Size‐Dependent Competition: Effects on the Dynamics Vs. The End Point of Mussel Bed Succession

I manipulated relative body size of mussels (Mytilus californianus) and gull (Larus glaucescens) predation pressure within gaps in a mussel bed to determine (1) whether size differences could cause shifts in competitive dominance between goose barnacles (Pollicipes polymerus) and Mytilus, species with indeterminate growth, and (2) if the late recruiting Mytilus required gull predation on established adult Pollicipes to attain dominance. Increasing relative body size enhanced the area and the rate of change in area covered by both Pollicipes and Mytilus. Preventing gull feeding enhanced Pollicipes and reduced Mytilus cover, slowing the rates of succession and reducing the frequency of disturbance. However, even in the absence of predators on Pollicipes and with an initial size disadvantage, Mytilus maintained positive rates of change in cover, probably because adult recruitment into the gap altered the initial size hierarchy of competitors. These results indicate that bird predation and initial size asymmetries can alter the dynamics, but not the eventual end point, of intertidal succession. They also highlight the importance of choosing the right comparison (differences between treatments vs. deviations in rates from 0) when testing alternative modes of succession (i.e., facilitation, tolerance, or inhibition). Although gull predation does not change the end point of succession locally, it may have important regional effects by altering the dynamics of disturbance and recovery. Such regional effects cannot be determined by experiments on a local scale.