GEOLOGICAL AND CLIMATIC CHANGE DEFINE ECOLOGICAL LIMITS TO SPECIES DIVERSITY

Biotic and abiotic factors both act to regulate biodiversity in deep time, but their mode of action is hard to discern because geological, biological and climatic changes are not independent. This challenge is complicated further by the different dynamics that biotic competition can generate. Here, using Cenozoic macroperforate planktonic foraminifera as a case study, we compare the statistical evidence for different forms of biotic competition. The models are drawn from population ecology, encoding zero net change at high species abundance, as Sepkoski conceptualised, as well as alternative forms for negative and positive (but decreasing) net change under the same circumstances. We find clear evidence for a finite upper ecological limit to species diversity that varies systematically with geological and climatic change. In the best-supported model, a temperature/ice volume proxy affects the per-lineage diversification rate, while the number of siliceous and carboniferous rock packages affects the upper limit to species richness. The latter dependence is unlikely to be a sampling effect in such a well-sampled clade, so probably reflects “common-cause” processes driving both biological evolution and geological sedimentation. These models emphasise how the interrelated climatic, geological and biological system provides a dynamic, but bounded, arena for biotic competition among species.