Extinction: _{When species come to an end}

What is extinction?

“Extinction” is the phenomenon of a a species no longer existing on Earth because all its members have died and not been replaced by new ones. We know of long-extinct species from clues they left behind, often fossils. But only a fraction of the species that lived on Earth are represented in fossilized remains because fossilization requires certain conditions, like an oxygen-free burial environment. Even under the right conditions, fossils degrade over time.

Similarly, out-of-print books become increasingly rare as the remaining copies become worn and lost. Only a fraction of the original inventory of books will survive years later. Sometimes, we uncover out-of-print books that were thought to be lost forever under a dusty pile in a used book shop. Likewise, we continue to discover fossils of species never previously described.

Extinction is always happening

Scientists estimate the biodiversity we see on Earth today is less than 5% of the species that have ever lived. The rise and fall of “lineages” (groups of related species) is a constant process. Usually, species die out at a relatively low “background rate.” The background rate represents the number of species expected to go extinct over time. Since the rate varies a lot across species and time, it’s taken as an average. We estimate that background extinction rates are 0.1-1 extinction per million species yearly. The background rate contrasts with the occasional periods of exponentially higher rates—the mass extinctions (see our Mass Extinctions module).

With the exception of the past five mass extinction events, estimates from the fossil record suggest that an approximate background rate is one extinction per million species-years.

Stuart Pimm, Conservation Ecologist, 2007

Causes for species turnover

But why do lineages rise and fall? Research on past extinctions suggests that the changes are not random, and some species are more at risk of extinction than others. Various features contribute to a species’ extinction risk. Some are “intrinsic traits” (characteristics of the species itself) and others are “extrinsic variables” (environmental features).

Intrinsic

• Body size
• Life history
• Species abundance

Extrinsic

• Climate change
• Ecological niche
• Interactions with other species

Body size

Body size has been shown to correlate with the extinction risk of a species. Colombian biologist Catalina Pimiento and colleagues (Pimiento et al., 2020) studied a huge sample of Caribbean fossilized mollusks (shelled organisms such as clams). They found that mollusks had diversified into more species from 23 - 5 million years ago but lost about half their species during the past five million years. The researchers mapped the mollusks’ traits (body size, life habits, locomotion, and habitat) against their extinctions. Body size stood out as an indicator of whether species went extinct.

Looking at the data plotted in Figure 2, which size mollusks were most extinction-prone?

Very small mollusks were more extinction-prone, with the largest (>80 mm) showing the most resilience over this timeframe. Why? The study authors hypothesized that smaller mollusk species were at a disadvantage because there are more smaller species, increasing competition among them (see our Trophic Ecology module).

However, how individual traits such as body size relate to extinction risk varies across groups of animals. Mexican biologist Rodolfo Dirzo and colleagues (Dirzo et al., 2014) mapped geologically recent extinctions of terrestrial mammals. The top row of Figure 3 (in red) shows extinctions of mammals during the Pleistocene (2.58 million years ago - 11,700 years ago), while the other rows show mammals that are extinct (orange), threatened (yellow), or non-threatened (green) during our current era, the Anthropocene. In each row, the mammals are categorized by body mass (x-axis) and by frequency of extinction for their category (y-axis).

What can you conclude about the relationship of body mass to extinction rates?

Their data shows that the largest mammals went extinct during the Pleistocene. The median size of extinct mammals is 182 kg (about 400 pounds). It includes some of the largest animals, like giant ground sloths, cave bears, and mammoths, suggesting a higher vulnerability of bigger mammals. Looking at the three Anthropocene graphs, you’ll also see a body size trend, with the median size of extinct mammals (70 kg; 150 pounds) larger than the median size of threatened mammals (44 kg; 97 pounds); non-threatened animals are smaller still (6 kg; 13 pounds). One factor explaining the trend is that, since the emergence of modern humans, larger-bodied land mammals are much more likely to be hunted and thus driven to extinction.

This pattern contrasts with what is observed in marine mollusks over millions of years (see Figure 2), suggesting that extinction risk factors are different in different kinds of animals.

Life history

In addition to size, other traits have also been implicated in extinction risk, such as life history characteristics (e.g., growth rates, reproductive age, lifespan). Generally, studies have found that species that mature later—reproducing later and having longer generation times—are more extinction prone. This correlates with body size because larger organisms like elephants or tortoises have long life histories, while small animals live fast and die young!

But distinct trends show up in plants. Ecologist Haydée Hernández-Yáñez and colleagues investigated patterns in the vulnerability of plant species to extinction relative to their life history traits. (Hernández-Yáñez et al., 2022). Using 14 life history measures, they modeled which were the best predictors of the plants’ current risks of extinction (see Figure 4).

What would you conclude about extinction risk from Figure 4?

