Illustration depicting herbivorous (green), carnivorous (purple), carnivorous (yellow), and carnivorous (red) mammals. Each column includes species of mammals that have become extinct within the past 2.58 million years (light shade); those that are expected to be lost in the near future (medium shade, probability of extinction >50%); and those that are likely to persist (dark color, probability of extinction less than 20%). A new study from the University of Nebraska-Lincoln and institutions on four continents shows that human-related extinctions of the largest herbivores and carnivores are disrupting what appears to be a key feature of past and present ecosystems. Credit: Julius Csotonyi/Ecology and Natural Evolution
The diet-size relationship has been found across deep time, many groups of vertebrates.
According to a new study, the U-shaped correlation between diet and body size in modern terrestrial mammals could also mean “global,” 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 produces a U-shaped curve when these mammals are aligned on a plant protein gradient. As this curve illustrates, the leftmost herbivores and the rightmost carnivores tend to grow much larger than the carnivores and the invertebrate invertebrates in the middle.
“We don’t know what will happen because it hasn’t happened before.” – Will Gerty
To date, almost no research has looked for a pattern beyond mammals or modern times. In a new study, researchers from the University of Nebraska-Lincoln and institutions on four continents conclude that the pattern actually dates back to ancient times and applies to wild birds, reptiles, and even saltwater fish.
However, the study also notes that human-caused extinctions of large herbivores and carnivores are causing disruption to what appears to be an essential component of past and present ecosystems, with potentially unpredictable ramifications.
“We don’t know what’s going to happen because it hasn’t happened before,” said Will Gerty, a postdoctoral researcher in Nebraska and co-author of the study, which was published April 21 in the journal Nature Ecology and Evolution. “But since the systems have been in a state that appears to be very stable for a very long time, it comes down to what might happen when they leave that state.”
Size up, size smaller
Gertie said the evolutionary and ecological stories of animal species can be told in part through the intertwining influences of diet and size. The species’ diet determines its energy consumption, which in turn stimulates growth and ultimately helps determine its size. However, this size can also limit the quality and quantity of food available to a species, even when it sets thresholds for the quality and quantity needed for survival.
“You can be as big as your food allows you to be,” Gerty said. “At the same time, you are often about as large as you need to catch and process your food. So there is an evolutionary interaction there.
Figure showing the U-shaped relationship between diet and size (or mass in kilograms) in a terrestrial mammal. The gray parts of the bars represent species that are currently endangered, and the white parts represent species that have already disappeared. Credit: Nature Ecology and Evolution / Springer Nature
Since the diets of herbivores of plant origin are relatively poor in nutrients, they often get huge in an effort to cover more ground to get more food – and to adapt to the resulting long and complex digestive tracts. Extracting the maximum amount of nutrients. On the other hand, carnivores must grow large enough to track and eliminate these herbivores. Although a buffet-style menu for carnivores usually keeps their stomachs full, their high energy needs usually cause them to focus on nuts, insects, and other small, energy-dense foods. And while reflectors primarily enjoy protein-rich prey, the small nature of these prey, combined with fierce competition from many other reflectors, relegates them to the smallest of all.
The end result: a U-shaped distribution of average and maximum body sizes in mammals. To analyze the generalizability of this model in the modern era, 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 exciting, and really surprising, to see that this relationship continues even when you have other dominant animals.” – Will Gerty
Although this pattern was absent in marine mammals and seabirds, likely due to the unique requirements for life in the water, it appeared in other vertebrate groups — reptiles, saltwater fish and land birds — that the team reviewed. This pattern even walked across various biomes—forests versus grasslands versus deserts, for example, or the tropical Atlantic versus temperate North Pacific—when analyzing land mammals, land birds, and aquatic fish.
“Demonstrating its presence in all of these different groups suggests that it’s something fundamental about how vertebrates gain energy, how they interact with each other, how they co-exist,” said co-author Kate Lyons, an assistant professor of biological sciences in Nebraska. . . “We don’t know if this is necessary – there may be other ways to organize vertebrate communities in terms of body size and diet – but it is certainly sufficient.”
But the researchers also wanted to know how long the U-curve might last. So they analyzed the fossil record of 5,427 mammal species, some dating back to early times.
” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”> Tabasheri Period from 145 million to 100 million years ago. Lyons and colleagues initially collected fossil data as part of the 2018 file studying the extinction of large mammals at the hands of humans and their new ancestors.
“To my knowledge, this is the most comprehensive investigation of the evolution of body size and especially diet in mammals over time,” Gertie said.
He found that the U-curve dates back at least 66 million years, when the non-avian dinosaurs had just been wiped out, but mammals had not yet branched out into the dominant animal class that they are today.
“It’s really exciting and amazing, to see that this relationship continues even when you have other dominant animals,” Gertie said.
“We suspect it has actually been around since the emergence of mammals as a group.”
The shape of things to come
After indexing the present and past U-curve, Gertie, Lyons, and their colleagues turned to its possible future or lack thereof. The average height of herbivores and carnivores has decreased about 100 times since the emergence of Neanderthals and sane man The team reports that over the past hundreds of thousands of years, the size of the carnivores has decreased about 10 times over the same period. As a result, the long-running U-curve began to flatten noticeably, Gertie said.
In this context, the team predicted a more than 50% chance of extinction of many large and medium-sized mammals – including the tiger and the Javan rhinoceros, both of which consider humans as their sole predators – within the next 200 years. These projected extinctions will 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 large carnivores that feed on them.
“It’s certainly possible that by removing some of these animals from the top (the U-curve) and reducing some of these ranges of body sizes, we’re changing the way the energy is distributed,” Gerty said. referred. “It could have fundamental effects on the environment and the ecosystem as a whole.”
The researchers concluded that it is also possible that the upcoming decline in mammalian body size will even exceed the unprecedented decline observed over the past hundreds of thousands of years.
“You keep seeing, in the environmental literature, people speculating about how ecosystems are now less stable, less resilient, and more prone to collapse,” Lyons said. “I think this is just another source of evidence to suggest that this may indeed be the case in the future.”
Reference: “Anthrogenic perturbations of long-term patterns of food size structure in vertebrates” by Rob Cook, William Gerty, Abbey S.A. Chapman, Gillian Dunnick, Graham J. — Smith, S. Kathleen Lyons and Amanda E. Bates, April 21, 2022, Available here. Nature and Evolution.
DOI: 10.1038 / s41559-022-01726-x
Gertie and Lyons co-authored the study with Robert Cook, the UK’s Center for Environment and Hydrology. Amanda Bates from Victoria University (Canada). Abby Chapman of University College London; Gillian Donek, Simon Fraser University (Canada); Graham Edgar and Rick Stewart Smith from the University of Tasmania (Australia); Jonathan Levchek, Smithsonian Center for Environmental Research; Craig MacLean, Naval Association of Universities of Louisiana; and Gil Rilov of the Research Institute of Hydrology and Oceanography in Israel.