Hippo poop cycles silicon through the East African environment

Hippos keep the nutrient silicon on the move through the East African environment.

Each day, the giant grazers transport nearly half a metric ton of silicon, an important nutrient for both plants and animals, from land to water, scientists report online May 1 in Science Advances. The hippos forage for silicon-bearing grass on land and then excrete it into the waters where they lounge.

A team led by biologist Jonas Schoelynck of the University of Antwerp in Belgium tracked silicon moving through Kenya’s Mara River, a hippo hangout, by analyzing ratios of two silicon isotopes — versions of the element with different masses — in grasses, hippo feces, soil and waters. Those ratios are modified by different biological and chemical processes, so can act as fingerprints for the different sources of silicon.

The team found that hippos play an outsized role in cycling silicon through the local ecosystem. Hippos grazing on grasses in the savanna can consume about 800 kilograms of silicon daily through the plants. As a hippo lingers in the water, it can excrete about half of the silicon it consumed. All told, the animals “pumped” 0.4 metric tons of silicon from the grasslands into the Mara River daily, increasing the total amount of silicon measured in the water by more than 76 percent, the team estimates.

Having more silicon available is particularly important for tiny floating alga called diatoms to build their silica shells (SN: 7/17/04, p. 42), the researchers note. But hippos are threatened by hunting and habitat loss: Populations decreased by up to 20 percent from 1996 to 2004. If the animals were to vanish from the Mara River, diatom growth in the water could decrease dramatically. Because the single-cell diatoms are at the base of the food web, that, in turn, could cause a cascade of food shortages across the ecosystem.