Mechanisms of Competitive Dominance Between Crustose Coralline Algae: An Herbivore‐Mediated Competitive Reversal

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Abstract
Competitive superiority among encrusting species is established when the margin of one species consistently overgrows another. By measuring overgrowth patterns for the two most abundant tidepool species of crustose coralline algae (Lithophyllum impressum and Pseudolithophyllum whidbeyense) in the San Juan Islands of Washington State, we documented a reversal in competitive dominance that occurs at about the +1 m level; L. impressum wins in upper zones and P. whidbeyense in lower zones. Regardless of tidepool elevation or species, thicker crusts overgrow thinner ones and crusts of equal thickness are competitively equal. Both crust species are grazed by limpets (primarily Lottia pelta and Tectura scutum), which are the dominant herbivores in tidepools. Laboratory experiments show that limpet grazing can reduce crust thickness. Laboratory and field analyses of limpet grazing indicate that corallines in tidepools in the upper intertidal zone are subject to high frequency (bite rate per unit area) and low intensity (penetration depth per bite) limpet grazing, whereas those in low tidepools are subjected to opposite grazing characteristics. L. impressum is a thick crust, which has a multicellular covering over its meristem. This covering protects the growing portion of the plant from the frequent, low—intensity herbivory that occurs in the higher tidepools. In contrast, P. whidbeyense lacks a multilayered epithallus and is more susceptible to injury in high tidepools. In low pools, meristems of both crusts are injured. L. impressum heals these deep wounds by regenerating vertically, but the net result is a thinner plant that is easily overgrown. P. whidbeyense dominates the low zone because it is capable of much more rapid lateral growth including over its own deep wounds; the crust overgrows itself, thereby developing a thicker thallus and improving its competitive success over L. impressum. We suggest that a continuum of defensive and regenerative characteristics exists among organisms; the adaptive nature of these depends on the levels of production and the frequency and intensity of disturbance in their environment. It is evident that herbivory is important in this system, not by the traditional mechanism of releasing limiting resources (e.g., primary space), but by mediating competitive abilities through the relative susceptibility to, and recovery from, herbivory—induced disturbance.

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