This text was initially printed by Hakai Journal.
When a real-estate growth threatened the stays of a Thirteenth-century coastal fishing website on southern Brazil’s Santa Catarina Island in 1996, archaeologists rushed to excavate. They quickly collected what they may from the Rio do Meio website—pottery, instruments, animal stays. The historic website now sits below a well-liked beachfront property. Luckily, although, the artifacts are tucked safely away within the museum on the Federal College of Santa Catarina (UFSC).
About 750 to 500 years in the past, Indigenous peoples journeyed to the realm to fish. Sharks have been an necessary a part of their weight-reduction plan, and, after a profitable hunt, the fishers would butcher bull sharks, nice white sharks, sand tiger sharks, and different species at Rio do Meio earlier than transporting the meat away. Fortunately for Guilherme Burg Mayer, a graduate scholar in ecology at UFSC, the hunters left the sharks’ severed heads behind. The college museum’s assortment from Rio do Meio consists of enamel from at the least eight completely different species—and shark enamel are a treasure chest of data.
In a current paper, Burg Mayer and his colleague in contrast these excavated enamel with trendy shark enamel collected in the identical area from 1975 to 2010. Utilizing a way referred to as stable-isotope evaluation, which yields insights into an animal’s weight-reduction plan and atmosphere, the crew recognized variations between the 2 units of enamel that probably replicate how the area’s marine ecosystem has modified over the centuries, and the way sharks have tailored.
“I feel quite a lot of of us simply suppose that shark enamel are triangular pointy issues,” says Sora Kim, a stable-isotope biogeochemist at UC Merced who additionally research shark enamel however wasn’t concerned within the Rio do Meio evaluation. This method is comparatively new, Kim says, and scientists can be taught so much from the form and chemistry of shark enamel.
Like human enamel, shark chompers include collagen, a sort of protein that features completely different parts, akin to nitrogen and carbon that animals get from their meals. Though scientists can’t inform precisely what an animal has eaten from analyzing its enamel, Burg Mayer’s crew was capable of glean clues in regards to the sharks’ diets and their rankings as predators by evaluating completely different types of nitrogen, referred to as isotopes, between the 2 dental units. Larger nitrogen-isotope values recommend that an animal is feeding greater on the meals chain—that’s, extra more likely to be consuming carnivorous or herbivorous prey than they’re to be consuming vegetation.
The researchers discovered that, centuries in the past, various kinds of sharks have been feeding on animals that occupied the identical stage of the meals net—prey that lived close to each other and ate comparable issues. Ecologists see this sort of weight-reduction plan overlap as factor, Burg Mayer says, as a result of it factors to a extra resilient meals net the place predators have plenty of completely different prey to munch on. If one kind of tasty fish disappears, for instance, sharks have loads of others to eat.
In distinction, the diets of Brazil’s trendy sharks have little overlap. As a substitute, sharks look like both chowing down on prey that will get little consideration from different sharks or consuming a extra numerous mixture of prey from completely different ranges of the meals net. A doable rationalization for this shift, Burg Mayer says, is that sharks have turn into much less choosy as fish species have declined in quantity, forcing them to snack on new meals. To the researchers, this variation alerts a much less resilient ecosystem.
When the researchers zeroed in on one species—the sand tiger shark—they discovered that it has moved up the meals chain over time to turn into one of many ecosystem’s high predators. Burg Mayer says that trendy members of the species are choosing their prey from greater on the meals chain as a result of it’s turn into simpler; there are fewer sand tiger sharks left to compete for the most important and finest meal choices. Sand tiger sharks have been overfished in Brazil for many years and, in 2018, have been declared critically endangered within the area. (Fishing for the species was banned a couple of years earlier than, although the meat remains to be bought illegally.)
However, as Kim factors out, stable-isotope evaluation can present solely that adjustments have occurred; it will probably’t clarify why. Overfishing could or is probably not in charge for sharks’ altering diets. And since sharks recurrently lose and regrow their enamel, a single tooth is extra like a snapshot masking a couple of weeks or months of the animal’s life, not its entire life story, she provides. Finally, Kim says, with out further analysis, it’s tough to say precisely why sharks are consuming various things now than they have been a number of centuries in the past.
Regardless, the findings replicate a disturbance within the ecosystem that’s price additional investigation, Burg Mayer says. The research additionally showcases the resilience of sand tiger sharks even below difficult situations.
“This explicit group is adaptive to the adjustments,” Burg Mayer says. “It’s attempting to outlive.”