Figure 4 shows a negative correlation between how long it takes plants to mature (y-axis) and their level of endangerment (x-axis). Plants that mature more quickly (reaching maturity at a younger age) tend to be more endangered. Compare this trend in herbaceous plants to the data on mammals in Figure 3. It again drives home the point that life history traits correlate with extinction risk. But the direction and strength of the correlation are different for distinct groups of organisms.

Geographic distribution

Where animals live on the globe may also be a determining factor in their risks of extinction.

Australian evolutionary biologist Marcel Cardillo and colleagues (Cardillo et al., 2008) modeled species-level extinction risks of mammals currently on Earth. They used data from the IUCN Red List, which catalogs species and includes their status as stable, threatened, or endangered. They then looked for correlations with various variables. Like Dirzo (2014), they found body mass positively correlated with extinction risk, but they also found geographic range to be a significant predictor. Mammals with larger geographic ranges (i.e., more widely distributed) were at lower risk of extinction, perhaps because some populations may survive even if others are extirpated (see our Population Biology module).

But, within a species, geographic range may change by location, season, or other variables. Australian biologist Claire Runge and colleagues studied nomadic birds (i.e., species with a roaming lifestyle) (Runge et al., 2015). By modeling the distributions of 43 bird species over 11 years, they detected huge fluctuations in their geographic ranges. During times of resource shortage, such as droughts, some birds’ ranges shrunk to levels that could put them at high risk of extinction. So, geographic range size is not a fixed characteristic but one that varies.

Still, geographic range is one of the most consistent predictors of extinction risk over time, across different groups of organisms, and in different environmental contexts. As a species range varies, its risk of extinction varies in tandem.

Climate change

Since climate influences resource availability, it makes sense that climate changes would affect extinction risk.

Major and sudden climate changes have been implicated in mass extinctions (see our Mass Extinctions module), but gradual extinctions can also be linked to climate. German geoscientist Gregor H. Mathes and colleagues (Mathes et al., 2021) mapped extinction risk against a combination of short and long-term climate changes for major groups of animals (e.g., gastropods, mammals, reptiles) in the geological past. They hypothesized that shorter-term stressful temperatures would have caused more extinctions when coupled with ongoing climate trends in the same direction (e.g., a heat spike amid a warming trend). Their data supported the hypothesis, with the strongest trend when a cold snap coupled with a gradually cooling climate increased extinction risks for mammals by 40%.

Ecological niche

How much climate change or other changes interact with extinction risk also relates to the “ecological niche” of a species or the set of resources it needs to exist (see our Animal Ecology module). Multiple studies have detected that species with smaller niches (reliant on a narrower range of resources) are more vulnerable to extinction. Scientists believe such “specialist” species are less resilient to change because their requirements are more specific.

Species with smaller ranges, and those with narrow habitat breadths are more at risk than others, regardless of the taxon or geographic distribution.

Filipe Chichorro Finnish biologist, 2019

For example, Finnish biologist Janne S. Kotiaho and colleagues compared ecological niches between nearly 100 threatened and non-threatened butterfly species in Finland (Kotiaho et al., 2005). They found that the level of specialization of both larvae and adults could predict extinction risk. Threatened species had narrower habitat breadth as adults, and their larvae tended to rely on more restricted plant species, supporting the premise that more specialized species are more vulnerable (see Figure 5).

The breadth of a species’ ecological niche stems from its habitat needs, available resources, and competition (see our Animal Ecology module). So, you would expect it to change as resources flux or competitors go extinct. The degree to which a species can shift or expand its ecological niche to respond to changing conditions may shape its chance of survival. In the butterfly study (Kotiaho et al., 2005), species with better dispersal abilities (i.e., able to move to new locations) were less likely to be threatened with extinction.

Complex of factors

To look broadly at extinction risk, American biologist Ana D. Davidson and colleagues modeled the determinants of extinction in marine mammals. They compiled a database of traits of 125 marine mammal species along with habitat use variables based on information to date. Using IUCN Red List data, they calculated the relative contribution of each variable to the risk of extinction.

Looking at Figure 5, how important were aspects of a species’ ecological niche?

Figure 6 shows that many variables weigh into extinction risk, in this case for marine mammals. Ecological niche includes things like habitat and foraging location, which have secondary importance to things like geographic range (species with smaller ranges are more vulnerable to extinction; Davidson et al., 2014).

Chinese biologist Yuxi Zhong and colleagues studied extinction risks in 54 Chinese lizards. As in the marine mammal study, Zhong and colleagues found extinction risk predictors included body size, geographic range, and habitat specialization (Zhong et al., 2017). A classic study by British biologist Andy Purvis and colleagues of carnivores and primates found that nearly half of the variation in extinction risk was explained by geographic range, reproductive rates, population density, and trophic niche (Purvis et al., 2000).

So, a species extinction rate correlates with a unique combination of both “intrinsic traits” (characteristics of the species) and “extrinsic variables” (environmental features). The combination depends on species, although certain variables have emerged as consistently important. For example, when extinction risk results from overexploitation, larger organisms are consistently at greater risk, whether they are marine or aquatic.

