THE IMPORTANCE OF ABUNDANCE DATA FOR RECONSTRUCTING ANCIENT COMMUNITIES

One of the major goals of paleoecology is the reconstruction of community interactions. Most paleoecological studies are based only on occurrences of taxa. However diversity is only one component of a community, and data on the relative abundance of different taxa and trophic groups can broaden the scope of community reconstruction. Estimating abundance is more difficult, and can be influenced by numerous factors. Poor fossilization is a well-recognized source of error in estimates of abundance, but sampling technique also plays an important role in the accuracy of abundance estimates. We examined differences in both taxonomic diversity and abundance between surface (outcrop) samples and bulk samples collected as part of the Panama Paleontology Project. Statistical analysis on both the generic and species level show that while estimates of species diversity may be similar between surface samples and bulk samples, surface samples often underestimate the abundance of taxa. This pattern holds true for analyses of all the bivalve genera within a few samples, or for a specific group such as the scallops (Pectinidae) across all samples. These differences are not just an artifact of higher total abundance in the bulk samples. Collectors making surface samples inadvertently or deliberately focus on maximizing diversity, consequently losing the ability to accurately estimate abundance.

Other measures of abundance can provide additional ecological information. In the case of molluscs and other carbonate secreting organisms, the weight of the taxon measured as a percentage of the total weight of skeletal carbonate within a sample has important implications for energy allocation, productivity, and growth within a community. This is especially important because the most abundant taxa are typically small, whereas large taxa are commonly less abundant. Comparisons of abundance of taxa based on counts versus weights may give strikingly different results that provide additional insights into how organisms react to changing environments. However, none of these metrics can be reliably obtained from surface samples. Paleoecologists interested in community structure and interactions should use bulk samples as opposed to museum collections whenever possible.