Humans have disrupted 66 million years of ecosystem stability

A new study from the University of Nebraska-Lincoln has found the U-shaped relationship between diet and size in modern land mammals spans at least 66 million years across a range of vertebrate animal groups and ecosystems.

Decades ago, ecologists realized that graphing the diet-size relationship of terrestrial mammals on a plant-to-protein gradient yields a U-shaped curve. For example, the plant-eating herbivores on the far left of the U-curve and meat-eating carnivores on the far right grow larger in size than those of the all-consuming omnivores and the invertebrate-feasting invertivores in the middle.

However, key features of past and present ecosystems are being disrupted by human-caused extinctions of the largest herbivores and carnivores. The study reveals that this comes with unpredictable consequences.

“We’re not sure what’s going to happen, because this hasn’t happened before,” said study co-author Will Gearty. “But because the systems have been in what seems to be a very steady state for a very long time, it’s concerning what might happen when they leave that state.”

The evolutionary history of a species can be told through the influences of diet and size. This is because diet influences energy consumption, which drives growth and development. Size can also limit the food available to a species, “you can be as big as your food will allow you to be,” Gearty said. “At the same time, you’re often as big as you need to be to catch and process your food. So there’s an evolutionary interplay there.”

The plant-based diet of herbivores is relatively poor in nutrition, they often grow large to cover more ground to forage more food. Herbivores often have long, complex digestive tracts to extract maximum nutrients from what they eat. Carnivores, on the other hand, must grow large to take down those herbivores. For omnivores, their selection of food is vast, their high energy demands require a diet of nuts, insects and other small, energy-dense foods. Lastly, invertivores enjoy mostly protein-rich prey, but are in stiff competition with many other invertivores, leading these species to be the smallest of all.

The U-shaped pattern was also found in land mammals, birds and salt water fish in biomes like forests, grasslands, deserts, or the tropical Atlantic Ocean and even the North Pacific.

“Showing that this exists across all these different groups does suggest that it is something fundamental about how vertebrates acquire energy, how they interact with one another, and how they coexist,” said co-author Kate Lyons.

The researchers were also interested in how long the U-curve may have endured. Fossil records from 5,427 mammal species were analyzed. Some dating back as far back as the Early Cretaceous Period of 145 to 100 million years ago. By doing so, the team conducted the most extensive study of the evolution of body, size and diet in mammals over time.

This revealed that the U-curve stretches back at least 66 million years, after the dinosaur extinction. “We suspect that it’s actually existed since the inception of mammals as a group.” said Gearty.

Next, the team wanted to understand the U-shape presence in the future. They found that the median sizes of herbivores and omnivores have decreased 100 times since the emergence of Neanderthals and Homo sapiens over the past few hundred thousand years. The size of carnivores has also dropped by 10 times in that same span. The U-curve that has persisted for 66 millions of years has begun to flatten. 

This led to the team projecting a 50 percent chance that large and medium mammals such as the tiger and rhinoceros will go extinct within the next 200 years. These mammals all have one thing in common – their only predators are human. 

The predicted extinctions would further exacerbate the disruption of the U-curve – the loss of large herbivores could trigger or accelerate the loss of the large carnivores that prey on them.

“That could perhaps have fundamental repercussions for the environment and ecosystem as a whole.” said Gearty.

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

The research is published in the journal Nature Ecology & Evolution.

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By Katherine Bucko, Earth.com Staff Writer