In the animal kingdom, it seems big is out and little is in.
New research suggests that large long-lived birds and land mammals will face extinction over the next century as small insect-eating animals that reproduce quickly and die young will predominate. Among the likely losers in the emerging world will be rhinos, hippos, gorillas, giraffes and caribou as well as large birds like eagles, condors and vultures.
The likely winners? Rodents and songbirds.
The research, described in a paper published May 23 in the journal Nature Communications, points to several causes of the looming shift in the world's fauna, including climate change, deforestation, hunting and increasing urbanization and agriculture. As their world changes, large birds and land mammals — which are known to be less adaptable to changing conditions than their smaller peers — will seemingly have a hard time surviving.
"By far the biggest threat to birds and animals is humankind — with habitats being destroyed due to our impact on our planet," Rob Cooke, a postgraduate researcher at the University of Southampton in England and the leader of the research, said in a statement.
The downsizing of the world's animals might sound like welcome news if you're fond of teensy creatures like dwarf gerbils and the white-browed sparrow-weaver — not so welcome if you're particular to rhinos and other "charismatic" species. But since big mammals often "engineer" their habitats in important ways — think of elephants uprooting trees to create open areas and condors scavenging carcasses that might otherwise spread disease to other species — their loss might put other animal species in jeopardy.
"If we lose these 'engineers,' other species that depend on them might also go extinct," Cooke said in a conversation with NBC News MACH. "It's great to have the diversity of life on the planet that we have," he added. "The idea of losing it ... is kind of scary."
William Ripple, a professor of ecology at Oregon State University in Corvallis who was not involved in the new research, echoed Cooke's concerns about the loss of large animals. "It's important for us not just to think about the extinctions by themselves, but we should start thinking about the niches that these species have and their interactions with other species," he said. "And especially the ecological services that they provide to humans," such as the wide dispersal of seeds that help important plants regenerate their numbers.
Ripple called the research "important" and added that the looking loss of biodiversity could be even greater. "These authors only examined [the] imperiled situation for birds and mammals, but we are finding similar threats to amphibians, reptiles and fish," he said.
For the research, Cooke and collaborators from the University of Southampton and Memorial University of Newfoundland considered all 15,484 species of birds and land mammals and looked at five characteristics for each: body mass, diet, breadth of habitat, typical number of offspring and the length of time between generations. They used this information along with data from a widely recognized list of threatened species and ran hundreds of computer simulations showing various extinction patterns and how they would affect global biodiversity.
The simulations showed that the average body mass of mammals will likely fall by 25 percent over the next century, a number that reflects the loss of bigger birds and animals. That might not sound like a big number, but the researchers say in their paper that mammals' average body mass fell just 14 percent over the last 130,000 years.
Of course, the projected downsizing of the world's animals reflects a "business as usual" approach to human activity. The outlook for large species could improve if we took aggressive steps to rein in climate change and the other factors that will put big birds and animals under increasing pressure.
"We need a groundswell of support from the public to put pressure on policymakers so they will do what we need to protect the species" from excessive hunting, trapping, fishing and other examples of overexploitation, Ripple said.
"These extinctions haven't happened yet," Cooke said. "They can still be stopped. It's not a done deal."
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