LONGITUDINAL TRENDS IN THE EVENNESS OF BIVALVE ASSOCIATIONS ACROSS PACIFIC ISLAND REEFS

Bivalves are important to paleontologists because they are one of the most dominant groups of macro-organism found in the fossil record. Consequently, their evolutionary history has contributed significantly to geologists’ understanding of past climate and geography. Although extremely high bivalve diversity in the modern Indo-Pacific is well documented, certain aspects of this diversity are still poorly known, and some localities display unique patterns in bivalve community compositions due to historical factors and local environmental variables.

Using bulk sampling of fore reef sediments, we estimated bivalve diversity from Indo-Pacific island localities along a longitudinal transect. These samples were obtained from comparable latitudes and reef sub-environments, but varied in longitude. In order to ensure that both small and large bivalve specimens were collected, the sediment was sieved into three size fractions and bivalves were picked proportionally from each size fraction.

The samples not only differ in species richness, showing the well-documented trend of declining species richness eastward away from the Indo-Pacific biodiversity hotspot, but they also vary considerably in evenness. For example, a sediment sample from the Gilbert island archipelago contains over 70 bivalve species, with specimen abundances being relatively evenly distributed across species. In contrast, a sample from the Palmyra fore reef (of the Line island archipelago) contains only ~50 bivalve species and is dominated by two species: a lucinid bivalve Ctena bella, and a giant clam Tridacna maxima.

This difference in species dominance patterns across the sampled sites points to the magnitude of the underlying complexity of the Indo-Pacific biodiversity hotspot. Both historical and local factors may have played a role in shaping these patterns, and need to be taken into account in conservation paleobiology studies in which modern ecosystems are used as a benchmark for past ecosystem biodiversity.