Survival of the blandest: Unusual sharks face highest extinction risk

If current extinction trends continue, global shark populations will lose much of their variety, thereby threatening ecosystems where specialized species serve vital roles, researchers have found.

In prowling Earth’s oceans for more than 400 million years, sharks have evolved into a delightfully diverse group. As popularly conveyed through children’s books, the famous fish range in size from palm-sized dwarf lanternsharks to school-bus-length whale sharks. Great whites, hammerheads, and many others serve key ecological roles as apex predators at the top of the food chain.

According to previous research, one-third of Earth’s 500 shark species are on the brink of extinction, largely due to human activity. A new Stanford-led analysis reveals the most threatened species tend to have uncommon physiologies and specialized ecological roles – broadly, those living at the ocean’s surface or its deepest depths. Losing these species to extinction would diminish trait diversity until the array of shark body types and habitats dwindles to only medium-sized sharks in a narrow band of mid-ocean depths.

“Our study illustrates that if these major shark extinctions do happen, sharks will become more alike and simplified, and you end up with a more boring world with less diversity of forms,” said lead study author Mohamad Bazzi, a postdoctoral scholar in Earth and planetary sciences at the Stanford Doerr School of Sustainability. “Even small differences between species matter. They each bring something distinct and important.”

As has been documented in other habitats, the loss of trait diversity in key species groups can have far-reaching impacts. For example, a global decline in vulture species is damaging ecosystems, and surging sea urchin populations are putting coral reefs at risk and impacting fisheries.

“This broader erosion of unique shark morphologies would mean that many distinctive features of each species, and the functions that they have in a given ecosystem, would go away,” said senior study author Jonathan Payne, the Dorrell William Kirby Professor and professor of Earth and planetary sciences at the Stanford Doerr School of Sustainability. “Those popular shark books for kids would become a lot less fun and interesting.”

Diminished diversity

For the study, the Stanford researchers consulted the Red List of Threatened Species compiled by the International Union for Conservation of Nature (IUCN), the world’s largest conservation network, to identify a shark genus under notable duress, Carcharhinus. Of the 35 IUCN-recognized Carcharhinus species, 25 are considered either “Vulnerable,” “Endangered,” or “Critically Endangered.” The bull shark and oceanic whitetip shark are two of the biggest, best-known, and at-risk members of this genus.

To search for any non-random patterns of extinction bias, the researchers performed a statistical analysis on the form and structure of more than 1,200 teeth from 30 Carcharhinus present-day species documented in scientific literature. Shark teeth serve as a reliable proxy for a species’ overall size and diet. For instance, tooth sizes generally correlate with body size, while shape and edge features such as serrations can reveal the sharks’ choice of prey.

The researchers found that the more physically and ecologically divergent species tended to also be those at higher risk of extinction. Species in the genus with closer to average sizes, about 3 to 15 feet long, and generalist diets tend to be less threatened than species with more specialized diets and ecological niche-tailored bodies. For example, larger sharks benefit from being hefty by having less risk of predation, but they also must obtain enough food to maintain their bulk.

For Carcharhinus, the findings indicate that if extinctions of some of its member species do occur, the survivors will be more similar than the current group. Bazzi and Payne expect that their findings extend to other shark groups, suggesting that extinction pressure promotes “phenotypic homogenization” – animals’ observable characteristics, such as appearance and behavior, becoming more similar.

Such an outcome would deprive the planet of special and unique creatures, along with potentially useful bio-inspired products.

“With this huge loss of shark traits, humans would be undoing all of this evolutionary work that’s gone on for millions and millions of years,” said Payne. “When we undo all of this work, we’re not only losing things that give us joy, but we’re also losing potential practical evolutionary solutions to problems, such as disease treatments or insights into new materials.”

“We lose in basically every way when we drive species extinct,” Payne added.

Path to positive change

Overall, the findings align with a widely observed pattern that extinctions often reduce trait diversity in a way that favors the survival of “average” generalist organisms over specialists.

The researchers emphasize that the hollowing out of shark diversity predicted by their study, as well as that of other threatened animal groups, can still likely be reversed. Although a confluence of factors endangers shark populations worldwide, including pollution and habitat loss, the single biggest driver – overfishing – can be addressed through more stringent laws, enforcement, and human restraint.

Recent history offers numerous encouraging instances where direct harvesting of an animal group mostly halted, and populations spectacularly rebounded. A prime example: the northern elephant seal, once slaughtered regularly for lamp oil blubber. By the late 1800s, only a single population numbering as few as 20 individuals off Baja California survived. Nowadays, a century after the U.S. and Mexico banned hunting of elephant seals, upward of 150,000 of the seals inhabit West Coast waters, serving their critical role as top predators that regulate overpopulation of prey and help distribute nutrients to promote greater biodiversity and a more resilient ecosystem.

“People don’t need to think about conservation of species as something theoretical, where if we make this change, only our great-great-grandkids might see a different world,” said Payne. “Over the course of just a few decades for some of these threatened sharks, you could already see positive change.”

Jonathan Payne is also a senior fellow at the Stanford Woods Institute for the Environment, a professor, by courtesy, of biology in the School of Humanities and Sciences, and a member of Stanford Bio-X.

This research was supported by the Wallenberg Foundation Postdoctoral Scholarship.