Every week, it seems, we hear about a new way that humans are screwing up the oceans. Too much plastic. Too much sunscreen. Too much fishing. Next to these problems, “not enough whale poop” does sounds a bit silly, but I assure you, it’s a serious consequence of human exploitation.
Over the last few centuries, whale populations have been decimated by hunting, with global population densities declining anywhere between 66 and 99%. And that’s leading to widespread changes across food webs and ecosystems. Because whales, with their tremendous appetites and equally impressive plumes of excrement, literally fertilize the Earth, bringing nutrients like phosphorus up from the ocean’s depths and transporting these critical resources across the world.
According to a study published this week in the Proceedings of the National Academies of Sciences, the loss of Earth’s largest poop machines, as well as certain smaller fish and seabirds, has had a profound impact on global nutrient cycles, potentially weakening ecosystem health, fisheries and agriculture. Let’s explore how it works.
As any farmer will tell you, dung is an excellent source of fertilizer. So it should come as no surprise that the animals with the largest defecations are giving the most back to the land, so to speak. Take, for instance, filter feeding baleen whales, which slurp down nearly two tons of krill in the deep ocean every day. As these behemoths rise to the surface, that shrimpy meal is digested, all the useful bits are soaked up, and everything else — including phosphorus, iron, zinc and other trace metals — is spewed out the other end in a literal shit-storm.
Marine biologists even have a fond nickname for whale excretions: “poonamis.”
Just as spraying fertilizer on a field will help crops grow, whale poop nourishes the phytoplankton that form the base of marine food chains. Fewer whales could mean fewer phytoplankton, with cascading effects across the entire ecological communities.
The impact of whales on nutrient cycles may seem obvious when you think about it, but large animals weren’t considered an important factor until very recently. It was long held that nutrients like phosphorus, which ultimately comes from rocks, are cycled by weathering, tectonic activity, and other geologic processes. But study co-author and conservation biologist Joe Roman believes this thinking is a consequence of the strange times we live in. “We’re looking at a depauperate planet where these large herbivores and mammals are absent,” Roman told Gizmodo in a phone interview. “So of course we think that way. But if you look back in time, you start to realize these animals could have played a large role.”
Indeed, since the last Ice Age, an estimated 150 megafauna species have vanished—in part due to climate change, but humans have almost certainly played a major role too. Blue whales were once a mighty force to reckon with in the Southern Ocean, but thanks to aggressive commercial whaling throughout the 19th and 20th centuries, their populations have been reduced to 1% of historic numbers. And you kind of have to wonder: With all those epic excretions lost to history, how has Earth’s nutrient recycling system been impacted?
In the new study, researchers took a first stab at quantifying how global nutrient cycles have been affected by the loss of whales and other key species, by drawing on datasets of historic and current animal populations. The researchers plugged their data into a series of mathematical models to estimate how the vertical and lateral movement of nutrients throughout the ocean, and the transport of nutrients from ocean to land, has changed in modern times.
The results were dramatic. Considering lateral nutrient transport by 13 species of gray whales, the study found that the Southern Ocean — a vast region of low biological productivity surrounding Antarctica — is operating today at 2% of its historic nutrient cycling capacity. The North Pacific and North Atlantic are faring only slightly better, with nutrient transport across the ocean surface occurring at 10 — 14% of its former capacity.
Conceptual model of an interlinked system of animals that carry nutrient from the deep ocean to the ocean’s surface and inland. Red arrows show the estimated amounts of phosphorus and other nutrients that were moved or diffused historically, and how much these flows have been reduced. Image Credit: PNAS / Renate Helmiss
The movement of nutrients from the deep ocean to more productive surface waters has also been decimated in modern times. In particular, phosphorus, a nutrient that often limits the growth of plants and marine phytoplankton, is not cycling nearly as efficiently as it once was. According to the researchers’ models, nine marine mammals used to bring 340 million kilos of phosphorus up from the deep ocean each year; that figure has since been reduced to 75 million. The transfer of phosphorus from the ocean to land via migratory fish and birds has also declined to less than 4% of its historical value.
“Fish like salmon carry nutrients up river when they breed,” Roman explained. “They die, and those nutrients can be passed through scavengers far inland.” In certain cases, the transfer of nutrients can be traced from river-breeding fish all the way to mountaintops.
“These aren’t just small, insignificant numbers,” he continued. “We’re talking thousands of tons of phosphorus going from the oceans onto land each year. That would have been much larger before the mass extinction of large herbivores on land and declines in whale and fish populations in the ocean.”
While the paper doesn’t directly address the ecological consequences of weakened nutrient cycles, we know that the abundance of elements like phosphorus influences everything from marine and terrestrial carbon sequestration to the structures of food webs. The Southern Ocean today is considered a biological desert, but could that be a consequence of a weakened biological nutrient pump? What about the fact that many rainforests worldwide, including the Amazon, are thought to be limited by phosphorus?
A school of Big Eyed Scad feeding on phytoplankton off the Hawaiian coast at night. Image Credit: Wikimedia
These are questions Roman and his colleagues are just beginning to answer. But there are also some straightforward ways we can begin to reverse the widespread changes we’ve caused.
“How can we restore these pathways? In the oceans, it’s a matter of restricting kills,” Roman said. “We’ve already done a great job of that, and we’ve seen humpbacks and sperm whales rebounding. But on land, too, we can think of something like the buffalo commons. We should be thinking about trying to restore species where they’re ecologically relevant.”
“This is still a relatively new idea,” he continued. “But we’re gaining some traction — I think conservation organizations are beginning to consider this, focusing on the rarest species.”
Personally, I think “Save The Whale Poop to Save the Oceans” is a great environmental campaign slogan.
[Read the full scientific paper at PNAS]
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Top image: Shutterstock