"Diversity-Dependent Diversification in the History of Marine Animals"
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Throughout Earth’s history, the number of animal species has been in constant flux as species originate and go extinct. What are the evolutionary forces controlling the total number of species? A new study by Dr Michael J. Foote, Louis Block Distinguished Service Professor at the University of Chicago, shows that one important factor could have affected diversification and extinction rates through time: species diversity itself.
Every species fills a distinct ecological niche. When open niches are available, species may diversify and adapt to these roles. Ecosystems with many species have fewer niches available, and thus, fewer opportunities for new species to arise. This theory is applicable at a global scale, implying that throughout Earth’s history, the total number of species could have influenced the number of species originating and going extinct.
The fossil record offers a unique opportunity to test this theory with actual data across the Phanerozoic. Authors such as J. John Sepkoski Jr. and John Alroy have already looked at the fossil record of marine animals for this purpose, but researchers have not reached a consensus. In this issue of American Naturalist, Prof. Michael J. Foote revisits this long-standing theory, testing whether time periods across the Phanerozoic with high species diversity were followed by low rates of diversification and high rates of extinction, as previously theorized. “What I have done is follow this tradition, but I did not feel the questions had been answered”- said Prof. Foote.
Foote’s study focuses on the entire fossil record of marine animals, a dataset that is currently only available thanks to digital repositories like the Paleobiology database. Critical to this study is its novel approach to calculating diversity and the correction of the statistical effect known as “regression to the mean”. This is a common statistical problem: after sampling a random value and getting an extreme result, the next sample is generally more likely to fall closer average. This phenomenon could have skewed previous studies, as high diversity levels at a previous time point would normally tend to be followed by more average diversification rates. Prof. Foote says: “This was the main limitation of these previous studies […] a vexing problem that has not been addressed this way, before.”
The results of Foote’s analysis show that time periods with low diversity are indeed followed by high diversification rates, as theorized. Niches are not filled immediately, and thus, this effect can persist for a long time. On the other hand, extinction rates have not been highly influenced by global diversity, comparatively. Extinctions largely depend on environmental perturbations, and thus, the total number of niches filled may not have been a highly important factor. One unusual result is that diversity-diversification correlations lose strength after the Paleozoic (543-251 million years ago). “This is a puzzle to me, I don’t even have a speculation”, says Prof. Foote.
The results of this study further support the idea that global diversity played a major role in the total number of animal species across Earth’s history. It also highlights the value of large paleontological databases to test these questions and offers new methods to calculate diversity and reduce statistical artifacts. With these tools we can now test new questions on diversity through time, such as changes in diversity on land or even the total number of animal species that could theoretically inhabit Earth.
Abstract
By comparing detrended estimates of diversity (taxonomic richness) and rates of origination, extinction, and net diversification, I show that at the global scale over the course of the Phanerozoic eon, rates of diversification and origination are negatively correlated with diversity. By contrast, extinction rates are only weakly correlated with diversity for the most part. These results hold for both genus- and species-level data and for many alternative analytical protocols. The asymmetry between extinction on the one hand and origination and net diversification on the other hand supports a model whereby extinction is largely driven by abiotic perturbations, with subsequent origination filling the void left by depleted diversity. Diversity dependence is somewhat weaker, but still evident, if the initial Ordovician radiation or rebounds from major mass extinctions are omitted from analysis; thus, diversity dependence is influenced, but not dominated, by these special intervals of Earth history. In the transition from Paleozoic to post-Paleozoic time, diversity dependence of origination weakens while that of extinction strengthens; however, diversity dependence of net diversification barely changes in strength. Despite nuances, individual clades largely yield results consistent with those for the aggregate data on all animals. On the whole, diversity-dependent diversification appears to be a pervasive factor in the macroevolution of marine animal life.
Photo credit: Alejandro Izquierdo López
Author Bio:
Alejandro Izquierdo López is a recent graduate with a PhD in Ecology and Evolutionary Biology from the University of Toronto in Canada. His research focuses on the taxonomy, ecology and morphological evolution of arthropods, and has described several new species from the world-renowned Cambrian Burgess Shale fossil site. He is also a freelance science writer that loves topics at the intersection between sciences and culture.