Project: Spatial Realism in the Mussel Bed Disturbance Paradigm

Project description from NSF award abstract:
Our understanding of how physical disturbance shapes the structure of populations and communities owes much to field studies of wave-generated gap formation in mussel beds. Prior studies depict mussel beds as a non-equilibrium system, in which disturbance is spatially unpredictable, generating a random patchwork of mussel cover and gaps. This project will test assumptions and predictions of an alternative view -- that disturbance shows predictable landscape patterns that depend not merely on spatial distribution of external forcing (wave stress) but also on biological processes determining the structure of the aggregation. Specifically, spatially varying mussel productivity (recruitment and growth), physiological stress, and predation interact to produce landscape patterns in the structure of the mussel cover. Certain regions of the mussel bed develop as mono-layers attached directly to the rock, resisting disturbance. Other regions develop in multi-layered configurations that when very deep force superficial mussels to attach solely to adjacent mussels instead of the rock surface, and cause interior mussels to only weakly attach to either rock or one another, favoring propagating disturbances. Therefore, spatial patterns of gap formation and recovery emerge from a unified landscape process.

Field work for this project emphasizes construction of a detailed GIS database using some innovative sampling methods applied to >10 mussel bed sites in Barkley Sound, British Columbia. GIS data layers for each site include wave force, topography (tidal height, slope, and aspect), mussel size structure, mussel bed thickness, differentiation of layering, and size-specific attachment strengths stratified by layer. GIS interpolations and regression analyses will be used to first examine assumptions of the hypothetical landscape process and then test specific predictions regarding spatial patterns in the occurrence of disturbance and recovery. Finally, controlled field experiments will test the key proposition that different mussel bed structures cause different resistance to-, extent of-, and recovery from disturbance.

The landscape disturbance hypothesis includes several processes that differ from previous conceptualizations of disturbance in this system: steady state (equilibrium) structures influence the likelihood that hydrodynamic stresses initiate a disturbance and whether once initiated the disturbance will propagate. Thus, equilibrium processes condition essential features of the disturbance process. Disturbance is predictable within probabilistic limits, and the landscape patterns of disturbance reflect in considerable measure the biological characteristics of a foundation species. This reframing of the disturbance paradigm may apply to a diverse array of layered ecosystems subject to perturbations (e.g. biofilms, coral reefs, forests).