Native urchins as potential agents of biotic resistance to the introduced alga Kappaphycus alvarezii in a tropical lagoon

Biotic resistance from native consumers can reduce the abundance and impacts of introduced species. Previous studies documented the escape of the introduced alga Kappaphycus alvarezii from abandoned farms in Bocas del Toro, Panama. Both attached and unattached aggregations of this invasive alga accumulated on and smothered native corals and seagrasses. However, native urchins and parrotfish were also observed feeding on the alga in the field suggesting that native herbivores may act as agents of biotic resistance. In this study, we conducted an herbivore-exclusion experiment in the field to determine the effect of native herbivory on K. alvarezii and a laboratory experiment to measure the rate of herbivory on the introduced alga by two common native urchins ( Lytechinus variegatus and Echinometra lucunter ). Consistent with the biotic resistance hypothesis, native herbivores rapidly consumed algae in both field and lab studies. Loss of algal biomass was approximately nine times higher in herbivore-exposed treatments than in herbivore-exclusion treatments in the field. Lab experiments revealed L. variegatus ate 3.5 times more algae than E. lucunter . While K. alvarezii was abundant in surveys during 2014, we did not detect any remaining individuals in our field sites during a return visit one year later. Thus, both native urchins consume K. alvarezii and, along with other herbivores, are likely important agents of biotic resistance. However, longer-term studies are needed to test if native herbivores can control the introduced algae still escaping from active farms.