Diet-Size Relationship in Animals Disturbed by Humans

The diet-size relationship is a universal principle in the animal kingdom that dates back millions of years. Find out how human activity is disturbing that principle’s balance and the implications for species extinction.

Spending my summers at our family cottage growing up gave me a profound appreciation for Nature. The diversity of animal life was especially intriguing.

One thing that always struck me was that larger animals like deer or coyotes were much harder to spot than smaller creatures like chipmunks and squirrels. I learned that the reason is larger animals need larger territories to provide all the food they need.

This diet-size relationship holds true for herbivores and carnivores. As a rule the larger the diet, the larger the animal, and vice versa.

Animal Diet-Size Relationship Results in U-Shaped Graph

If we plot the various species of animals in terms of what they eat versus their mass, the resulting graph is U-shaped. We end up with large herbivores on the left, smaller omnivores and animals that eat invertebrates (invertivores) in the middle and large carnivores on the right. Here’s an example.

Scientists have known that size was a function of diet for mammals for a few decades. Now, a study published in the journal Nature Ecology and Evolution shows that this pattern applies all over the globe and goes back to the dawn of life on Earth.

Beyond that, it also applies to animals other than mammals. Land-dwelling birds, reptiles and saltwater fishes also display the same U-shaped diet-size relationship. The researchers’ motto has become “U stands for “Universal.”

Diet a Species Eats Drives Its Energy Consumption

The diet a species eats drives its overally energy consumption. This determines the species’ rate of growth and, over time, the average size of its specimens.

The study also contains some familiar bad news. As in so many areas of ecology, human activity is disrupting this fundamental diet-size relationship in ecosystems.

Will Gearty is a postdoctoral researcher at the University of Nebraska-Lincoln and a co-author of the study. As he put it, “The systems have been in what seems to be a steady state for a very long time. It’s concerning what might happen when they leave that state.”

Connection Between Diet and Size Is a Balance

Like most things in Nature, the connection between what a species eats and how large it gets is a balance. Diet drives size, but at the same time, the availability of that diet constrains the amount of food that’s available in the habitat.

“You can be as big as your food will allow you to be,” Professor Gearty explained. “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.”

If we think about a herbivore like a cow or even an elephant, we can see that grazing animals can grow to massive sizes. This enables them to forage over very large tracts of land.

Plants Are Relatively Low in Nutritive Value

Since plants are relatively low in nutritive value, herbivores have evolved longer and more complicated digestive tracts. These enable plant-eaters to draw as many nutrients as possible from their food.

Carnivores need to be able to prey on these ever-larger herbivores. That leads to a sort of arms race as natural selection favours bigger and stronger specimens who can catch up to and bring down oversized grazing animals.

Omnivores like raccoons have more choice in their diets, but they tend to focus on small, highly concentrated foods like nuts and small insects. This limits their potential size.

Animals that Eat Invertebrates Are the Smallest Species

Invertivores include a few animals like robins. Because their prey are so small, and their habitats so competitive, they end up being the smallest species.

The researchers wanted to find out if the U-shaped diet-size relationship applies to species other than mammals. So, they looked at body-size statistics for roughly 5,000 species of mammals, 9,000 bird species, over 7,000 kinds of reptiles and 2,800 fish species.

Most vetebrate animals followed the same pattern. The exceptions were seabirds and marine mammals like whales and dolphins. It seems as if the Universal U-curve doesn’t apply to species that divide their time between air and water environments.

Connection Seems to Apply to a Wide Range of Habitats

The connection between size and food choices also seems to apply to a wide range of habitats. It turns up in forests, deserts and grasslands. It also affects fish species in both tropical and temperate ocean environments.

Biology Professor Kate Lyons is a co-author of the study. On the topic of the universality of the diet-size relationship, she offered these thoughts. “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.”

Having established that the Universal U-curve applies all over the modern world, the team decided to look back in time as well. So they analyzed the fossil record for more than 5,000 species of mammals, dating back as far as 145 million years.

Diet-Size Relationship Has Applied for 66 Million Years

The researchers found that the diet-size relationship has applied for at least 66 million years. This takes us back to the end of the age of the dinosaurs and the rise of land mammal species.

Professor Gearty added, “This relationship persists even when you have other dominant animals around. We suspect that it’s actually existed since the inception of mammals as a group.”

The team also noticed the disruptive influence of human activity on other species. It dates all the way back to the ermergence of Neanderthals and early Homo sapiens several hundred thousand years ago.

Herbivores and Omnivores Now About 100 Times Smaller

Herbivores and omnivores are now on average about 100 times smaller than they once were. Typical carnivores are about one-tenth of earlier sizes. We humans have been flattening the Universal U-curve through our big-game hunting and by encroaching on animal habitats.

A growing number of scientists are calling our current geological era “the anthropocene” epoch. They’re adopting this term to reflect how humans are now the dominant influence on the global environment rather than natural forces, causing our planet’s sixth mass extinction.

At the same time, many are calling for a transition from the anthropocene into a new “ecozoic” epoch. The idea is that Humanity would shift from a position of dominance toward a strategy of mutually enhancing Human-Earth relations.

Mass Extinction Crisis From Anthropocene Worldview

The mass extinction crisis we’re experiencing today has multiple causes, but they all stem from human activity driven by our anthropocene worldview. The study authors view their findings as potential confirmation of our predicament.

According to the researchers, there’s more than a 50/50 chance that a number of large and midsize mammals will go extinct over the next two centuries. The tiger and the Javan rhinoceros are especially vulnerable.

This would likely trigger a chain reaction. Losing large herbivores will lead to the loss of large carnivores that rely on them for food, and so on.

“Fundamental Repercussions for the Environment”

“We’re altering the way the energy is divvied up,” explained Professor Gearty. “That could perhaps have fundamental repercussions for the environment and ecosystem as a whole.”

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

We always have more to learn if we dare to know.
Learn more:
Humans Disrupting 66 million-year-old feature of ecosystems
Anthropogenic disruptions to longstanding patterns of trophic-size structure in vertebrates
Mass Extinction Happening Again
Biodiversity Always Wins
Animal Movement Disruptions Theaten Biodiversity

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