COS 23-1
Small-scale patchiness dominates algal community structure on coral reefs
Quantifying variability over multiple spatial scales is a fundamental goal in ecology. Settlement, herbivory, and competition are considered important processes that influence the structure of coral reef benthic communities. Small-scale patchiness on reefs would indicate either that these processes are variable at small scales, or that additional processes structure benthic communities. On coral reefs, turf algae are the major food source for herbivores and the primary competitors with corals. They are also potentially the most widespread benthic component on coral reefs and are likely to increase in the future, because turf algae thrive under conditions that reduce coral cover.
While turfs are typically treated as a single homogenous functional group, analyzing them as a species assemblage is more informative. Here, we use a fully nested hierarchical sampling design to measure scales of variability in turf communities from centimeters (within single dead coral heads) to kilometers (across islands) on Lhaviyani Atoll, Maldives. To assess variability in these turf communities we used four metrics related to ecological function: percent cover, canopy height, taxonomic richness, and community composition.
Results/Conclusions
The smallest scale (centimeters) explained the greatest percent of variability for all metrics: 77% of variability in cover, 56% of variability in canopy height, 62% of variability in richness, and 32% of variability in community composition was explained by differences among samples. The least variability in cover, canopy height, and richness was explained by differences among sites (100’s meters). In contrast, community composition was least variable at the largest scale, among islands (kilometers).
These different spatial patterns for physical characteristics and community assemblage suggest that both small- and large-scale processes play roles in structuring algal communities on coral reefs. At scales of <100 meters, processes such as competition, predation, and vegetative growth are heterogeneous and drive variability. At larger scales, oceanography reduces variability, and the supply of algal propagules is well mixed.
The extreme small-scale variability in turf algae presents a suite of implications for how we study coral reef communities. With declining coral and increasing turf cover on reefs worldwide, coral-turf interactions and turf consumption by herbivores will become increasingly central to coral reef ecology and conservation. However, because turf communities are variable at scales smaller than a single coral colony, these interactions deserve more detailed consideration.