Global, Regional, and Local Patterns in Species Richness and Abundance of Butterflyfishes

Abstract

Butterflyfish communities vary in richness longitudinally from highs of >30 species to lows of 2–3 species. We investigated this gradient because it provides an opportunity to address some basic questions relating to determinants of community structure, e.g., are richness and abundance functions of geography or of local conditions? How are richness and density of species populations related? Do community-level processes, such as competition, determine community structure? In a broader sense, to what extent are community properties internally deterministic as opposed to being functions of higher-level processes? From 1981 through 1997 we counted kinds and number of butterflyfishes on over 2000 10-min transects distributed from the eastern Caribbean to the western Indian Ocean and Red Sea. We grouped transects at the same site into stations, stations in the same local area into islands, and islands into 18 geographic regions, thus organizing the data into four spatial levels. For each station, extent of live coral cover and degree of open-ocean influence were estimated. Richness is highest in the Philippine–Bornean–New Guinean region and declines radially from that center. Total number of individual butterflyfish also declines over that gradient. On more local scales, increased open-ocean exposure and live-coral cover enhance both richness and numerical abundance. Because species richness declines more rapidly with distance from the richness center than does abundance, across the same gradient average abundance per benthivore species rises. In peripheral, species-poor reaches of the Indopacific, the Eastern Tropical Pacific, and the Caribbean the communities are dominated by one or a few very common species. Butterflyfishes respond to changing richness of their communities by adjusting the abundance of individual species. Use of space by individual species decreases with increasing richness. Global richness is determined chiefly by geography; within local areas local factors are more important. Community structure is predominantly organized by richness, hence first by geography and then by habitat characteristics.