Morphology and herbivory of Egregia menziesii at sites from California to Washington

Herbivores can drastically alter the morphology of macroalgae by directly
consuming tissue and by inflicting structural wounds.  Wounds, in
particular, can result in large amounts of tissue breaking away from
macroalgae, amplifying the damage initially caused by
herbivores.  Herbivores that commonly wound macroalgae often only
occur over a portion of a macroalga’s lifespan or geographic
range.  However, we know little about the influence of these
periodic or regional occurrences of herbivores on the large-scale seasonal
and geographical patterns of macroalgal morphology.  We used the
intertidal kelp Egregia menziesii to investigate how the kelp’s morphology
and the prevalence of two prominent kelp-wounding herbivores (limpets and
gammarid amphipods) changed over two seasons (spring and summer) and over
the northern extent of the kelp’s geographic range (six sites from central
California to northern Washington).  Wounds from limpets and
amphipods often result in the kelp’s fronds being pruned (intercalary
meristem broken away), so kelp morphology was quantified as size (combined
length of all fronds) and pruning (proportion of broken fronds). 
In both seasons, limpets were the dominant herbivore at southern sites
while amphipods were dominant at northern sites.  This pattern
was likely driven by each herbivore species’ unique response to local wave
action, temperature, and tidal regimes.  Within each season, most
kelp had a similar morphology and collective herbivore prevalence (limpets
and/or amphipods).  Our results suggest that large-scale
geographic similarities in macroalgal-wounding, despite regional variation
in the herbivores inflicting the wounds, can maintain similar macroalgal
morphologies over large geographic areas.