Imagine an asteroid striking Earth and wiping out most of the human population. Even if some lucky people survived the impact, Homo sapiens might still face extinction, because the social networks humans rely on would collapse.
This dynamic also plays out in the wild.
Social interactions are essential for many animals, helping them to locate food, spot predators and raise offspring. Without such connections, individuals can struggle to survive.
In a new study, researchers at the University of Colorado Boulder challenge a long-held assumption that social connections matter most for "highly social species", like humans and wolves. They show that much more common "loosely social species," those that make temporary friends rather than living in stable groups, might be more vulnerable to extinction due to population declines that limit social interactions. Deer, squirrels, chickadees and a whole host of other animals, including invertebrates, all fall into this category.
The study was published in Trends in Ecology & Evolution.
"This finding comes at a moment when many wildlife populations are shrinking or fragmenting due to climate change, habitat loss and exploitation," said senior author Michael Gil in the Department of Ecology and Evolutionary Biology in the College of Arts and Sciences. "We provide a new framework for predicting which species are most susceptible to collapse so we can better forecast risk."
The extroverts
Nearly a century ago, American ecologist Warder Clyde Allee showed that animals often do better when they are in larger groups, a phenomenon known as an Allee effect.
Studies have since linked larger group sizes to higher reproductive success and survival in many highly social animals, which are those that live in a fixed group. For example, meerkats with more group mates tend to have more offspring, and more of those offspring survive.
Having more individuals in a group means the group members can get more help when needed, said Samantha Rothberg, the paper's first author and a doctoral student in Gil's lab in the Department of Ecology and Evolutionary Biology.
"We can relate to that as humans, because we can benefit a lot from the information provided by individuals around us," she added.
While many explanations for Allee effects point to benefits from social interactions, research to date has failed to show that social behavior, or the loss of it, can tip a species' chances of survival.
For example, in African wild dogs, larger packs often have more pups per animal. But when wild dog populations decline across the region, the remaining dogs form new social groups, allowing group sizes and overall survival rates to remain unchanged.
Confused by the inconsistency, Rothberg, Gil and Ella Henry, another doctoral student in Gil's lab, reviewed decades of ecological theory, models and case studies on social interactions and survival.
What they found suggested that ecologists might have been looking at the wrong animals.
The introverts
For decades, ecologists assumed that if social interactions are driving Allee effects, it would be the most pronounced in highly social species like meerkats and wild dogs. But those animals, Gil said, appear to have a built-in buffer against the loss of social interactions.
"It's intuitive that we think the more social a species is, the more vulnerable it is to losing those interactions," Gil said. But it turns out, highly social animals can actively compensate.
Much like extroverted humans who have no trouble making friends when they move to a new city, wild dogs seek out new members to restore their group size when they lose members of their pack.
Loosely social animals, by contrast, are more like introverted people. They make friends, but they don't always have to hang out with them. These species don't go out of their way to replace lost companions to maintain their social interactions. As a result, when their populations decline, they lose social benefits from experiencing fewer interactions.
"When you remove individuals, you're not just removing those individuals from the population, you're also removing the benefits that they conferred on surviving individuals. That creates a feedback loop," Rothberg said.
A driver of collapse
Gil said the study highlights a glaring possibility that more species are susceptible to population collapse than previously thought.
According to the World Wildlife Fund , global wildlife populations have declined by at least 73% in the past 50 years. Many scientists have declared this period the "sixth mass extinction," with human resource extraction wiping out species hundreds of times faster than they would otherwise disappear.
"I'm looking out my window right now, and there are a couple of birds sitting on branches. They're being social. But those moment-to-moment interactions are easy to take for granted. We now realize that, in aggregate, they can determine whether a population survives or collapses," Gil said.
