Understanding and predicting how natural multi-species systems respond to disturbance is a key goal for community ecology. Ideally, an understanding of the system is developed through experimental species manipulations and controlled disturbances; however, comprehensive sets of manipulations are logistically impractical in complex communities. We examined multi-species time series collected over 15 years on the rocky intertidal shores of the Pacific Northwest. Our goal was to identify direct and indirect species interactions in this intertidal community using observational time series of fixed quadrats. These data are particularly challenging to work with because they are percent cover (therefore bounded between zero and 100) and exhibit variability that changes as a function of the density of the dominant mussel species (<i>Mytilus californianus</i>). This density-dependent stochasticity is of biological interest because it represents variability in immigration at low density and increased susceptibility to wave disturbances at higher density. We modeled this system as a series of non-linear betabinomial mixture models that describe the system dynamics and identify how species interactions change through time.
Results/Conclusions
Several coarse patterns emerge from the data. First, the return interval of wave disturbance is 5-7 years and is linked closely with the density of <i>M. californianus</i>. Following large disturbances (i.e., where > 50% of a quadrat is cleared), sub-dominant species compete for space and typically reach peak density 2-3 years post-disturbance. Using score functions to measure the response of each species to community structure, we found that the growth of <i>M. californianus</i> was relatively insensitive to the density of other species; however, as expected, all other species declined as the density of this mussel increased. Surprisingly, the growth rates of several sub-dominant species were inhibited by a coralline seaweed (<i>Corallina vancouveriensis</i>). Although this pattern is very subtle in the observational time series, it suggests the outcome of experimental mussel removals where <i>C. vancouveriensis</i> tends to dominate. The interaction between wave disturbance and the density of <i>M. californianus</i> is clearly one of the most important factors in determining local community structure; however, observations of multi-species time series can provide insights into how the system will respond to changing disturbance regimes and species extinctions.
