DEAD OR ALIVE, (MESH) SIZE MATTERS

Since the 1950s, marine benthic biologists have explicitly recognized the effect of mesh size on ecometrics: as finer sieves are used additional species are encountered, and with these additions species can shift from relative dominance to rarity and vice versa. Mesh-size thus affects the known composition, richness, and structure of a given fauna, with implications for comparisons over space and time. This effect has been rediscovered – and largely ignored – by each generation of biologists, who still have no mesh-standards for bio-surveys; except for microfossil specialists, paleontologists have been largely oblivious to these pitfalls. Moreover, despite four decades of taphonomic studies demonstrating that large specimens are more durable than small, and that damage levels are highest among large shells, the sensitivity of the quality of paleoecologic data to mesh-size remains unexplored.

Based on meta-analysis of 85 molluscan datasets, I find that live-dead agreement in species composition, richness, spatial distribution, and rank-order is very good, but declines as finer sized specimens are included in the analysis. In the number of ecometrics affected and in the degree of taphonomic offset, mesh-size is in fact more important than sample-size, sediment grain-size, and environment. The critical threshold for molluscan data lies between 1 and 2 mm: at 2 mm and coarser, most specimens are late juveniles and adults, which are ecologically more stable and taphonomically more durable than the larvae and early juveniles that can dominate sieves 1 mm and finer. Ongoing work on species’ proportional abundance and evenness (with T. Olszewski) suggests an opposite effect for these metrics – less live-dead discrepancy in fine- than in coarse-mesh datasets – consistent with the lower durability, shorter taphonomic half-lives, and presumably less time-averaging that characterize small shells. Thus, although there are many pros and cons to establishing standard sizes for paleobiologic analysis, and the taphonomic effects apparently vary among metrics, mesh choice clearly matters: whether your fauna are dead or alive, document – and think about – your mesh size.