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Abstract
This paper compares the relative contributions of within-habitat diversity [alpha-diversity] and between-habitat-diversity [beta-diversity] to regional diversity [gamma-diversity] in marine benthic communities of the western US before and after the end-Permian mass extinction. We found that presumably cool-water faunas from the Permian Gerster Limestone and the Park City Formation had low alpha- and beta-diversities, comparable to those of low diverse faunas of the Early Triassic. In contrast, tropical Permian faunas had much higher alpha-diversities and a variable pattern of beta-diversity: Whereas faunas of space-limited bioherms show a positive correlation between beta-diversity and gamma-diversity, beta-diversity in level-bottom faunas is elevated only when gamma-diversity is very high (>250 species). This contrasting pattern probably reflects differential effects of interspecific competition on habitat partitioning. In low-competitive level-bottom faunas, species are able to coexist until competition forces species into their ecological optima, thereby increasing beta-diversity. This effect occurs at much lower gamma-diversities in more competitive reef-bound faunas, causing the observed positive correlation between beta- and gamma-diversity. We suggest that differences in the level of interspecific competition and hence diversity partitioning between Permian and Triassic benthic communities result from the higher average metabolic rates in the Mesozoic mollusc-dominated benthos in contrast to their Permian counterparts.
Acknowledgements
R.H. is particularly indebted to the Jim Alan Family (Diamond 4 Ranch, Wyoming) for their support in recovering my stranded vehicle near Sheep Mountain. Ruby and Jim Jenks (West Jordan, Utah) are thanked for their warm hospitality and providing a home base for the field campaign conducted by R.H. The field localities visited for this study are located on US public land under the stewardship of the Bureau of Land Management (BLM) of the US Department of the Interior; their management and access to this land are greatly acknowledged. We thank Historical Biology editor Gareth Dyke, and the valuable comments from two anonymous reviewers.
