The impact of transplanted sea urchins on alien and native flora

Abstract

After fish, urchins are the most conspicuous herbivores on coral reefs in Hawai'i, as elsewhere. In Kane'ohe Bay, O'ahu, the native urchin Tripneustes gratilla is present in small numbers, but is easily transplanted to patch reefs with high algal cover. Because it readily consumes several of the invasive algae in the bay, it has been proposed as a biological control agent for these algae. To determine its usefulness as a control of invasive algae and to examine its impact on the native benthic invertebrate and algal communities, urchins were placed in 1m^2 cages on the reef flat in varying densities and the substrate composition was monitored over time. After three months, high densities of urchins (6 urchins m^-2) were able to significantly reduce high cover of Gracilaria salicornia and the medium density of urchins (3 urchins m^-2) were able to further reduce and control the invasive alga in sites where it had previously been brought down manually. Background herbivory by fishes was not able to reduce or control G. salicornia, even in sites where the algal cover had been reduced manually first. Fish, but not urchins, seem to have an effect on the community composition of native turf algae. On reefs where invasive algae competes with corals, transplanting urchins to that coral-algal interface holds promise for controlling and reducing algal cover, especially in cases where algal biomass is physically reduced first.