How fast can life recover after a mass extinction?

Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction

Christopher M. Lowery and Andrew J. Fraass
Summarized by Adriane Lam

The Problem: There is no doubt that species today are going extinct due to human activities, such as habitat loss, climate change, and the introduction of invasive species that take over areas. For example, the Florida Panther used to range throughout the southeastern U.S., but due to humans expanding into their habitat, they now only occupy a mere 5% of their former range. Polar bears are also facing loss of their habitat due to melting ice and snow caused by human-induced warming. But if humans were to disappear tomorrow, how long would it take for Earth’s flora and fauna to bounce back to the number of species before humans were here? This is a hard question to answer, but to begin to quantify this, paleontologists can use the fossil record.

In this study, the scientists looked at the time before, during, and after the end-Cretaceous mass extinction event that took place ~66 million years ago. This was one of the largest mass extinction events in Earth’s history, where about 75% of all species on Earth went extinct, including the non-avian dinosaurs. It is also an important mass extinction event to study because the event that wiped out all those species was very rapid. During other mass extinction events in Earth’s history, the extinction events themselves took on the order of millions to hundreds of thousands of years (read more about extinctions here).

An overly-dramatic image of dinosaurs during the end-Cretaceous mass extinction about 66 million years ago.

The rate at which humans are altering the Earth today is unprecedented to any climate change event in the geologic past (see our ‘CO2: Past, Present, & Future’ page for more details). Thus, scientists need to compare the rate at which we are losing species today under a very fast climate change scenario to another event that was also very fast. Therefore, studying the end-Cretaceous mass extinction is particularly valuable: it was a very quick event and one that is most comparable to the rate at which we are losing species today.

Data Used: In this study, the scientists used the fossil record of planktic foraminifera to see how long it took for life to recovered after a mass extinction event, the end-Cretaceous mass extinction. Planktic foraminifera are single-celled protists (not animals) that live in open-marine environments. They have occupied our oceans for the past ~165 million years. These protists produce a calcium carbonate (same material seashells are made of) shell, or “test,” that grows to be about the size of a grain of sand. When the foraminifera die, its shell sinks to the seafloor. Over millions of years, these shells are preserved at the bottom of the ocean. Making the fossil record of planktic foraminifera an archive of extinction and evolution events for the past 165 million years of Earth’s history! The scientists who conducted this study used this amazing fossil archive to see how long it took these marine protists to return to pre-extinction levels after the end-Cretaceous mass extinction.

Figure 1. The number of planktic foraminifera species (=diversity estimate) from 80 million years ago to 50 million years ago. The end-Cretaceous mass extinction occurred at the end of the Cretaceous, and can be identified by the major drop in diversity at this time. ‘Throw back’ indicates species that survived the mass extinction event and gave rise to new species; ‘Spinose’ indicates species of foraminifera that evolved to have spines; and ‘Symbiont bearing’ indicates species that have photosymbionts (algae that live on the spines of certain foraminifera).

Methods: As part of his master’s project, Andy Fraass compiled a database of first and last occurrences of planktic foraminifera species. First and last occurrence datums are often used in paleontology to examine how long in geologic time a species existed. The authors used these data to examine when planktic foraminifera species evolved and went extinct (Figure 1).

This dataset collected, but left unpublished until this paper, also included measurements of the species’ tests, such as the number of chambers in the shell, how quickly the chambers expanded from the earliest chamber to the last, etc. From these measurements, the authors calculated test complexity. This is a metric that shows how ‘complex’ planktic foraminifera shells became through time. For example, a species with a simple shell might have simple chambers arranged in a spiral pattern. A more complex species might have a more extreme test (Figure 2).  The test complexity of each species was then given a score, with 1 being the simplest, and 4 being the most complex or extreme.

