Positive interactions are sensitive to human activities, necessitating synthetic approaches to elucidate broad patterns and predict future changes if these interactions are altered or lost. General understanding of freshwater positive interactions has been far outpaced by knowledge of these important relationships in terrestrial and marine ecosystems. We conducted a global meta‐analysis to evaluate the magnitude of positive interactions across freshwater habitats. In 340 studies, we found substantial positive effects, with facilitators increasing beneficiaries by, on average, 81% across all taxa and response variables. Mollusks in particular were commonly studied as both facilitators and beneficiaries. Amphibians were one group benefiting the most from positive interactions, yet few studies investigated amphibians. Invasive facilitators had stronger positive effects on beneficiaries than non‐invasive facilitators. We compared positive effects between high‐ and low‐stress conditions and found no difference in the magnitude of benefit in the subset of studies that manipulated stressors. Future areas of research include understudied facilitators and beneficiaries, the stress gradient hypothesis, patterns across space or time and the influence of declining taxa whose elimination would jeopardise fragile positive interaction networks. Freshwater positive interactions occur among a wide range of taxa, influence populations, communities and ecosystem processes and deserve further exploration.

Hydropsychid (net‐spinning caddisfly) larvae are aquatic macroinvertebrate ecosystem engineers, altering streambed properties by creating silk nets and retreats. Recent research has shown that caddisfly silk binds stream sediments and alters streambed cohesion, yet potential influences of caddisfly activity on streambed hydrologic properties have not received similar scrutiny. Utilizing a novel downward flow permeameter, we demonstrate how net‐spinning caddisfly colonization of the streambed interstitia at moderate but common densities (2000 m−2) can reduce vertical hydraulic conductivity (KV) by up to 55% in streambed sediments (coarse sand—medium gravel; median diameter = 12.91 mm). Additionally, organic matter content within sediment columns showed that occupation by caddisflies resulted in twice the ash‐free dry mass (AFDM) of noncaddisfly (primarily microbial) organic matter at the end of our experiments relative to control systems that lack caddisflies. These results suggest that net‐spinning caddisflies may restrict hyporheic exchange, alter streambed hydrologic residence time distributions and increase per‐unit‐volume biotic metabolic demand for stream solutes in the hyporheic zone, with the potential to alter whole‐stream biogeochemical processes.

Given unprecedented rates of biodiversity loss, there is an urgency to better understand the ecological consequences of interactions among organisms that may lost or altered. Positive interactions among organisms of the same or different species that directly or indirectly improve performance of at least one participant can structure populations and communities and control ecosystem process. However, we are still in need of synthetic approaches to better understand how positive interactions scale spatio‐temporally across a range of taxa and ecosystems. Here, we synthesize two complementary approaches to more rigorously describe positive interactions and their consequences among organisms, across taxa, and over spatio‐temporal scales. In the first approach, which we call the mechanistic approach, we make a distinction between two principal mechanisms of facilitation—habitat modification and resource modification. Considering the differences in these two mechanisms is critical because it delineates the potential spatio‐temporal bounds over which a positive interaction can occur. We offer guidance on improved sampling regimes for quantification of these mechanistic interactions and their consequences. Second, we present a trait‐based approach in which traits of facilitators or traits of beneficiaries can modulate their magnitude of effect or how they respond to either of the positive interaction mechanisms, respectively. Therefore, both approaches can be integrated together by quantifying the degree to which a focal facilitator's or beneficiary's traits explain the magnitude of a positive effect in space and time. Furthermore, we demonstrate how field measurements and analytical techniques can be used to collect and analyze data to test the predictions presented herein. We conclude by discussing how these approaches can be applied to contemporary challenges in ecology, such as conservation and restoration and suggest avenues for future research.

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