Comparing taxonomic and functional change through time in terrestrial mammal paleo-communities in Pliocene-Pleistocene East Africa
Recent studies have observed that taxonomic turnover (across space or through time) is higher than its functional counterpart with the latter reflecting ecosystem processes and stability. However, it is well known that ecological patterns and processes change across spatial and temporal scales. Given the current biodiversity crisis, one might be more interested in larger-scale turnover processes such as speciation, extinction, and range shift responses to global climate change. We take advantage of the larger time scale afforded by the fossil record and compare taxonomic and functional temporal turnover in the Pliocene-Pleistocene (4 – 1 million years ago) mammalian fossil record of East Africa at four sites: Koobi Fora, West Turkana, Omo, and Hadar. Taxonomic diversity was analyzed at the species level, and functional diversity was operationalized as unique category combinations of body size, diet, habitat preference, locomotor behavior, and digestion mode for each species. We looked at patterns of turnover across abundance classes using Hill numbers (alpha diversity) and Sorenson, Horn, and Morisita-Horn similarity measures (beta diversity).
Taxonomic alpha turnover through time was higher than functional alpha turnover, reflecting what is found in modern systems. In common species and functional groups, alpha turnover was lower with taxonomic and functional trends more tightly coupled (higher r2) compared to their rarer counterparts. Functional beta diversity through time was always lower than that of taxonomic beta diversity, which corresponds to the alpha diversity results but also takes taxonomic/functional composition into account. Comparing beta diversity across abundance classes was more complex but during periods of high turnover coincident with basin reorganization and major habitat changes, common species and functional groups experienced higher turnover than rarer groups. These results demonstrate an interaction between the core-transient species and functional trait paradigms. Previous researchers have shown abundant core species are more affected by local environmental conditions and density-dependent processes, whereas regional processes influence rare transient species. This study shows this pattern is reflected in the taxonomic and functional trait structure of communities on 104-5 year time scales. These results are important for understanding the nuances of community assembly processes and informing conservation practices at larger time scales with future biodiversity in mind.