| dc.description.abstract | To assess the potential of herbivory in regulating the invasive green alga Codium fragile ssp.
tomentosoides, field and laboratory experiments were conducted with the green sea urchin
Strongylocentrotus droebachiensis in Nova Scotia, Canada. In the field, urchins at different
densities (0, 50 and 100 urchins m(-2)) were caged on boulders covered with a canopy of either the
kelp Laminaria longicruris, Codium, or a mixture of both species for 13 wk. In the treatment with
Laminaria only, similar to 90% of the canopy was removed within 34 and 75 d in cages with 100 and 50
urchins m(-2), respectively. In contrast, Codium cover decreased by similar to 20% at both levels of
urchin density in the treatment with Codium only, and did not differ significantly from the control
(no urchins) at the end of the experiment. In the mixed canopy treatment, urchins showed a
preference for Laminaria, consuming 90 % of kelp cover within 3 9 and 54 d (at 100 and 50 urchins
m(-2), respectively), while Codium cover increased gradually. Urchins grazed turf-forming red algae
in all treatments, although in treatments with Laminaria, intensive grazing of turf only occurred
once kelp was completely consumed. In the laboratory, urchins fed single diets of Laminaria or
Codium for 8 wk had similar grazing rates (similar to 0.20 dry weight g urchin(-1) d(-1)), while
urchins fed a mixed diet consumed 2 times more kelp (0.15 g urchin(-1) d(-1)) than Codium (0.08 g
urchin(-1) d(-1)). These experimental results indicate that urchins prefer kelp but will consume
Codium when other algal foods are not available. We predict that urchin aggregations encountering
mixed stands of kelp and Codium will initially graze the kelp and turf algae, creating patches of
Codium that ultimately will be consumed as well. | en_US |
