Research concepts

Streams constitute highly dynamic ecosystems and are – worldwide – affected by multiple stressors. These stressors often interact in complex, nonlinear ways leading to the degradation of biodiversity and ecosystem functions. Therefore, predictions about changes in biodiversity and ecosystem function are difficult, yet critically important. Similarly, ecosystem recovery following the release from stressors now increasingly receives attention. A mechanistic understanding of the processes underlying degradation and recovery, however, has been long missing. Such mechanistic insight is required to accurately predict stressor effects, their interactions and impacts on biodiversity and ecosystem functioning, and eventually to manage the recovery of degraded freshwater ecosystems.

In Phase I of the RESIST project, we have published and tested a novel theoretical framework to disentangle mechanisms causing stressor interactions during phases of ecosystem degradation and recovery: the Asymmetric Response Concept (ARC) (see Vos et al., 2023). The ARC concept holds that three key mechanisms influence population, community and ecosystem trajectories:

• tolerance
• dispersal
• biotic interactions

As the central tenet of the ARC, the significance of tolerances, dispersal and biotic interactions differs fundamentally between phases of degradation and recovery. The shifting importance of these mechanisms creates asymmetries between the ecological trajectories that follow increasing and decreasing stressor intensities. This recognition helps to understand multiple stressors’ impacts and to predict which measures will restore communities that are resistant to restoration.

In RESIST, we test three main hypotheses addressing the role of the ARC processes during phases of degradation and recovery - the response of different organism groups to these processes, and the rates of recovery of community composition and ecosystem functioning:

• Main Hypothesis 1 (MH1): For the process of community assembly, the organisms’ tolerances are most relevant during phases of degradation, whereas dispersal and biotic interactions are more relevant during phases of recovery
• Main Hypothesis 2 (MH2): The importance of the ARC’s processes (tolerance, dispersal, biotic interactions) differs in relation to organism size (micro- vs. macro-organisms)
• Main Hypothesis 3 (MH3): Functions recover faster than community composition to a pre-degradation stage, due to partial functional redundancy among taxa

To test those hypotheses, RESIST focuses on three crucial stressors and their effects on various organism groups and ecosystem functions using a variety of research approaches including large-scale experimental setups (ExStream, AquaFlow) and field studies, focusing on two selected catchments. In turn, experiments and field studies provide knowledge rules for developing statistical and mechanistic models. Those results in addition to external information, provide the basis to elaborate the overall synthesis of RESIST and effectively, to test the Asymmetric Response Concept.