GLOBAL RESPONSE OF BETA DIVERSITY TO PULSES OF ORDOVICIAN-SILURIAN EXTINCTION

Fossil deposits that record the biogeographic fabric of extinctions are potentially invaluable to interpreting the consequences of present-day and future ecosystem collapse. In particular, beta (between site) diversity describes turnover in species composition across a wide range of temporal and spatial scales, and underpins much of conservation theory and practice. Two questions that have yet to be rigorously examined using the fossil record, and yet have immediate application in informing conservation efforts, are: 1) what are the effects of extinction on beta (between site) diversity? And, 2) are these effects consistent across extinction events? We test these hypotheses by examining the changing contributions of alpha (local) and beta (between-site) diversity to overall changes in generic richness (gamma diversity) across both pulses of Ordovician-Silurian mass extinction, using a large georeferenced database of brachiopod occurrences from the Paleobiology Database (PBDB). We also furthermore reconstruct geographic ranges for well-sampled brachiopod genera over this interval, in order to assess the contributions of range size expansion and contraction on changing patterns of beta diversity. We find: 1) the two pulses of extinction had opposite effects on beta diversity – the first pulse produced a global decrease, while the second pulse resulted in a dramatic increase; 2) decreased beta diversity in the Hirnantian is primarily driven by a small number of genera, which undergo dramatic range expansion; 3) if these wide-ranging genera are excluded, regional disparity in the Hirnantian changed relatively little; and, 4) these patterns likely reflect a combination of extinction dynamics, and eustatic sea-level rise and fall. We also find that changes in beta diversity over the two pulses of extinction are consistent with changes in reconstructed geographic range size for studied taxa; intervals of high beta diversity are correlated with small geographic ranges, and intervals of low beta diversity are reflected in larger ranges. In a conservation paleobiology context, these data suggest that the warning signs for impending ecological crisis may not be changing taxonomic disparity among regional assemblages, but rather the appearance of wide-ranging invasive species and ecological ‘opportunists’.