Giants once roamed the earth. Oceans teemed with
ninety-foot-long whales. Huge land animals--like truck-sized sloths and ten-ton
mammoths--ate vast quantities of food, and, yes, deposited vast quantities of
A new study shows that these whales and outsized land
mammals--as well as seabirds and migrating fish--played a vital role in keeping
the planet fertile by transporting nutrients from ocean depths and spreading
them across seas, up rivers, and deep inland, even to mountaintops.
However, massive declines and extinctions of many of these
animals has deeply damaged this planetary nutrient recycling system, a team of
scientists reported October 26 in the Proceedings of the National Academy of
"This broken global cycle may weaken ecosystem health,
fisheries, and agriculture," says Joe Roman, a biologist at the University
of Vermont and co-author of the new study.
On land, the capacity of animals to carry nutrients away
from concentrated "hotspots," the team writes, has plummeted to eight
percent of what it was in the past – before the extinction of some 150 species
of mammal "megafauna" at the end of the last ice age.
And, largely because of human hunting over the last few centuries,
the capacity of whales, and other marine mammals, to move one vital
nutrient--phosphorous--from deep ocean waters to the surface has been reduced
by more than seventy-five percent, the new study shows.
"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 the new study shows that animals are a crucial
"distribution pump," the scientists write, transporting masses of
fecal matter to fertilize many places that would otherwise be less productive,
including ocean surface waters and the interior of continents.
These fertilized ecosystems, in turn, maintain natural
functions vital to people. For example, the new study notes that restoring
whale populations could help increase the ocean's capacity to absorb
climate-warming carbon dioxide.
Traditionally, scientists studying nutrient cycling have
focused on weathering of rocks and nitrogen collection by bacteria--largely
ignoring animals. This view assumed that the role of animals was minor, and
mostly that of a passive consumer of nutrients. "However, this notion may
be a peculiar world view that comes from living in an age where the number and
size of animals have been drastically reduced from their former bounty,"
the team of nine scientists write.
"This study challenges the bottom-up bias that some
scientists have--that microbes are running the show, and phytoplankton and
plants are all that matter," says Joe Roman, a whale expert in UVM's
Rubenstein School of Environment and Natural Resources and the Gund Institute
for Ecological Economics.
"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," says Roman.
On land, before the rise of modern humans, there were
elephant-like gomphotheres the size of a backhoe, deer with twelve-foot wide
antlers, and bison herds to the horizon. These were just a few of the huge
animals that could eat huge amounts of plant matter, accelerating the release
of nutrients through digestion and carrying these nutrients away from feeding
areas to higher ground through their deposit of feces, urine and, upon death,
Overall, the scientists calculate that this animal-powered,
planetary pump may have dropped to just six percent of its former capacity to
spread nutrients away from concentrated sources on both land and sea.
A series of recent studies show that large animals appear to
disproportionately drive nutrient movements. To make their new study, the
team--including scientists from University of Oxford, University of Vermont,
Harvard University, Aarhus University in Denmark, Princeton University,
Netherlands Institute of Ecology, and Purdue University – used these findings
and other existing data about historic and current animal populations. Then
they 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.
For example, whale densities are estimated to have declined
by between 66% and 90% over the last three centuries due to commercial hunting,
the study notes. Most grievously, 350,000 blue whales, many over one hundred
tons, used to inhabit oceans around the globe. Only a few thousand now remain.
These and other great whales feed in the depths and then defecate at the
sun-lit surface "in a flocculent, liquidy cloud," says Roman.
In particular, the new study examined phosphorous, a
nutrient critical for plant growth. Prior to the era of commercial hunting, the
scientists estimate that whales and other marine mammals annually moved around
750 million pounds of phosphorus from the depths to the surface. Now that
figure is about 165 million pounds – some 23% of former capacity.
The team also gathered data on seabird and fish populations
that feed in the sea and then come onto land, like ocean-going salmon that move
up rivers to defecate, spawn, and die. Movements by these birds and fish once
carried more than 300 million pounds of phosphorus onto land each year, but
that number has declined to less than four percent of past values as a result
of destroyed seabird colonies, habitat loss, and overfishing.
"Phosphorus is a key element in fertilizers and easily
accessible phosphate supplies may run out in as little as fifty years,"
says Oxford's Chris Doughty. "Restoring populations of animals to their
former bounty could help to recycle phosphorus from the sea to land, increasing
global stocks of available phosphorus in the future."
The world of giants came to an end on land after the
megafauna extinctions that began some 12,000 years ago that were driven by a
complex array of forces including climate change and Neolithic hunters. And it
ended in the oceans in the wake of whale and other mammal hunting in the
industrial era of humans.
"But recovery is possible and important," says
UVM's Roman. He points to 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."
And many whale and marine mammal populations are also
recovering, Roman notes. "We can imagine a world with relatively abundant
whale populations again," he says.
But have domestic animals, like cows, taken over the
nutrient distribution role of now-extinct large land animals? No, the new study
shows. Though there are many cows, fences constrain the movement of domestic
animals and their nutrients. "Future pastures could be set up with fewer
fences and with a wider range of species," the team writes.
"The typical flow of nutrients is down mountains to the
oceans," says Joe 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."