The loss of big animals to overhunting and human-driven pollution has meant a drastic drop in the amount of poo, which is critical to nourishing the Earth, researchers said Monday.

Restoring the populations of large mammals and sea creatures through conservation could help fight the damage being caused by global warming, by absorbing more carbon dioxide in the oceans and replenishing landscapes across the planet, said the findings in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.

“This once 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,” said Joe Roman, a biologist at the University of Vermont and co-author of the study.

Their declines have damaged the Earth’s nutrient cycle, the process by which animal waste moves from the ocean depths to deep inland via migrating seabirds and fish, he said.

“This broken global cycle may weaken ecosystem health, fisheries, and agriculture,” said Roman.

Researchers found that the capacity of animals to circulate nutrients has dropped to eight percent of what it was in the past, before some 150 species of “megafauna” went extinct at the end of the last ice age about 12,000 years ago.

Hunting by humans has also been a driving force in severely diminishing the capacity — by more than 75 percent — of marine mammals to move the vital nutrient phosphorous from the deep ocean to land surfaces.

“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.

But researchers say animals are a critical “distribution pump,” moving fecal matter around to fertilize places that would otherwise be barren of life.

The paper argues that this animal-powered fertilizer machine has decreased to six percent of its one-time capacity.

The research is based on a series of recent studies on large creatures, coupled with mathematical models to estimate how nutrients move across land and oceans, and how this has changed along with declining animal populations.

For instance, researchers estimate that before the era of commercial hunting, whales and other marine mammals moved around 750 million pounds (340 million kilograms) of phosphorus from the depths to the surface each year.

Now, they move about 165 million pounds, just 23 percent of their former capacity.

Seabirds and salmon used to carry more than 300 million pounds of phosphorus onto land each year.

Now, that number is less than four percent of past capacity, largely because of habitat loss and overfishing.

Since phosphorus is a key element in fertilizers — and its supply may run critically short in the next 50 years — scientists say conservation efforts could help recycle much-needed phosphorus from the sea to land.