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.

Ecosystem engineers transform habitats in ways that facilitate a diversity of species; however, few investigations have isolated short‐term effects of engineers from the longer‐term legacy effects of their engineered structures. We investigated how initial presence of net‐spinning caddisflies (Hydropsychidae) and their structures that provide and modify habitat differentially influence benthic community colonization in a headwater stream by conducting an in situ experiment that included three treatments: (1) initial engineering organism with its habitat modification structure occupied (hereafter caddisfly); (2) initial habitat modification structure alone (hereafter silk); and (3) a control with the initial absence of both engineer and habitat modification structure (hereafter control). Total invertebrate colonization density and biomass was higher in caddisfly and silk treatments compared to controls (~25% and 35%, respectively). However, finer‐scale patterns of taxonomy revealed that density for one of the taxa, Chironomidae, was ~19% higher in caddisfly compared to silk treatments. Additionally, conspecific biomass was higher by an average of 50% in silk treatments compared to controls; however, no differences in Hydropsyche sp. biomass were detected between caddisfly treatments and controls, indicating initially abandoned silk structures elevated conspecific biomass. These findings suggest that the positive effects of the habitat modification structures that were occupied for the entirety of the experiment may outweigh any potential negative impacts from the engineer, which is known to be territorial. Importantly, these results reveal that the initial presence of the engineer itself may be important in maintaining the ecological significance of habitat modifications. Furthermore, the habitat modifications that were initially abandoned (silk) had similar positive effects on conspecific biomass compared to caddisfly treatments, suggesting legacy effects of these engineering structures may have pertinent intraspecific feedbacks of the same magnitude to that of occupied habitat modifications. Elucidating how engineers and their habitat modifications differentially facilitate organisms will allow for a clearer mechanistic understanding of the extent to which animal engineers and their actions influence aspects of community organization such as colonization.

Animals and plants engineer their physical environment by building structures that create or modify habitat. Biotic effects on physical habitats can influence community composition, trophic dynamics, and ecosystem processes, however the scales and mechanisms regulating the importance of biotic engineering effects are not well documented. We used a laboratory experiment with common and abundant silk net-spinning caddisflies (Trichoptera:Hydropsychidae) to investigate how biotic structures built in riverbeds influence fluid dynamics at micro spatial scales (1 cm) over two months. We made velocity measurements with accoustic doppler velocimetery around caddisfly silk structures to test how they influence flow velocity and whether these effects are maintained after the structure is abandoned. We found that caddisfly retreats reduced flow downstream by 85% and upstream by 17% compared to gravels without caddisfly retreats. We also found that experimentally abandoned caddisfly retreats could persist for at least 60 days, suggesting legacy effects of the structures. Although aquatic insects are rarely accounted for in hydrological models, our study suggests that small but numerous caddisfly larvae could have substantial hydraulic effects. Future work could address variation in the magnitude and duration of biotic engineering among different silk-producing species, densities through space or time, and hydrologic regimes.

Invasive species are capable of altering ecosystems through the consumption of basal resources. However, quantifying the effects of invasive species in large ecosystems is challenging. Measuring changes in basal resources (i.e., phytoplankton) at an ecosystem scale is an important and potentially translatable response vital to the understanding of how introduced species influence ecosystems. In this study, we analyzed patterns of early summer chlorophyll-a in a large-river reservoir in response to invasion of silver carp (Hypophthalmichthys molitrix). We used 25 years of ecological data from a 30-km reach of Kentucky Lake collected before and after silver carp establishment. We found significant decreases in chlorophyll-a within certain reservoir habitats since establishment of silver carp. Additionally, environmental and biological drivers of phytoplankton production showed no significant differences before and after invasion. These results suggest seasonal, and habitat-specific consumptive effects of invasive silver carp on an important basal food web resource. Further, our results convey the utility of long-term quantitative biological and physiochemical data in understanding ecosystem responses to elements of global change (i.e., species invasions). Importantly, the observed changes in the basal food web resource of Kentucky Lake may apply to other ecosystems facing invasion by silver carp (e.g., Laurentian Great Lakes). Our study offers insight into the mechanisms by which silver carp may influence ecosystems and furthers our understanding of invasive omnivores.

Ecosystem engineers can generate hotspots of ecological structure and function by facilitating the aggregation of both resources and consumers.However, nearly all examples of such engineered hotspots come from long-lived foundation species, such as marine and freshwater mussels, intertidal cordgrasses, and alpine cushion plants, with less attention given to small-bodied and short-lived animals. Insects often have rapid life cycles and high population densities and are among the most diverse and ubiquitous animals on earth. Although these taxa have the potential to generate hotspots and