Down in the dumps: Loss of big animals and their poop damages Earth – study
Before the rise of modern humans, there were “deer with twelve-foot antlers, and bison herds to the horizon” that ate huge amounts of plant matter, moving these nutrients to higher ground “through their deposit of feces, urine and, upon death, decomposing bodies.”
"I wanted to know whether the world of the past with all the endemic animals was more fertile than our current world," lead study author Chris Doughty of Oxford University told The Washington Post.
Doughty’s team applied a set of mathematical models to estimate the movement of nutrients vertically in the oceans and across the land, and how this movement changed with extinctions and declining animal populations. What researchers found was that these megafauna, or large animals, played a greater role in the spread of nutrients across the planet than scientists realized.
Equally, whales and other marine mammals moved phosphorous from deep ocean water to the surface, which was then spread by seabirds and migrating fish across seas, up rivers, and deep inland to the mountaintops.
Previously, scientists studied nutrient cycling related to the weathering of rocks, which broke down and left nutrients in the soil. They also found that microbes and bacteria contributed to nutrient cycling. The team’s new finding adds another dimension to the science, finding that fertilized ecosystems maintain natural functions vital to people.
“Previously, animals were not thought to play an important role in nutrient movement,” said Doughty in a statement.
"Large free-ranging animals are much less abundant than they once were. Today, if scientists were to study the role of animals they would find that it is important but small," Doughty explained. "However, in the past, we hypothesize that it would have been at least an order of magnitude larger than today. Essentially, we have replaced wild free-roaming animals with fenced domestic cattle that cannot move nutrients in the same way."
Prior to the era of commercial whale hunting, which saw populations cut between 66 to 90 percent, scientists estimated that whales and other marine mammals annually “moved around 750 million pounds of phosphorous from the depths to the surface.” Now that figure is about 165 million pounds, representing a 75 percent reduction.
“This broken global cycle may weaken ecosystem health, fisheries, and agriculture,” Joe Roman, a biologist at the University Vermont and co-author of the study, said in a press release. “This was a world that had ten times more whales; twenty times more anadromous fish, like salmon, double the number of seabirds, and ten times more large herbivores – giant sloths and mastodons and mammoths.”
The team also gathered data on seabird and fish populations that fed in the sea and then came into land – like the ocean-going salmon that move upriver to defecate, spawn and die. These movements carried more than 300 million pounds of phosphorous, which is vital to fertilization, onto land each year. With the destruction of seabird colonies, habitat loss and overfishing, that figure has declined to 12 million pounds.
Scientists say the “animal-powered, planetary pump may have dropped to just six percent” of its former capacity to spread nutrients on land and throughout the sea.
The study’s authors warn that easily accessible phosphorus could disappear in as little as 50 years. Continuing to push for the recovery of these whales, and other large animals, could make a big difference.
"A world bereft of large wild animals, whether they are whales, salmon, albatrosses, or elephants, is a less productive place – and one that has lost much of its magic," Roman said.
“Recovery is possible and important,” said Roman, pointing to the return of the bison as an example. “That’s achievable. It might be a challenge policy-wise, but it’s certainly within our power to bring back herds of bison to North America. That’s one way we could restore an essential nutrient pathway.”
The study was published in the Proceedings of the National Academy of Science on Monday.