Changes in benthic community composition associated with the outbreak of the corallimorph, Rhodactis howesii, at Palmyra Atoll

Abstract

Few studies have documented the spatial and temporal dynamics of highly invasive species in coral reef benthic communities. Here, we quantified the ecological dynamics of invasion by a corallimorph, Rhodactis howesii, at Palmyra Atoll in the central Pacific. A localized outbreak of this species was first observed following a shipwreck at Palmyra in 1991 and has subsequently spread across hectares, reaching 100% cover in some areas. We examined the spatial and temporal dynamics of this invasion, and its impact on the benthic community, using a combination of permanent photoquadrats and large-scale photomosaic imagery. Our data revealed two distinct patterns in the spatial dynamics of R. howesii on the reef. First, following the removal of the shipwreck in 2013, the cover of the corallimorph in the immediate vicinity of the wreck decreased markedly, with crustose coralline algae (CCA), an important reef-builder, dominating the newly available substrate. However, in contrast to the decline at the epicenter of the invasion, the corallimorph has spread to additional sites around the atoll where increases in abundance have been associated with decreases in hard coral cover. Reductions in percent cover and corallimorph patch size near the epicenter of the outbreak, coupled with increases in cover and patch size and appearance of the corallimorph at other locations around Palmyra, demonstrate the dynamic nature of this “invasion.” Further, we found that the corallimorph settled disproportionately often on patches of turf or CCA cover, but can then overgrow all benthic competitors following establishment. This study provides evidence that R. howesii has the capacity to be highly invasive on coral reefs and highlights the importance of large-scale, long-term monitoring efforts to capture the dynamic nature of such invasions.