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Patterns and processes of species diversification after mass extinctions: A case study on Triassic Pectinoidea (Mollusca, Bivalvia)

Final Activity Report Summary - TRIASDIVERSITY (Patterns and processes of species diversification after mass extinctions: A case study on Triassic Pectinoidea (Mollusca, Bivalvia))

Understanding how life diversified in the geological past is a key to predict future changes in biosphere. In order to reveal general insight into the controls of global biodiversification, we reconstructed global species diversification of pectinoid bivalves ('scallops') during the Triassic period (250-200 million years before present), which followed the greatest mass extinction event in Earth's history.

We found that the number of pectinoid species increased exponentially during the initial 35 million years after the extinction event, which suggests that global diversity was not damped by competition for a considerable period of geological time. Rather, the exponential diversity increase supports a prediction of so-called expansion models of biodiversification, which emphasise the positive role of biotic interaction on the number of niches and hence the number of species that an ecosystem carries. However, the number of pectinoid species within habitats (alpha diversity) showed a much shorter increase (ca. 6 million years) and then remained more or less constant until the end of the period. This is a prediction of equilibrium models of biodiversification, which postulate the prevalence of competition in the biotic interaction between organisms, posing limits on carrying capacity of ecosystems.

The contrasting pattern of global and alpha diversity in the Triassic expansion of pectinoid bivalves suggests that equilibrium models of biodiversification might explain diversity patterns at the level of local ecosystems but can probably not be projected to the global scale. Here, differentiation between habitats (beta diversity) and biogeographical regions (gamma diversity) plays a key role, on which competition between groups with similar ecological demands has little effect. Towards the end of the Triassic period, the number of known pectinoid species declined, which probably reflects the combined effect of a poorer fossil record and worsening environmental conditions that finally culminated in the end-Triassic mass extinction event.