A new study from the University of Nebraska-Lincoln has found that the U-shaped relationship between diet and body size in modern land mammals spans at least 66 million years across a range of animal groups. vertebrate animals and ecosystems.
Decades ago, ecologists realized that plotting the diet-size relationship of land mammals on a plant-protein gradient yielded a U-shaped curve. For example, the herbivorous herbivores on the far left of the U-curve and carnivorous carnivores on the far right become larger than those of all-consuming omnivores and invertebrate-eating invertivores in the middle.
However, key features of past and present ecosystems are being disrupted by human-caused extinctions of larger herbivores and carnivores. The study reveals that this has unpredictable consequences.
“We don’t know what’s going to happen because it’s never happened before,” study co-author Will Gearty said. “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.”
The evolutionary story of a species can be told through the influences of diet and body size. Indeed, food influences energy consumption, which stimulates growth and development. Size can also limit the food available to a species, “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.
The plant-based diet of herbivores is relatively poor in nutrition, they often grow taller to cover more ground and forage for more food. Herbivores often have long and complex digestive tracts to extract maximum nutrients from what they eat. Carnivores, on the other hand, must grow to take down these herbivores. For omnivores, their choice of food is vast, their high energy needs necessitating a diet of nuts, insects and other small foods with high energy density. Finally, invertivores primarily enjoy protein-rich prey, but compete fiercely with many other invertivores, making these species the smallest of all.
The U-shaped pattern has also been found in land mammals, birds, and saltwater fish in biomes like forests, grasslands, deserts, or the tropical Atlantic Ocean and even the North Pacific.
“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.
The researchers were also interested in the duration of the U-curve. The fossil record of 5,427 species of mammals was analyzed. Some date back to the Lower Cretaceous period 145 to 100 million years ago. In doing so, the team conducted the most in-depth study of the evolution of mammalian body, size and diet over time.
This revealed that the U-curve dates back at least 66 million years, after the extinction of the dinosaurs. “We suspect it’s actually been around since the inception of mammals as a group.” said Gearty.
Next, the team wanted to figure out the U-shaped presence in the future. They found that the median height of herbivores and omnivores had decreased 100 times since the emergence of Neanderthals and Homo sapiens over the past hundreds of thousands of years. The size of carnivores also decreased 10 times over the same period. The U-curve that has persisted for 66 million years has begun to flatten.
This led the team to predict a 50% chance that large and medium-sized mammals such as tigers and rhinos will become extinct over the next 200 years. These mammals all have one thing in common: their only predators are humans.
Predicted extinctions would further aggravate the disruption of the U-curve – the loss of large herbivores could trigger or accelerate the loss of the large carnivores that feed on them.
“It could possibly have fundamental repercussions for the environment and the ecosystem as a whole.” said Gearty.
“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.”
The research is published in the journal Nature ecology and evolution.
By Katherine Bucko, Terre.com Personal editor