Relationship between species diversity and reef growth in the Holocene at Ishigaki Island, Pacific Ocean

Coral reefs are influenced by global and local factors, and living corals are currently faced with a potential loss of species diversity. Knowledge of the relationship between species diversity and reef growth during the Holocene is important in terms of accurately reconstructing natural conditions prior to recent disturbances (e.g., human impact, pollution, and over-harvesting) and in predicting future scenarios (e.g., abrupt sea-level rise, coastal change, and economic services). This study seeks to characterize the Holocene and present-day reef at Ishigaki Island in the Ryukyu Islands, focusing on spatial and temporal variations in the relationship between species diversity and reef growth. The analysis is based on a drilling core obtained for the Holocene reefs and quantitative species-diversity data (Shannon and Weaver's diversity index, H′) obtained for the present-day reef. H′ was calculated for four coral communities surveyed at the Ibaruma and Fukido reefs. The Holocene sequence was dominated by the corymbose coral community (e.g., Acropora digitifera, A. hyacinthus, Goniastrea retiformis, and Platygyra ryukyuensis), yielding an H′ value of 1.6. The encrusting coral community (e.g., Echinopora lamellose and Pachyseris rugosa) showed the highest diversity at the reef (H′ = 2.2); however, this community was not one of the main reef builders during the Holocene. The massive coral community (e.g., Porites lutea and Favites chinensis) showed the lowest diversity (H′ = 0.6). It also made a minor contribution to reef building; this community appeared in a shallow lagoon once sea level had stabilized. The arborescent coral community (e.g., A. formosa and A. nobilis) was one of the main reef builders, although yielding an H′ value of much less than 1.0. Species diversity is not a prerequisite in terms of Holocene reef growth. Thus, a few species (e.g., A. digitifera, A. hyacinthus, A. formosa, A. nobilis, G. retiformis, and P. ryukyuensis) from two main reef-building communities are important for reef growth. These corals that act as reefs are characterized by high growth rates, having an upward reef growth rate of 2–12 m kyr− 1 in the Indo-Pacific during the Holocene.