Quantifying scales of spatial variability in algal turf assemblages on coral reefs

Abstract

Quantifying variability over multiple spatial scales is a fundamental goal in ecology,providing insight into which scale-dependent processes most strongly influence community structure.On coral reefs, the ubiquitous turf algae are the primary food source for herbivores and competitorsfor space with corals. Turf algae will likely increase in the future, because they thriveunder conditions that reduce coral cover. Turfs are typically treated as a single homogeneousfunctional group, but analyzing them as a variable assemblage is more informative. We used ahierarchical sampling design to quantify 4 scales of variability in turf assemblages from centimeters(within single dead coral heads) to kilometers (across islands) on the rarely studied LhaviyaniAtoll, Maldives. We used 4 metrics, each reflecting different ecological processes: percent cover,canopy height, richness, and assemblage composition. For most of these metrics, variability wassignificant at multiple spatial scales. However, for all metrics, the smallest scale (centimeters)explained the greatest proportion of overall variability. The least variability in cover, canopyheight, and richness occurred among sites (100s meters), suggesting that processes such as competition,predation, and vegetative growth are heterogeneous at small scales. In contrast, assemblagecomposition was least variable at the largest scale (kilometers), suggesting that oceanographicprocesses or a well-mixed propagule supply reduce variability. With declining coral andincreasing cover of turf on reefs worldwide, it will become increasingly important to understandthe dynamics of coral−turf competitive interactions. However, because turf assemblages arehighly variable at small spatial scales, these interactions require more detailed consideration.