Tracing an Invasion Paradox Across Scales: Patterns and Tests for the Effects of the Introduced Predatory Grouper, Roi (Cephalopholis Argus) in Hawaii
Theory, observation, and experimental studies in invasion ecology have led to what is known as the ‘invasion paradox’, where both the factors that determine the invasability of ecosystems, and the invasiveness of species are context specific. As the patterns observed and the underlying processes are sensitive to the extent and resolution of inquiry, paradox can best be understood by tracing patterns and processes across scales. In Hawai‘i, a mid-sized predatory grouper, roi (Cephalopholis argus) was introduced during the 1950s, and subsequently established and spread throughout the main archipelago. Yet, the seascape factors that drive their distribution, a determination of their impact on the native reef fish assemblage, and methods for assessing and managing roi populations, were previously unknown. To address this gap in knowledge, I conducted studies of roi in Hawai‘i at three levels of organization: 1) field observations at the population level; 2) field manipulative experiments at the community level; and 3) species distribution modeling at the seascape level. I trace salient factors of roi invasiveness and community invasability across the three scales, and relate these to the human social system, as the roi introduction effects, and is affected by human communities. I found that with low population mortality rates, introduced roi has the potential to be an effective invader. Yet, over a long-term predator removal experiment, roi had no effect on the abundance of their prey. Likewise, in the seascape context, populations of roi declined in relation to increasing densities of native fish species. In the broadest sense, this introduced species has inspired community-led conservation action in Hawai‘i through roi fishing tournaments and thus, roi present an opportunity to engage across sectors and strengthen collaborative ocean management.