Now imagine a world ruled by rats. Parasite-carrying rats that spread disease far and wide into the (remaining) human population.
That's the future sketched by Stanford University biologist Rodolfo Dirzo and his colleagues in a paper published in the journal Science. The researchers conclude that we're on the brink of a mass extinction—the sixth in Earth's history. This time the culprit is not a catastrophic asteroid strike or "natural planetary transformations" such as ice ages or volcanic eruptions.
This time the cause is us, as species face oblivion from human-triggered climate change, rampant development, and habitat destruction. So-called anthropogenic, or human-caused, extinction is not a new idea. But in the Stanford researchers' review of the scientific literature and current data, they document that the sixth mass extinction is now underway, and they explore the effects on human health of "anthropocene defaunation."
For instance, between 23 percent and 36 percent of all birds, mammals, and amphibians we use for food or medicine are now threatened with extinction, according to the paper. Declines in wild-animal food sources will cause human death and suffering in many countries. In Madagascar, loss of wildlife will increase hunger by 30 percent, the scientists estimated.
"Systematic defaunation clearly threatens to fundamentally alter basic ecological functions and is contributing to push us toward global-scale 'tipping points' from which we may not be able to return," the researchers wrote.
More than 320 terrestrial vertebrates have become extinct since 1500, according to the paper. Populations of those that survived have declined by 25 percent. That's "due to anthropogenic destruction of animals' natural habitats, overexploitation, and, increasingly now, invasive species," Dirzo stated in an email. "More critically, going forward, it will be the combined effect of all those factors."
Certain animals and geographic regions have been hit hardest. The largest animals—"megafauna" in biologist lingo—such as elephants, rhinoceroses, polar bears, and other mammals, have slow reproductive rates and require large habitats.
Take the big animals out of the food chain, and what are you left with?
Studies from Kenya show that the disappearance of megafauna like lions, zebras, giraffes, and elephants can throw an ecosystem completely off-balance. As animals that eat rats disappear, rodent numbers will explode.
As species that prey on agricultural pests decline, crop damage will increase up to 37 percent, Dirzo and his colleagues predict.
"In the United States alone, the value of pest control by native predators is estimated at $4.5 billion annually," wrote the scientists in the paper.
Insects are disappearing quickly too, with an estimated 33 percent of species in decline. And we need bees, as pollinators play a role in the production of up to 75 percent of the world's food supply.
Many animal species also decompose organic material and disperse their nutrients, and some contribute to clean water. Amphibians, for example, eat algae and suspended organic particles, while other animals, such as crocodiles, aerate water as they splash around.
It may all seem overwhelming and hopelessly depressing, but Dirzo says citizens can help alert policy makers to this imminent but largely ignored threat.
"Be educated about the problem. Assess what is at stake. Demand and vote for politicians who contemplate being globally environmentally friendly," he said. "These animals are our collective responsibility. They are products of 3.5 billion years of evolution and our companions on the planet."