Testing the Asymmetric Response Concept in disturbed and recovering stream ecosystems: integrating the contributions of multi-stressor tolerance, dispersal and biotic interactions to (a)symmetry of response
The post-disturbance recovery of stream communities often gets mysteriously stunted. This is most clearly seen in the benthic invertebrate fauna. For as yet unresolved reasons their succession may not progress beyond early and mid-stages of community re-assembly even when abiotic conditions have been fully restored. The phenomenon has been called negative resistance and, in contexts of ecological restoration, resistance of the degraded state to restorative interventions (Lake, 2013).
Project A19 aims to help resolve this issue by analysis of mathematical models of stream communities and food webs. We will test which ecological mechanisms differentially govern the alternative degradation and reassembly trajectories that lead either to full recovery or to the system getting locked in a depauperate state with limited levels of function. The proposed work thus tests the central idea of RESIST: the Asymmetric Response Concept (ARC).
I hypothesise that the stage of degradation is first set by multiple stressors (through their additive, synergistic and antagonistic effects) causing specific patterns of species loss. These primary local extinctions are predictable from species-specific tolerance traits. This may then result in a series of secondary local extinctions that cannot be predicted on basis of species tolerance traits alone, as they depend on the configuration of the stream food web. Primary extinctions will additionally lead to changes in relative abundances of the remaining resident species. To predict and quantify these secondary extinctions and/or density changes, we perform dynamic community viability analyses and simulations of stream food webs.
These analyses serve to predict both the sequence and number of secondary extinctions and thus the set of degraded states recovery will have to start from. We will then test a suite of ecological mechanisms for their respective contributions to either stunted or full recovery. These mechanisms include variation in tolerances, dispersal and biotic interactions. The latter include Allee effects, indirect effects (and the associated positive feedbacks) as well as priority effects that may result from shifts in the relative forces of competition, predation and parasitism. We use an approach that strongly links our stream food web modelling to outcomes of the experiments, field work and other modelling projects in RESIST and we will directly contribute to the work of the synthesis committee (Z03).