Down in the Dumps

Remember learning about the nutrient cycle in biology class and thinking, Snooze...When can we talk about poop? Well, that time has come. A recent study in the Proceedings of the National Academy of Sciences is spicing up the lesson with a little potty humor. The study shows that large animals, via their massive feces, play an important role in moving nutrients like phosphorus and nitrogen around the globe. In recent times, however, their ability to do so has been severely curtailed by extinction and overhunting.

Nutrients can move about the world in lots of ways, such as mineral erosion, surface runoff, and dust clouds. But while rock weathers at a snail’s pace, animals can move nutrients through the environment pretty quickly, and unlike runoff, this flow can go against the grain. By traveling upstream or uphill to make their deposits, animals bring nutrients to places that might otherwise be severely lacking in nitrogen and phosphorus.

Whales in the Gulf of Mexico, for instance, move nitrogen from the nutrient-rich depths of the ocean to less fertile surface waters through their sizable urine and fecal “plumes.” By eating fish, seabirds bring ocean nutrients to land as they deposit guano all over the rocks. On some islands, plants’ natural phosphorus levels can be twice as much as usual thanks to seabird droppings. Anadromous fish (which migrate between rivers and the ocean) have historically transported 165,000 tons of phosphorus a year inland from the sea. Terrestrial and coastal predators help mix it up, too: bears, otters, and eagles that feed on fish like salmon or shad enrich soil by pooping out sea-based nutrients after a tasty meal. Other land animals can then move those nutrients deeper inland. For instance, wading moose transfer nitrogen from river to shore, and bathing hippos bring land nutrients into waterways.

But much of the megafauna that has historically recycled tremendous amounts of nutrients through ecosystems is now gone. Mass extinctions during the last Ice Age did away with about 150 species of large animals (think mammoths and giant sloths). In the past few centuries, commercial whaling lowered blue whale populations to 1 percent of their historic numbers. Smaller species aren’t doing too well, either. In the Pacific Northwest and Atlantic Ocean, anadromous fish have declined to 10 percent of their previous ranks, with their nutrient-transport capacity at just 4 percent of what it once was. Currently, 27 percent of seabirds are classified as threatened. All in all, the new study shows that nutrient recycling globally is currently at 6 percent of what it was in the late Pleistocene epoch, or what it could be if more whales, seabirds, and fish were allowed to thrive.

In some of Africa’s protected areas, and in other places that have managed to keep their big beasts, the poop cycle still runs smoothly. Nutrient flow within South Africa’s Kruger National Park, for instance, remains exactly what it was in the late Pleistocene. But in regions that have maintained far fewer megafauna—essentially every continent except Africa—wildlife now moves just 5 percent of the nutrients it once did.

Today, humans move a lot more nutrients than we used to, but we do so in ways that aren’t particularly helpful to ecosystems. In 2010, farmers used 53 million tons of phosphoric acid for fertilizer, and that number grows by 1.9 percent a year. To put that in perspective, the most phosphorus seabirds and fish moved from sea to land during their late-Pleistocene poop peak was about 161,000 tons a year. Runoff from our farms eventually washes into waterways where it can create oxygen-depleted dead zones. If we keep burning through phosphorus at current rates, scientists predict that we may run out of global stocks of phosphate rock in as little as 50 years, which would make farming very difficult.

This is why it’s better to recycle nutrients than to plunder them. So, what can be done? The PNAS authors suggest diversifying our livestock beyond mostly cows, and creating more fenceless pastures so domesticated animals can play a bigger role in moving crap around. Bringing back big animals that have been largely hunted out—say bison, elephants, or whales—to some areas might also help. At sea and on the coasts, returning populations of seabirds and anadromous fish to healthier levels could also give a boost to phosphorus availability in years to come.

There is an age-old question, "Does a bear shit in the woods?" For the sake of feeding the world’s humans, we’d better hope so.