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Ecosystem engineers in rivers: An introduction to how and where organisms create positive biogeomorphic feedbacks

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Ecosystem engineers substantially alter physical flow characteristics and shape a river's form and function. Because the recurrence interval of geomorphic processes and disturbances in rivers commonly match the temporal scale of plants’ life cycles or alterations by animals, the resulting feedbacks are an important component of rivers. In this review, we focus on biota that directly or indirectly induce a physical change in rivers and cause positive feedbacks on the functioning of that organism. We provide an overview of how various ecosystem engineers affect rivers at different temporal and spatial scales and plot them on a conceptual gradient of river types. Various plants engineer the river environment through stabilizing sediment and reducing flow velocities, including macrophytes, woody plants, and algal mats and biofilms. Among animals that engineer, beaver that build dams cause substantial changes to river dynamics. In addition, benthic macroinvertebrates and mussels can stabilize sediment and reduce velocities, and aquatic and riparian grazers modulate the effect of plants. Humans are also considered river ecosystem engineers. Most of the ecosystem engineers reported in literature occur in rivers with low to intermediate relative stability, intermediate channel widths, and small to intermediate grain sizes. Ecosystem engineers that create positive biogeomorphic feedbacks are important to take into account when managing river systems, as many common invasive species are successful due to their engineering capabilities. River restoration can use ecosystem engineers to spur holistic recovery. Future research points towards examining ecosystem engineers on longer spatial and temporal scales and understanding the co‐evolution of organisms and landforms through engineering. WIREs Water 2018, 5:e1271. doi: 10.1002/wat2.1271 This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Water and Life > Conservation, Management, and Awareness
Schematic overview of different ecosystem engineers in river systems, categorized according to their engineering effect. Gray shading indicates the type of ecosystem engineers described in this review. The position of the “ecosystem engineers,” “niche construction,” and “keystone species” concepts are only meant to show that there is overlap and not the degree to which the concepts overlap. A full reference list is provided in Appendix S1, Supporting information. References for ecosystem engineers with positive feedbacks can be found in the text
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Diagrams denoting where different ecosystem engineers actively engineer the environment and create positive biogeomorphic feedbacks in rivers based on three process‐ and form‐based geomorphic factors: relative stability, channel width, and grain size. Note that these organisms may be present in other parts of the river system but may not act as ecosystem engineers there (e.g., beaver occur in wide channels with fine sediment, but they do not build dams there), and that the ecosystem engineering capacity may be context dependent within the approximate space shown in the diagrams, depending on other, nongeomorphic environmental factors (e.g., temperature, nutrient availability)
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Water and Life > Conservation, Management, and Awareness
Water and Life > Nature of Freshwater Ecosystems

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