Background

Humans have interfered with the natural hydromorphology of streams and rivers for centuries, altering the flow regime, channel structure, and connection to floodplains. These impacts have cumulatively reduced the hydromorphological complexity and integrity of river-floodplain ecosystems. Thereby, they contribute to the ongoing decline of freshwater biodiversity.
The management of freshwater ecosystems has addressed this problem initiating efforts to restore the hydromorphology of running waters. Such activities can include reconfigurations of the stream channel by installing boulders and wood, removing dams and weirs or building new channels, as well as riparian reforestation. The underlying assumption behind these efforts is that once the hydromorphology is restored, freshwater biodiversity and ecosystem functions will follow.
With RESTOLINK, we argue that the potential for hydromorphological restoration to improve biodiversity and function is not fully explored. Through an effort for assessing stream restoration initiatives across a latitudinal gradient (Brazil, Spain, Germany, and Sweden), we will tackle two significant shortcomings in current restoration practices:
1) There is an apparent mismatch between the spatial scales at which hydromorphology is restored and scales at which biodiversity and ecosystem functions are controlled.
2) Most indicators of restoration success are structural components of communities (e.g., diversity, abundance) and a restoration measure is deemed successful if a particular set of species occurs at a restored site. We argue that the functional role of organisms is a crucial parameter besides their taxonomic identity and that certain ecosystem functions may already recover in a restored reach even if the species community has not yet reached the target.

Objectives and hypothesis

The overall aim of RESTOLINK is to advance a novel mechanistic framework for quantifying restoration success that interlinks hydromorphological heterogeneity at relevant spatial scales with multi-group biodiversity and essential ecosystem functions. Our framework will advise managers to select the most effective restoration measure at ecologically relevant scales.

Hypothesis 1: Biodiversity is generally positively related to hydromorphological heterogeneity for micro- and macroorganisms in line with the habitat heterogeneity hypothesis (H1). We expect differences in the shape of both trajectories, given that the body size and mobility of a given species drive its niche requirements. Moreover, we predict that biome-specific differences in biodiversity should be larger for macroorganisms than for microorganisms due to the ubiquitous distribution range of most microbes versus the distinct biogeographic distribution ranges of larger organisms.
 
Hypothesis 2: Biodiversity scales asymptotically with multifunctionality (H2) because few species contribute disproportionately to ecosystem functioning. This hypothesis is based on empirical observations demonstrating that certain species are responsible for most processes. We predict that such functionally key species are primarily microbes and that the inter-biome variability is primarily driven by the degree of functional redundancy.
 
Hypothesis 3: Trajectories of ecosystem functioning and hydromorphological heterogeneity are asymptotic but differ in shape between microbially and macrobially driven functions due to differences in the niches experienced by these different groups (H3). We furthermore expect that microbially-driven functions may benefit from mass transfer processes, which are related to temporal variances of flow and can be high already at small spatial scales.
 
Hypothesis 4: Success in restoring hydromorphological heterogeneity is achieved earlier for ecosystem multifunctionality than for biodiversity (H4). This implies that restoration can have two outcomes, i.e., one where ecosystem multifunctionality is fully recovered and one where both biodiversity and multifunctionality are fully recovered.