 retreat and rock size. Diameter measurements of rocks and retreats were
 used to convert to surface area of m<sup>2</sup> assuming
 rocks and retreats had a uniform circular shape, so that densities and
 biomasses could be assessed across consistent aerial estimates. Total
 invertebrate biomass in each sample was estimated as mg ash-free dry mass
 (AFDM) m<sup>-2</sup> using established taxon-specific
 length–mass relationships (Benke et al. 1999) and scaled to the aerial
 measures of retreat and rock size. 
description: minumum of R version 4.0.2