Measuring relative abundance in fossil and living assemblages

Paleontologists increasingly appreciate the importance of studying the ecological context of fossil species and communities. Measuring abundance is a vital component not just for describing this context, but also for evaluating biases related to preservation and sampling and for estimating species richness (Jackson et al. 1999; Jackson and Johnson 2001; Kidwell 2001). Our purpose here is to identify a previously unrecognized problem that could lead to incorrect interpretation of observed patterns of abundance. Understanding ancient and living communities and ecosystems requires intimate knowledge of their members. We need to know who the members are, how many of them there are, how they interact, and how they collectively forge a workable, durable whole. Not all species contribute equally to ecosystems. Although some rare species have disproportionate effects because of their very high per capita power-large warm-blooded vertebrates come to mind as examples-many others are what Birkeland (1996) referred to as ornaments, pleasing and interesting but economically of little consequence. Abundance is one measure of a species' success. If a species is common, its characteristics-individual features as well as the attributes of populationsmust be responsible for its success. Jernvall and Fortelius (2002), in fact, applied this rationale to show that abundant and widespread species of Miocene mammals in Europe typically show a trend toward hypsodonty (the development of high-crowned cheek teeth), whereas rare ones often do not. They argued that abundance is a legitimate measure of suc