Humans are disrupting a 66 million year old fundamental feature of ecosystems – ‘This has never happened before’

An illustration depicting herbivorous (green), omnivorous (purple), invertivorous (yellow), and carnivorous (red) mammals. Each column includes mammalian species extinct over the past 2.58 million years (light shade); those expected to be lost in the near future (medium shade, probability of extinction > 50%); and those likely to persist (dark tint, probability of extinction

Diet-size relationship found across deep time, several vertebrate groups.

According to a new study, the U-shaped association between diet and body size in modern land mammals could also mean ‘universal’, as the relationship spans at least 66 million years and a range of vertebrate animal groups. .

It has been decades since ecologists realized that plotting the diet-size relationship of terrestrial mammals yields a U-shaped curve when aligning these mammals on a plant-protein gradient. As this curve illustrates, the plant-eating herbivores on the far left and carnivorous carnivores on the far right tend to grow much larger than those of the all-consuming omnivores and invertebrate-eating invertivores in the middle.

“We don’t know what will happen because it has never happened before.” — Will Gearty

To date, however, hardly any research has looked for the pattern beyond mammals or modern times. In a new study, researchers from the University of Nebraska-Lincoln and institutions on four continents have concluded that the pattern actually dates back to deep time and applies to land birds, reptiles and even water fish. salty.

However, the study also suggests that human-caused extinctions of larger herbivores and carnivores are causing disruption of what appears to be a fundamental component of past and present ecosystems, with potentially unpredictable implications.

“We don’t know what’s going to happen because it’s never happened before,” said Will Gearty, a Nebraska postdoctoral researcher and co-author of the study, published April 21 in the journal Nature Ecology and Evolution. “But because the systems have been in what seems like a very stable state for a very long time, it’s about what might happen when they leave that state.”

Size up, size down

The evolutionary and ecological stories of animal species can be told in part through the intertwined influences of diet and size, Gearty said. A species’ diet determines its energy consumption, which in turn stimulates growth and ultimately helps dictate its size. Yet this size can also limit the quality and quantity of food available to a species, even as it sets thresholds for the quality and quantity needed for survival.

“You can be as big as your food will let you,” Gearty said. “At the same time, you’re often as big as you need to catch and process your food. So there is an evolutionary interaction there.

U-shaped relationship between power supply and size

A figure illustrating the U-shaped relationship between diet and size (or mass, in kilograms) in land mammals. The gray parts of the bars represent species currently threatened with extinction, the white parts representing species that have already disappeared. Credit: Nature Ecology and Evolution / Springer Nature

Since the diets of plant-based herbivores are relatively poor in nutrients, they often become massive in an effort to cover more ground to forage for more food – and to adapt to the long and complex digestive tracts that result. extract maximum nutrients. Carnivores, on the other hand, must grow large enough to track and eliminate these herbivores. Although omnivores’ buffet-style menu usually keeps their stomachs full, their high energy needs usually leave them focusing on nuts, insects, and other small, energy-dense foods. And while invertivores primarily enjoy protein-rich prey, the tiny nature of these prey, combined with fierce competition from many other invertivores, relegates them to the smallest sizes of all.

The end result: a U-shaped distribution of mean and maximum body sizes in mammals. To analyze the generalizability of this model in modern times, the team compiled body size data for a large number of surviving species: 5,033 mammals, 8,991 birds, 7,356 reptiles and 2,795 fish.

“It’s really interesting, and really striking, to see that this relationship persists even when you have other dominant animals around.” — Will Gearty

Although the pattern was absent in marine mammals and seabirds, likely due to the unique demands of life in the water, it appeared in the other vertebrate groups – reptiles, saltwater fish, and landbirds. – reviewed by the team. The pattern even held across various biomes—forests versus grasslands versus deserts, for example, or the tropical Atlantic Ocean versus the temperate North Pacific—when analyzing land mammals, land birds, and water fish. salty.

“Showing that it exists in all of these different groups suggests that it’s something fundamental about how vertebrates acquire energy, how they interact with each other, and how they coexist,” said the co- author Kate Lyons, assistant professor of biological sciences at Nebraska. . “We don’t know if this is necessary – there could be other ways to organize vertebrate communities with respect to body size and diet – but it is certainly sufficient.”

But the researchers also wanted to know how long the U-curve might have lasted. So they analyzed the fossil record of 5,427 species of mammals, some of which date back to the early Cretaceous Period of 145 million to 100 million years. Lyons and his colleagues originally collected the fossil data as part of a 2018 study of the extinction of large mammals at the hands of humans and their recent ancestors.

“To my knowledge, this is the most thorough investigation of the evolution of body size and particularly diet in mammals over time,” Gearty said.

He found that the U-curve dates back at least 66 million years, when non-avian dinosaurs had just been wiped out, but mammals had yet to branch out into the dominant animal class they are today.

“It’s really interesting and really striking,” Gearty said, “to see that this relationship persists even when you have other dominant animals around.

“We suspect it’s actually been around since the inception of mammals as a group.”

The shape of things to come

After cataloging the present and past of the U-curve, Gearty, Lyons and their colleagues set their sights on its future, or potential lack thereof. The median height of herbivores and omnivores has dropped about 100 times since the emergence of Neanderthals and Homo sapiens over the past hundreds of thousands of years, the team reported, with the size of carnivores dropping about 10 times over the same period. As a result, the U-curve that has persisted for so long has begun to flatten noticeably, Gearty said.

In this vein, the team predicted more than a 50% chance that several large and medium-sized mammals – including the tiger and the Javan rhino, both of which count humans as their only predators – will go extinct within 200 years. coming years. These projected extinctions would only exacerbate the disruption of the U-curve, the researchers said, especially since the loss of large herbivores could trigger or accelerate the loss of the large carnivores that feed on them.

“It’s certainly possible that by removing some of these animals from the top (of the U-curve) and reducing some of these ranges of body sizes, we’re changing the way the energy is distributed,” Gearty said. said. “It could possibly have fundamental repercussions for the environment and the ecosystem as a whole.”

It’s also possible, the researchers concluded, that the coming decline in mammalian body size even exceeds the unprecedented decline observed over the past hundreds of thousands of years.

“You keep seeing, in the ecological literature, people speculating about how ecosystems are less stable now, less resilient, and more prone to collapse,” Lyons said. “I think this is just another source of evidence to suggest this may indeed be the case in the future.”

Reference: “Anthropogenic Disturbances of Longstanding Patterns of Trophic Size Structure in Vertebrates” by Rob Cooke, William Gearty, Abbie SA Chapman, Jillian Dunic, Graham J. Edgar, Jonathan S. Lefcheck, Gil Rilov, Craig R. McClain , Rick D Stuart-Smith, S. Kathleen Lyons and Amanda E. Bates, April 21, 2022, Nature ecology and evolution.
DOI: 10.1038/s41559-022-01726-x

Gearty and Lyons authored the study with Robert Cooke, UK Center for Ecology & Hydrology; Amanda Bates, from the University of Victoria (Canada); Abbie Chapman, of University College London; Jillian Dunic, Simon Fraser University (Canada); Graham Edgar and Rick Stuart-Smith, from the University of Tasmania (Australia); Jonathan Lefcheck, Smithsonian Environmental Research Center; Craig McClain, Louisiana Universities Marine Consortium; and Gil Rilov of Israel Limnological and Oceanographic Research.