Relationship of diet size found over time in several groups of vertebrates.

The U-shaped relationship between diet and body size in modern terrestrial mammals could also mean “universality,” which showed the relationship spans at least 66 million years and a range of vertebrate groups, according to the new study.

Decades have passed since ecologists realized that plotting the diet size ratio of terrestrial mammals produces a U-shaped curve when these mammals align with the plant protein gradient. As this curve shows, the herbivorous herbivores on the far left and the carnivorous carnivores on the far right tend to grow to much larger sizes than the omnivorous omnivores and invertebrate invertebrates in the middle.

To date, however, almost no research has looked for patterns other than mammals or modernity. 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 ancient times and applies to land-dwelling birds, reptiles and even salt water fish.

But the study also suggests that human-related extinctions of large herbivores and carnivores are disrupting what appears to be a fundamental feature of past and present ecosystems, with potentially unpredictable consequences.

“We don’t know what’s going to happen because it’s never happened before,” said Will Gerty, a Nebraska research assistant and co-author of the study, published April 21 in the journal. Ecology of nature and evolution. “But since the systems have been in a very stable state for a long time, it’s about what might happen when they leave that state. »

Size up, size down

The evolutionary and ecological histories of animal species can be partly told through the intertwining influences of diet and size, Gerty said. The diet of a species determines its energy intake, which in turn stimulates growth and ultimately determines its size. However, this size can also limit the quality and quantity of food available to a species, even if it sets thresholds for the quality and quantity needed for survival.

“You can only be as big as your food will allow you,” Gerty said. “At the same time, you are often as big as you need to be to catch and process food. So there is an evolutionary interaction here. »

Since the diet of herbivorous herbivores is relatively poor in nutrients, they often become massive to cover more land to forage for more food, and adapt to long, complex digestive tracts that extract the maximum nutrients from them. Carnivores, on the other hand, need to get big enough to track and destroy these herbivores. Although a smorgasbord of omnivores usually keeps their stomachs full, their high energy requirements usually cause them to focus on nuts, insects, and other small, high-calorie foods. And while invertivores primarily enjoy protein-rich prey, the diminutive nature of these prey, coupled with fierce competition from many other invertivores, ranks them among the smallest of all.

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

Although this pattern was absent in marine mammals and seabirds, probably due to the unique needs of life in water, it appeared in other groups of vertebrates – reptiles, marine fish and land birds. This pattern persisted even across biomes—for example, in forests, grasslands, and deserts, or in the tropics of the Atlantic Ocean and the temperate North Pacific—when analyzing land mammals, land birds, and waterfowl.

“Showing that it exists in all of these different groups suggests that it’s something fundamental to how vertebrates get energy, how they interact with each other, and how they coexist,” said co-author Kate Lyons, assistant professor of biological sciences. in Nebraska. . “We don’t know if this is necessary — there may be other ways that vertebrate communities can organize based on body size and diet — but it’s certainly enough. »

But the researchers also wanted to know how long the U-curve could have continued. So they analyzed the fossil record of 5,427 species of mammals, some of which date back to the Early Cretaceous, between 145 million and 100 million years ago. Lyons and colleagues initially collected fossil data as part of a 2018 study of large mammal extinctions at the hands of humans and their recent ancestors.

“To my knowledge, this is the most thorough study of the evolution of body size and especially mammalian diet over time,” Gerty said.

He found that the U-shaped curve dates back to at least 66 million years, when non-avian dinosaurs had just been wiped out and mammals had not yet evolved into the dominant animal class they are today.

“It’s really interesting and amazing,” Gerty said, “to see that these relationships persist even when there are other dominant animals around you.

“We suspect that it has indeed existed since the creation of mammals as a group. »

Shape of the future

By examining the present and past of the U-curve, Gerty, Lyons, and their colleagues set their sights on its future, or potential lack of it. The average height of herbivores and omnivores has decreased by about 100 times since the advent of Neanderthals and Homo sapiens The team reported that over the past hundreds of thousands of years, the size of predators has decreased by about 10 times over the same period. As a result, the U-curve that had persisted for so long began to noticeably flatten out, Gerty said.

In this vein, the team predicted a more than 50 percent chance that several large and medium-sized mammals, including the tiger and Javan rhinoceros, which consider humans their only predators, will disappear within the next 200 years. The researchers said these supposed extinctions would only exacerbate the disruption of the U-curve, especially since the extinction of large herbivores could trigger or accelerate the extinction of the large predators that feed on them.

“It’s possible that by removing some of these animals from the top (the U-curve) and shrinking some of those body size ranges, we’ll change the way energy is distributed,” Gerty said. mentioned. “Perhaps this could have fundamental implications for the environment and the ecosystem as a whole. »

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

“In the environmental literature, you keep seeing people talk about how ecosystems are now less stable, less resilient and more prone to destruction,” Lyons said. “I think this is just another source of evidence that this may indeed be the case in the future. »

Gerty and Lyons co-authored the study with Robert Cook, British Center for Ecology and Hydrology; Amanda Bates from the University of Victoria (Canada); Abby Chapman of University College London; Gillian Dunick, Simon Fraser University (Canada); Graham Edgar and Rick Stuart-Smith from the University of Tasmania (Australia); Jonathan Lefchek, Smithsonian Center for Environmental Research; Craig McClain, Louisiana Maritime Consortium; and Gil Rilov of Israel Limnological and Oceanographic Research.

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