Ecologically diverse clades dominate the oceans via extinction resistance

Ecologically diverse clades dominate the oceans via extinction resistance

Matthew L. Knope, Andrew M. Bush, Luke O. Frishkoff, Noel A. Heim, and Jonathan L. Payne

Summarized by Anna Geldert

What data were used? Researchers examined taxonomic data of marine organisms across 444 million years of geologic time. Taxonomic data relates to the level of biodiversity of organisms, and classifies them under different evolutionary categories (domain, kingdom, phylum, class, order, family, genus, and species). On the whole, this study examined 19,992 genera (species groups) from the fossil record and 30,074 genera of living marine species..

Methods: This study examined speciation (origination) and extinction rates of marine species over the past 444 million years. Speciation refers to the evolution of new species, while extinction occurs when a species dies out; both factors impact the overall level of biodiversity. Net diversification rates (i.e., the difference between speciation and extinction rates) were calculated for each period  of geologic time. Additionally, researchers graphed a relationship between the species richness and ecological diversity at different points in geological time. Species richness refers simply to the number of species in a group, while ecological diversity indicates the number of “modes of life” present, such as varying habitats, levels of mobility, and feeding methods.

Results: An examination of the fossil record found that a high biodiversity among species groups could be reached in two primary ways: firstly, by a relatively short period of high speciation, and secondly, by a gradual increase over time due to average speciation and low extinction. While the first category tended to reach high biodiversity faster, they were more vulnerable to mass extinctions than the second group. Most species groups alive today, therefore, evolved via the second route. With respect to the relationship between species richness and ecological diversity, this study found a positive correlation between the two factors, meaning that a variety of life modes can be tied to having more species. 

The figure compares ecological diversity and species richness over the past 444 million years of geologic time. Species richness is graphed as the log10 of the number of genera on the x-axis, while ecological diversity (in log10 of the number of modes of life) is on the x-axis. The x-axis spans from 0 to 4 in increments of 1, while the y-axis spans from 0.0 to 1.5 in increments of 0.5. Several slopes in different colors are shown, with a legend indicating the geologic time to which the slope corresponds. The geologic stages of time included are: Silurian to Devonian (443.4 to 358.9 million years ago), Carboniferous to Permian (358.9 to 252.2 mya), Triassic (252.2 to 201.3 mya), Jurassic to Cretaceous (201.3 to 66.0 mya), and Paleogene to Neogene (66.0 to 0.0117 mya). The slope of the modern relationship between species richness and ecological diversity is also shown. Slope values range from approximately 0.20 to 0.32 and appear generally to increase steadily over time, with some overlap between geologic stages. The modern slope is approximately 0.30, and lies in the middle of the range of slope values for the Paleogene to Neogene category.
Fig 1. Relationship between species richness and ecological diversity of marine species from 444 million years ago to present.

Why is this study important? The results from this study reveal that, in the long run, rapid diversification within a species group is not sustainable because the majority of this species group is likely to be wiped out during a mass extinction event. On the other hand, gradual diversification in species groups that are able to survive mass extinctions is a more probable explanation for modern levels of marine biodiversity. These species were most likely able to survive mass extinctions due to higher levels of ecological diversity, a theory which would also explain why ecological diversity has been increasing compared to species richness over more recent eras. This study is important because it calls into question an accepted theory that directly links ecological diversity to speciation rates. While the results from this study likewise recognizes a correlation between these factors, it also implies that the relationship between the two factors may be more complex. It is only because species groups with high ecological diversity were able to survive mass extinction events that this correlation is seen so clearly today.

The big picture: This study is important in the larger field of evolutionary ecology because it impacts our understanding of how species evolve and respond to extinction pressures over time. Researchers should not assume that the tight correlation between species richness and ecological biodiversity implies a direct causational relationship, because as this study reveals, in many cases the relationship is more complicated than that. Further research is needed to fully analyze the role that ecological diversity plays in survival of mass extinctions.

Citation: Knope, M. L., Bush, A. M., Frishkoff, L. O., Heim, N. A., & Payne, J. L. (2020). Ecologically diverse clades dominate the oceans via extinction resistance. Science, 367(6481), 1035–1038.


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