Riverine macrosystems ecology: sensitivity, resistance, and resilience of whole river basins with human alterations

www.frontiersinecology.org © The Ecological Society of America R ecosystems are some of the most diverse on Earth and provide important services (Palmer and Richardson 2009; Strayer and Dudgeon 2010). Understanding how they function is critical to sustainable management but challenging given their complex spatial and temporal structure and multi-scale processes. Riverine systems comprise hydrological–ecological networks organized by the flow of water, sediment, nutrients, and organisms downhill and downstream and the active movement of animals uphill and upstream. Rivers are multidimensional, including longitudinal (upstream–downstream), lateral (upland to channel), vertical (hyporheic, or the zone below the stream bed), and temporal components (Ward 1989; Fausch et al. 2002). Despite this multidimensionality, many ecological processes are influenced by the rapid flow of water downhill, providing strong directional connectivity (Wiens 2002). Rivers are also organized hierarchically, with fine-scale structures (eg gravel patches) embedded within channel bed features (eg riffles), which in turn are embedded within reaches, valley segments, basins, and regions (Table 1; Frissell et al. 1986; Thorp et al. 2008). Uplands are fundamental to riverine organization, with variations in land use, land cover, and soils influencing surface-water and groundwater flow paths, thereby altering water, nutrient, and sediment fluxes to rivers (eg Lewis and Grimm 2007). Rivers are also temporally variable, partially due to hydrology that varies within and across basins and climatic regions (Poff et al. 1997). Thus, we define riverine macrosystems as hierarchical dynamic networks, influenced by strong directional connectivity that integrates processes across multiple scales and broad distances through time (Figure 1; see Heffernan et al. [2014] for macrosystem definition). Ecologists have typically studied riverine ecosystems at the scale of bed features or reaches distributed longitudinally along rivers of varying size, in an attempt to understand the strong influences that upstream and watershed processes, including human modifications, can have (Poole 2010). Our conceptualization of rivers and watersheds as “macrosystems” is a logical extension of these approaches (Figure 1). We view riverine macrosystems as repeating, interacting MACROSYSTEMS ECOLOGY