SPATIAL VARIATION IN ENVIRONMENTAL AND FAUNAL GRADIENTS AS A SOURCE OF HIDDEN BETA DIVERSITY

Faunal variation along onshore-offshore or water depth gradients is thought to be a major source of beta diversity that contributes to global diversity. Nonetheless, in Sepkoski’s classic study of alpha, beta, and gamma diversity, he found that, after accounting for a 50-70% increase in alpha diversity, increasing beta diversity could only explain another 50% of the 300% increase in global diversity resulting from the Ordovician radiation. He argued for hidden sources of beta diversity to explain the rest. Sepkoski collated assemblages from across North America and arrayed them into a simple onshore-offshore scheme, thus masking any spatial variation in diversity across the continent. We argue that spatial variation in depth-related faunal gradients, both within and between depositional basins, may be a major contributor to Sepkoski’s hidden beta diversity. Comparing faunal gradients requires collecting faunal data from detailed measured sections to establish a well-constrained environmental framework. Differences between faunal gradients suggest environmental limits to dispersal of taxa, even within depositional basins. Comparing niche parameters of taxa between different geographic areas can also determine how well niche parameters are conserved. We illustrate our main point with two intrabasinal and one interbasinal comparison of faunal gradients from the Ordovician and the Devonian. We find that the taxonomic composition of faunal gradients varies even within depositional basins. Preferred environment is often conserved between faunal gradients, but other niche characteristics like depth tolerance, peak abundance, and occupancy are not conserved. These observations on faunal gradient variability suggest that environmental heterogeneity at this scale can be a significant source of beta diversity.