Ecology of the Invasive Red

The red alga Gracilaria salicornia (c. Agardh) E. Y. Dawson was introduced intentionally to two reefs on O'ahu, Hawai'i, in the 1970s for experimental aquaculture for the agar industry. Some 30 yr later, this species has spread from the initial sites of introduction and is now competing with native marine flora and fauna. The goals of this study were to quantify various aspects of G. salicornia ecology in Hawai'i in an effort to develop control or eradication tools. Experimental plots were established to determine cover and biomass of G. salicornia per square meter and to determine the amount of time and person hours needed to remove G. salicornia from these plots. Substantial amounts of G. salicornia become dislodged from the reef during large wave events and periodically become deposited onto the beach in front of the WaikIkI Aquarium. Algal beach wash biomass was quantified and positive relationships were established between swell height and the amount of algae that washed up onto the beach in this location. We then quantified the ability of G. salicornia vegetative fragments to regrow after desiccation to determine if algal biomass stranded on shore survives the tidal cycle until being washed back out on the reef at high tide. Gracilaria salicornia was remarkably resistant to temperature, salinity, and chemical treatments examined as possible in situ control options. Herbivore preference tests showed that a native Gracilaria species is consumed far more frequently than the alien congener. Finally, large-scale community volunteer efforts were organized to remove drifting G. salicornia fragments from the reef area in front of the WaikIkI Aquarium. Over 20,000 kg of alien algal fragments were removed from this location in five 4-hr cleanup events. However, based on G. salicornia growth rates, ability to fragment, physical tolerance, and low herbivory, it is clear that a large-scale dedicated effort will be needed to control this invasive species on WaikIkI's reefs. THE LARGE-SCALE introduction of nonindigenous species is among the greatest threats to the homogenization of the world's biota and subsequent reduction in global biodiversity (Carlton and Geller 1993, Maragos et al. 1996, Ruiz et al. 1999). Hawaiian ecosystems I Funding for this research was provided by the National Oceanic and Atmospheric Administration Hawai'i Coral Reef Initiative Research Program Grant no. NA160AI449 awarded to C. L. Hunter and C. M. Smith. Manuscript accepted 27 May 2003. 2 Corresponding author: Department of Botany, Pacific Science (2004), vol. 58, no. 2:325-343 © 2004 by University of Hawai'i Press have been particularly impacted by the introduction of nonindigeous plants, invertebrates, birds, and mammals, with approximately 22% of the state's biota considered to be alien (Eldredge and Smith 2001). In many cases, terrestrial introductions have either directly University of Hawai'i at Manoa, 3190 Maile Way, Honolulu, Hawai'i 96822 (phone: 808-956-3943; fax: 808956-3923; E-mail: [email protected]). J Waiidid Aquarium, University of Hawai'i, 2777 Kalakaua Avenue, Honolulu, Hawai'i 96815. 4 Department of Botany, University of Hawai'i at Manoa, 3190 Maile Way, Honolulu, Hawai'i 96822. 5 Department of Zoology, University of Hawai'i at Manoa, McCarthy Mall, Honolulu, Hawai'i 96822. 6 Department of Oceanography, University of Hawai'i at Manoa, 100 Pope Road, Honolulu, Hawai'i