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. 

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^-2 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 m² 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^-2 using established taxon-specific length–mass relationships (Benke et al. 1999) and scaled to the aerial measures of retreat and rock size. 

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