Abstract
The integrity of river systems is increasingly threatened by global environmental changes, including habitat degradation and climate change. Habitat degradation in floodplain river systems has been largely driven by human land-use conversion, including substantial engineering of river channels. Consequently, altered flow and sediment dynamics have increased flood risks to human life and property. These risks and associated economic and social costs have motivated a paradigm shift in river management toward ‘living with water’. However, management practices under this new paradigm have undervalued the ecological co-benefits that can be achieved by giving a river more room to move; humanity relies on healthy, functioning freshwater ecosystems for food, clean water, recreation, education, and social and cultural benefits. Furthermore, to support and motivate decision making that prioritises ecological resilience, we need a better understanding of the effects of channel complexity (such as is promoted by giving a river more room to move) and flow disturbance regimes on stability in biological communities.
First, I review riverscape ecology from the perspective of providing rivers with more room to move. Climate change is forcing us to rethink river management, as altered rainfall patterns are increasing flood risks. The Netherlands’ Room for the River project demonstrates the advantages of working with nature to reduce risks; however, the priorities of this initiative were driven by protecting human life and property. Giving a river more room – that is, allowing a greater area of the floodplain for natural processes – also has diverse and critical advantages for freshwater ecosystem functioning and resilience. I present a conceptual framework of the ecological benefits promoted by giving a river more room to move: 1. A dynamic equilibrium between the drivers of channel morphology (biology, geology, and hydrology), results in a river planform that reflects local conditions; 2. A spatiotemporally variable habitat mosaic that promote regional biodiversity; 3. The natural flow disturbance regime maintains a complex habitat mosaic and does not affect all habitats to the same degree, or simultaneously; 4. Variable hydrologic connectivity between habitat patches modulates community assembly and maintain the movement of organic and inorganic materials through the system; and 5. A functioning interface between aquatic and terrestrial ecosystems connects the river to a wider meta-ecosystem. The ecological features and processes within these categories interact to support species diversity and community stability, thereby bolstering ecosystem resilience. River management that considers these characteristics of natural systems will work to address declines in freshwater biodiversity and associated ecosystem services upon which humans depend, while also increasing societal adaptability and resilience against climate change.
Second, I analyse relationships between channel complexity, diversity (taxonomic and trait), and long-term community stability (aggregate and compositional), demonstrating the importance of physical complexity that is promoted by giving rivers more room to move. The shifting mosaic of habitat patches in floodplain river systems provides the foundation for a resilient ecosystem, including refuge from disturbances, availability of resources, and sufficient connectivity for recruitment. I analyse relationships at 60 New Zealand rivers along a gradient of channel complexity, using a 27-year time-series dataset of annual benthic macroinvertebrate surveys. Structural Equation Models were used to test the links between channel complexity (defined here as the length of the side-channel habitat), flow regime characteristics (disturbance and predictability), diversity (taxonomic and trait diversity), and long-term community aggregate and compositional stability. I found that a considerable amount of variability between sites in three measures of aggregate and compositional community stability (stability in aggregate abundance, community turnover, and mean rank shift) was explained by channel complexity and the frequency of flow disturbances. The relationship between channel complexity and community stability (aggregate and compositional) was mediated by the influence of channel complexity on taxonomic and trait diversity in the community, while the frequency of flow disturbance was directly related to aggregate stability. The community dynamics I found in my study sites reflect assemblages that are evolved to long-term conditions and thus could be susceptible to instability in ecosystem functionality under rapid environmental change.
As climate change and the concurrent biodiversity crisis threaten freshwater ecosystem functioning, this thesis provides conceptual and quantitative support for the conservation and restoration of lateral channel migration and floodplain processes in river systems.