Does speciation counterbalance extinction?

“Speciation” is when new species arise, like when new books get published and added to your library. If the number of books added is similar to the number lost, a library’s collection may stay roughly the same size for years. The continual turnover of books is comparable to the gradual turnover of species on Earth as new species evolve and old species go extinct.

The relationship of extinction rates to speciation rates determines the number of species at a given time. For example, using evolutionary trees of relationships, American evolutionary anthropologist Jeremiah E. Scott estimated speciation rates and extinction rates of primates still on Earth today. The relationship of speciation rates (λ; x-axis) and extinction rates (μ; y-axis) is plotted in Figure 7.

How do the speciation rates of Old World monkeys (living in Africa, Asia, and Europe) compare to the human lineage -”Apes”? How do their extinction rates compare?

Based on Scott’s data, the speciation rates of Old World monkeys have been higher (~0.36) than those of Apes (range ~ 0.25 - 0.30). Scientists explain the lower speciation rates of the human lineage as due to an overall slowdown in the molecular evolution that generates new species. However, the lower extinction rates of Old World monkeys are more dramatic than of Apes. The mean extinction rate for Old World Monkeys is just above zero (0.015 extinctions per million years). Because of their high speciation rates and low extinction rates, Old World monkeys boast almost 140 species to fewer than 30 species of Apes today (humans plus 29 others). Old World monkeys include many familiar captive species, such as baboons, macaques, and mandrills.

• Old Word Monkeys high speciation rate - Low extinction rate = Lots of species diversity
• Hominoidea moderate speciation rate - High extinction rate = Lower species diversity

So, the difference between speciation rates and extinction rates ultimately determines diversity.

Can new species evolve without older species going extinct?

To answer that question, let’s look at one of our now-extinct relatives, Homo naledi.

More than 1,500 H. naledi fossils were unearthed from a South African cave. H. naledi’s small brain suggested that it was an old species in our family tree that had evolved long before us. But its arrangement of brain parts suggested a more modern species. Figure 8 shows the uncertainty in placing Homo naledi on the human family tree.

Did H. naledi live way before the earliest H. sapiens or alongside them?

Using radiometric age dating, Australian geologist Paul Dirks and colleagues dated the H. naledi remains to be just 236,000 – 335,000 years old, which means they were alive at the same time as our H. sapiens ancestors (Dirks et al., 2017) in Africa. Indeed, looking at Figure 8, you can see the cooccurrence of multiple species in the human family tree.

Homo naledi highlights, once again, that we can't think of human evolution in terms of ape-like ancestors gradually evolving more modern features in a linear fashion. Instead, multiple human species evolved in parallel and coexisted, sometimes side-by-side.

Lisa Hendry Science content producer, Natural History Museum of London, 2019

Of the upwards of 20 different species of early humans that once lived on Earth, most left no descendants when they went extinct. Which ones survived to give rise to modern humans remains a matter of debate. Fossil and genetic evidence suggests that our lineage emerged in Africa about 300,000 years ago. But exactly how it evolved is a complex conundrum dubbed “the Muddle in the Middle” (Havarti and Centeno, 2022). In Figure 8, notice other species alongside Homo sapiens, including H. neanderthalensis and H. erectus.

“Paleo genetics” (the study of DNA from fossils) is helping resolve some of the uncertainty. For example, we now know that the modern human gene pool includes Neanderthal genes. German paleoanthropologist Mateja Hajdinjak and colleagues used genetics to study the overlap of modern humans and Neanderthals in Europe (Hajdinak et al., 2021). By analyzing remains of the earliest known Homo sapiens specimens in Europe, she confirmed that they were mixing (i.e., mating) with Neanderthals. As a result, the European people of today have as much as 2% Neanderthal DNA. Further, Neanderthal DNA has been found in modern African populations, which is best explained by ancient Europeans migrating back into Africa after mixing with Neanderthals (Chen et al., 2020).

That gene flow with Neanderthals exists in all modern humans, inside and outside of Africa, is a novel and elegant finding.

Michael Petraglia Anthropologist (Price, 2020)

So, while the species we know as Neanderthals went extinct, they did leave descendants, and some of their genes live on through human genomes worldwide. This phenomenon of “introgression” (the transfer of genetic material from one species to another) muddies the idea of extinction since some portion of a species’ genome lives on through other species. Indeed, some reproductive mixing between species may be key for introducing new genetic diversity, which leads to successful adaptations, allowing a species like ours to continue surviving. This is akin to how a particular book may be lost, but the inspiration it left on other authors and books can last forever.

Thinking back to the library, current books (species) may give rise to later books (descendants) in the form of serialized stories. An original book may eventually go out of print (extinct species), but its descendants live on through continued adaptations of the original text (genes). Still, many books that go out of print (extinct species) leave no descendants. Some may even be lost from history altogether.