Hippo Movements

Hippos move nutrients from grasslands to waterways.

Christopher Dutton

Excrement from large animals is nutrient-rich and often deposited some distance from where nutrients were ingested. This link in the nutrient cycle has an important effect on the ecosystems at the source and at the recipient locations. Studies have focused on carbon, nitrogen, and phosphorus cycles, but the silicon cycle has been largely overlooked. New research conducted in the Maasai Mara National Reserve in Kenya suggests hippos (Hippopotamus amphibius) are important to the savannah silicon cycle, with profound ecological consequences for aquatic environments.

Hippos are semi-aquatic animals that feed on silicon-containing savannah grasslands at night. During the day, they rest in river meanders, where they excrete large volumes of feces. Biologist Jonas Schoelynck of the University of Antwerp, Belgium and an international team of scientists determined just how much silicon hippos move from terrestrial to aquatic ecosystems. They sampled dissolved and particulate silicon at ten sites along the Mara River, home to about 4,000 hippos. The samples were analyzed to determine the overall silicon input from the study area to the river, and an isotopic analysis was used to trace the path of silicon cycling through the plants to the hippos to the water.

The team estimated that hippos in the study area excrete 11.1 metric tons of feces—including 400 kilograms of silicon—into the Mara River per day. They calculated that fresh hippo feces contributes nearly a third of the particulate silicon in the river. Furthermore, hippo-related activity affects over 76 percent of the increased silicon load in the river.

These findings may have important implications for the health of Lake Victoria, downstream from the Mara River. Silicon is an essential element in the outer cell wall of diatoms—microscopic organisms that are the foundation of aquatic food webs, and that help to prevent large algal blooms. As hippo populations decline from poaching and habitat loss, an important part of the silicon cycle is lost, and less silicon will be available to aquatic ecosystems downstream. The study “highlights how important wildlife is, in general, for sustaining transfers of elements,” said Schoelynck, who in future studies will try to determine how much silicon non-aquatic grazers in the same ecosystem contribute to the river. (Science Advances)