October 26, 2015
How missing mammal poop is weakening the world’s ecosystems
Long ago, giant whales and outsized land mammals roamed the Earth, transporting nutrients across the planet through their poop. However, according to the study in the Proceedings of the National Academy of Sciences, these animal populations' decline into extinction has damaged the Earth’s nutrient recycling system due to the missing poop.
“This broken global cycle may weaken ecosystem health, fisheries, and agriculture,” said Joe Roman, a biologist at the University of Vermont and co-author on the new study. Who knew poop could be so important?Animals’ role in nutrient movement
“Previously, animals were not thought to play an important role in nutrient movement,” said lead author Christopher Doughty, an ecologist at the University of Oxford.
Originally, scientists studied the weathering of rocks and bacteria’s nitrogen collection as the main nutrient provider. It is no wonder in the beginning animals were ignored—they’ve become so small their “bounty” was not as prevalent among ecosystems.
Now, the study challenges the bottom-up bias by analyzing the importance of animals in “nutrient distribution” by how their poop may fertilize areas that previously didn’t receive a lot of nutrients, such as ocean surfaces and interior areas of land.
Giant animals like the elephant-like gomphotheres, bison herds, and mammoth deer ate large amounts of plants—digesting them and releasing them back into environments with less nutrients through poop, urine, and even during decomposition in death.
Overall, the team’s calculations show that the animal-powered nutrient dump has dropped eight percent from what it once was—before the last ice age’s extinction of 150 “megafauna” animal species.
The excrement experiments
Using recent studies showing how large animals appeared to disproportionately drive nutrient movements along with existing data about past and present animal populations, the team applied mathematical models to estimate nutrient movement vertically in the oceans and across land, as well as how the declining animal populations changed its movement.
In particular, the changes in marine mammals such as whales gained the scientists’ attention in their decreases. According to the study:
- Whale densities have declined between 66% and 90% due to commercial hunting.
- Prior to commercial hunting, whales and marine mammals moved 750 million pounds of phosphorus (critical nutrient for plant growth) to the surface. Now, they only move around 165 million pounds—23% of the original capacity.
- Seabirds and fish once carried over 300 million pounds of phosphorus onto land every year. Now the amount is less than four percent of past values due to destroyed colonies, habitats, and overfishing.
“Phosphorus is a key element in fertilizers and easily accessible phosphate supplies may run out in as little as fifty years,” said Doughty. “Restoring populations of animals in their former bounty could help to recycle phosphorus from the sea to land, increasing global stocks of available phosphorus in the future.”
Recovering from this "bowel foul"
“…recovery is possible and important,” said Roman. He points to bison for proof. “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 typical flow of nutrients is down mountains to the oceans,” said Roman. “We are looking at ways that nutrients can go in the other direction—and that’s largely through foraging animals. They’re bringing nutrients from the deep sea that could eventually reach a mountain in British Columbia.”
Feature Image: The red arrows show the estimated amounts of nutrients that were moved or diffused. Diagram from PNAS; designed by Renate Helmiss