Connectivity, Fish Communities, and Human Impacts: an Integrated River Systems Perspective

The role of connectivity between aquatic habitats in sustaining productive and healthy fish communities has become a key conservation concern.  Changes in connectivity due to anthropogenic impacts can alter fish communities and put particular species at risk.  In order to understand how impacts to the watershed affect connectivity, and thereby fish populations, an integrated systems perspective is required.  We have developed a conceptual framework that explicitly considers all of the components of the river system and their spatiotemporal interactions.  This framework was developed to understand and evaluate the effects that wetlands and small streams have on larger river systems.  The framework addresses how components of the river system – including wetlands, small streams, and other aquatic habitat – alter the fluxes of materials (e.g., water, heat energy, sediment, wood, organic matter, nutrients, chemical contaminants, and organisms) that are transported through the river network, thereby affecting structure and function.  The framework considers two major factors:  functions that allow system components to alter material fluxes, and connectivity between those system components.  Components of river systems can affect material fluxes by serving as sources, sinks, refuges, lags, and/or transformers.  Movement of water, through surface or subsurface flows, is the primary mechanism for connectivity between river system components, but biological movement is also important.  However, in some cases the lack of connectivity (i.e., isolation) can also play an important role in sustaining fish populations, as in the case of restricting establishment by invasive species.  The framework does not consider connectivity as a fixed characteristic of the river system; rather, connectivity and isolation vary over space and time due to the longitudinal and lateral expansion and contraction of the river network and transient connection with other components of the river system.  Human impacts within the river network can increase or decrease connectivity.  For example, levee construction decreases connectivity with the adjacent riparian zone, but increases connectivity with downstream floodplains due to increased flow.  Human impacts in the terrestrial portion of the watershed can also affect connectivity in the river system, e.g., through construction of ditches to drain wetlands.  By taking an integrated, river system perspective that includes all components that interact and contribute to downstream waters, this conceptual framework should allow a better understanding of how anthropogenic alterations can affect connectivity and, ultimately, fish populations.