In foraminifera, the shape of the test can be assumed to have some sort of relationship to the organism’s life strategy, or its niche, basically. A species’ niche is where and how it can live and interact with the environment. For example, humans occupy a broad range of niches: our technology allows us to live in very hot to very cold climates. On the other hand, polar bears have a very narrow niche. These animals only live in the tundra biome in the Northern Hemisphere on the ice and hunt seals. A specific foraminifera might only live at a particular depth in the ocean, or in water that’s above or below a certain temperature, or in regions with a certain abundance of food, etc. These niches are the scaffolding on which species diversity is built.

During a mass extinction event niche spaces are often completely disrupted or destroyed along with the species that occupy them. Thus, paleontologists have hypothesized that after an extinction event, the number of species cannot simply bounce back to what it was before, but the number and size of niche spaces has to be rebuilt first. This may cause the observed delay in the recovery of species after mass extinction events. This paper provided the first test of that hypothesis with real data.

Figure 2. Some examples of a morphologically simple (images A and B) and complex (images C and D) planktic foraminifera. For the simple forms, A is Muricohedbergella monmouthensis, B is Muricohedbergella holmdelensis. Notice that these shell are very simple, with chambers added to the test in a spiral pattern. The complex forms include C, Hantkenina alabamensis and D, Morozovella velascoensis. Both of these species start out with smaller chambers in their shells, with larger and more complex shaped chambers added. In addition, Hantkenina alabamensis (C) also has very prominent spines that jut out of the test. All images from pforams @ mikrotax.

Because the shape and characteristics of a planktic foraminifera’s shell is related to its niche, the authors used average test complexity of all the foraminifera that were alive at different points in time to reconstruct how many niches were occupied by forams before and after the end-Cretaceous mass extinction. Higher average complexity suggests a wider variety of niches were occupied, while lower complexity suggests that fewer niches existed.

Results: The authors found that there was a huge drop in the number of species after the end-Cretaceous mass extinction (Figure 1), which was not a surprise and something we have know about for a while. But the other finding was that along with a huge drop in the number of species was also a huge drop in the test complexity (Figure 3). It took about 5 million years for test complexity to reach the levels it was at before the mass extinction event. That’s a really long time!

Figure 3. The results of the test complexity index (TCI) plotted against time. The horizontal black lines indicate how long each planktic foraminifera species lived through time, and their position on the y-axis (TCI) indicates how complex their test was. The red line is the median of this data, and the black line is the mean. At the end-Cretaceous mass extinction, both the red and black lines drop, but then begin to increase slowly. It takes about 5 million years for TCI to increase to pre-extinction levels.

Another interesting find is that test complexity increased before species diversified. That is, new niches were created faster than new species to fill them after the mass extinction event. This study shows that before a lot of new species can evolve, there need to be a few species that evolve and open new niche spaces first.

Why is this study important? Today, humans are having a huge effect on the ability of species to survive on our planet. Through destruction of species’ habitats and niche space, we are pushing more and more species to the brink of extinction. Importantly, there are also thousands of species that have already gone extinct from human activities (such as the Tasmanian Tiger, Passenger Pigeon, Sea Mink, Caribbean Monk Seal, Quagga, Elephant Bird, Haast’s Eagle, and many more). If we keep causing animals to go extinct, we may see a loss of biodiversity that rivals those of mass extinctions that have taken place in the geologic past. But until now, we didn’t really know how long it took for new niche spaces to be filled and how that would affect how fast new species can fill those niche spaces.

This study gives us a clue: it may take as long as 5 million years after a mass extinction event for new niche spaces to be created. It then takes additional five million years for diversity, or the number of species, to rebound to pre-extinction levels. The bottom line is that it takes 10 million years for the biosphere to recover from a mass extinction event. This means that even though humans have been on Earth for a very short period of time (geologically speaking), we will have a huge impact on the flora and fauna, even if we were to disappear tomorrow.

Citation: Lowery, C. M., and Fraass, A. J., 2019. Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction. Nature Ecology & Evolution https://doi.org/10.1038/s41559-019-0835-0

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