The diet-size relationship has been found across multiple vertebrate groups in deep time.
According to a new study, the U-shaped association between diet and size in modern terrestrial mammals could also represent “global,” with the relationship spanning at least 66 million years and a range of vertebrate animal groups.
It has been several decades since ecologists realized that a graph of the diet-size relationship of terrestrial mammals produces a U-shaped curve when those mammals are aligned on a plant-to-protein gradient. As can be seen from this curve, the leftmost herbivores and the rightmost carnivores tend to grow much larger than the middle-feeding herbivores and invertebrates.
“We’re not sure what’s going to happen, because this hasn’t happened before.” – Will Gerty
So far, though, 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 land-dwelling birds, reptiles, and even saltwater fish.
However, the study also notes that human-caused extinctions of the largest herbivores and carnivores are causing disruption in what appears to be an essential component of past and present ecosystems, with potentially unpredictable effects.
“We’re not sure what’s going to happen, because this 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 because the systems have been in a very stable state 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 history of animal species can be told in part through the intertwining influences of diet and size. A species’ diet determines its energy consumption, which in turn drives 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, it’s often just as big as you need to pick up 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) among land mammals. The gray parts of the bars represent species that are currently in danger of extinction, while the white parts represent species that are already extinct. Credit: Ecology and Evolution / Springer Nature
Since the vegetarian diets of herbivores are relatively poor in nutrition, they often grow large in order to cover more ground to feed more food – and to accommodate the long, complex digestive tracts from which they extract the maximum nutrients. In the meantime, carnivores must grow large enough to keep up with and eliminate those herbivores. Although the buffet menu for carnivores usually keeps their stomachs full, their high energy requirements generally cause them to focus on nuts, insects, and other small, energy-dense foods. And while reflectors have mostly protein-rich prey, the diminutive nature of that prey, combined with intense 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 pattern 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, 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, possibly due to the unique requirements of living in water, it appeared in other vertebrate groups — reptiles, saltwater fish and land birds — that the team examined. 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, wild birds, and saltwater fish.
“Showing that this is present in all of these different groups suggests that it’s something fundamental about how vertebrates acquire 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 were also interested in knowing how long the U-curve might take. So they analyzed fossil records from 5,427 species of mammals, 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 originally collected the fossil data as part of a 2018 study on 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.
It revealed that the U-curve stretches back at least 66 million years, when the non-avian dinosaurs had just been wiped out, but that mammals had not yet diversified into the dominant animal class that they are today.
Gertie said: “It’s really interesting, and really surprising, to see that this relationship continues even when you have other dominant animals.
“We suspect it has actually existed since mammals emerged 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. Average sizes of herbivores and carnivores have decreased nearly 100-fold since the emergence of Neanderthals and sane man Over the past hundreds of thousands of years, the team reports, with the size of the carnivores declining about 10 times in the same period. As a result, the long-running U curve began to noticeably flatten, Gertie said.
In this context, the team predicted that there is a more than 50% chance that many large and medium-sized mammals – including the dry tiger and rhinoceros, both of which consider humans as their only predators – will become extinct within the next 200 years. These projected extinctions would only exacerbate the perturbation of the U-curve, especially to the extent that the loss of large herbivores could result in or accelerate the loss of large carnivores that feed on them, the researchers said.
“It is certainly possible that when we take some of these animals off the top (the U-curve), and as we tear down some of these body size ranges, we change the way the energy is partitioned,” she said. “This may have fundamental implications for the environment and the ecosystem as a whole.”
It is also possible, the researchers conclude, that the upcoming decline in mammalian body sizes will even outpace 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 piece 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, Abby S. Chapman, Gillian Dunnick, Graham J. . Stuart-Smith, S. Kathleen Lyons and Amanda E. Bates, 21 April 2022, Available here. Ecology and Nature Evolution.
DOI: 10.1038 / s41559-022-01726-x
Gertie and Lyons co-authored the study with Robert Cook of the UK’s Center for Environment and Hydrology. Amanda Bates from Victoria University (Canada). Abby Chapman of University College London; Gillian Donek from Simon Fraser University (Canada); Graham Edgar and Rick Stewart Smith from the University of Tasmania (Australia); Jonathan Levchek of the Smithsonian Center for Environmental Research; Craig MacLean, of the Louisiana Maritime Universities Association; and Gil Riloff, of Israel Hydrobiology and Oceanography Research.