RELATIVE ABUNDANCE DISTRIBUTIONS OVER THE PHANEROZOIC: EVIDENCE FOR ELEVATED COMPLEXITY OF POST-PALEOZOIC MARINE ECOSYSTEMS

Many workers posit that marine ecosystem complexity increases after the Paleozoic, both from increasing numbers of common ecotypes and also by increasing numbers of high metabolism organisms that directly affect their environment. Ecological theory holds that the complexity of an ecosystem should affect relative abundance distributions (RADs) of species within a community. Simple models where species simply partition ecospace on a “first come, first serve” basis predict a simple exponential decay, and thus geometric RADs. Another simple model where extinction, origination and emigration rates alone affect abundances predicts a zero-sum multinomial RADs. Alternatively, species might directly affect ecospace through niche construction or by creating ecological opportunities for additional species. These models predict either Zipf or log-normal RADs. Even if species do not affect their environment greatly but there are multiple distinct ecospaces, then the Central Limit theorem predicts that the mixing of independent distributions will yield a log-normal RAD overall. We test this possibility using 1176 fossil collections from Paleobiology Database (http://paleodb.org), each of which has 10+ species and 100+ specimens, and which span the Cambrian through the Cenozoic. We determine best-fit geometric, zero-sum multinomial, log-normal and Zipf RADs for each locality based on the probability of observed numbers of species with 1, 2, 3, etc. specimens given the sample size. We then use Akaike's Weights to assess whether the information of the best “simple” models (geometric + zero sum multinomial) is significantly greater or less than the information of the best “complex” models. This shows a marked shift from a near 50:50 split of “simple” and “complex” RADs in the Paleozoic to a near 3:1 ratio of “complex:simple” RADs in the Meso-Cenozoic. This transition coincides not with any major change in the way fossils are preserved or collected, but with a shift from communities dominated by sessile epifaunal suspension feeders to communities with elevated diversities of mobile and infaunal taxa. This suggests that the end-Permian extinction permanently altered prevailing marine ecosystem structure and precipitated high levels of ecological complexity and alpha diversity in the Meso-Cenozoic.