Fine-scale effects of sedentary urchins on canopy and understory algae

Abstract

The effect of sea urchins on algae has often been studied when sea urchins forage over areas to create or maintain urchin barrens. The effect of sea urchins that are not associated with barrens, however, is not so clear. This study tested the hypothesis that sedentary and largely site attached sea urchins (Heliocidaris erythrogramma) have detectable effects on the covers of canopy-forming and understory algae, whether that be due to their mere presence or grazing. Algal cover was estimated beneath and immediately surrounding urchins in a location where they do not create barrens but attain densities at the upper end of their local range. The area immediately beneath the urchins was mostly devoid of visible algae. Experimental removal of urchins over 3years revealed that canopy-forming algae were able to develop in more extensive covers to match those in areas naturally lacking urchins. In the presence of urchins, the immediate area (i.e. first 5–6cm from urchins) was primarily comprised of turf-forming algae, thereafter the cover of canopy-forming algae sharply increased until at ~8cm their cover matched areas without urchins. These data support the model that composition and cover of algae can be altered in the presence of non-barren forming urchins, albeit only within a few centimetres of individual urchins. Where populations of sedentary urchins are dense and widespread (e.g. sheltered coasts for H. erythrogramma) such fine-scale effects may repeat over large areas such that urchins do not need to be associated with barrens before their effects are detectable.