Geographical partitioning of marine macrophyte assemblages in the tropical Pacific: a result of local and regional diversity processes

Aim Various coral reef organisms display distinct gradients in taxonomic turnover throughout the tropical Pacific Ocean. Marine macrophytes are one of the most dominant and ecologically important benthic components of tropical reefs, yet little is known about the ecological biogeography of the macrophyte assemblages throughout this biodiverse region. This study assessed: (1) the geographical clustering of macrophyte assemblages in the tropical Pacific; (2) the environmental/geographical factors that best explain the observed patterns in taxon richness and taxon composition; and (3) the validity of large-scale biogeographical hypotheses with respect to the distribution of macrophyte assemblages. Location Coral reefs of 39 US Pacific islands. Methods Surveys of reef macrophytes for all 39 Pacific islands were conducted from 2004 to 2007. Rank-order data of quadrats were transformed to proportional abundance data in order to compute site averages for each of the 84 macrophyte categories. Further data analysis employed taxon accumulation curves, generalized additive models, and multivariate techniques. Results Pacific macrophyte assemblages displayed consistently higher within-archipelago similarities than between-archipelago similarities, which is reflected in pronounced differences in functional group composition between archipelagos. The maximum land elevation, maximum seasonal sea surface temperature, reef extent, and longitude of the investigated islands were the predictor variables that best described the similarities in macrophyte assemblage structure among islands. Maximum land elevation and reef extent, however, were the two predictor variables that best explained macrophyte richness per island. Main conclusions Macrophyte assemblages of the Pacific Islands cluster in geographical groups, indicative of the importance of evolutionary factors related to dispersal and speciation. Whereas macrophyte assemblage structure is governed by both local (habitat diversity and availability) and regional (geographical and environmental descriptors related to oceanic isolation and latitude) variables, the macrophyte richness of these islands is defined primarily by local habitat diversity and availability. Biogeographical patterns of marine macrophyte assemblages in the tropical Pacific deviate from those of other well-studied marine organisms.