What If Gone

Species → Common hippopotamus

Common hippopotamus

Hippopotamus amphibius

122,500 individuals
Extinction risk Vulnerable
MegaherbivoreNutrient cyclingWaterwayEcosystem engineerScientific controversy
Common hippopotamus (Hippopotamus amphibius)
Photo: Bernard Gagnon, Wikimedia Commons (CC BY-SA 4.0)

What if it disappeared?

Impact High
Ongoing

By grazing on grassland at night and then defecating in the water by day, the hippopotamus alone transports up to 76% of the total silicon flux entering certain East African rivers — a nutrient essential to diatoms, at the base of the aquatic food chain.

Documented
Detail

Schoelynck, J., Subalusky, A.L., Struyf, E. et al. (2019). Hippos (Hippopotamus amphibius): The animal silicon pump. Science Advances, 5(5), eaav0395.

The study quantifies the hippopotamus's role in the silicon cycle across the East African savanna: by transporting roughly 0.4 tonnes of silicon per day from terrestrial grasslands directly into the water, it bypasses the slow retention processes of the soil. Silicon is essential for diatoms to build their shells — their decline, should this input be lost, could deprive the aquatic food chain of a significant share of its primary production.

During floods

The same organic input can turn lethal downstream: the roughly 4,000 hippos of the Mara River deposit about 8.5 tonnes of partially digested plant matter there per day, and when floods flush their pools all at once, the resulting oxygen-poor water has caused nine fish kills in five years.

Documented
Detail

Dutton, C.L., Subalusky, A.L., Hamilton, S.K., Rosi, E.J. et Post, D.M. (2018). Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills. Nature Communications, 9, 1951.

Of 49 flood events monitored over three years, 13 triggered a marked drop in dissolved oxygen down to hypoxic levels, and nine fish kills were documented over five years. Two distinct mechanisms are involved: the direct mixing of oxygen-poor water from hippo pools with the rest of the river, and the sediments from those pools, which continue consuming oxygen as they decompose downstream once resuspended.

If the decline continues

A sharply declining hippo population would reduce both the risk of fish kills from organic overload and the silicon input that feeds the base of the aquatic food chain — two effects documented separately, and no one has yet established which would dominate as the population shrinks.

Extrapolated
Detail

Extrapolation combinant Schoelynck et al. (2019) et Dutton et al. (2018), qui documentent chacun un mécanisme opposé sans les mettre en balance l'un par rapport à l'autre.

The hippopotamus is an unusual case in this repository: its presence is both beneficial (silicon input essential to diatoms) and potentially harmful in concentration (fish kills from hypoxia during floods). A population decline would shift the balance between these two effects, but in which direction, and past what population threshold, remains to be established.

What's missing to confirm it: No study models how the net balance of these two opposing effects would evolve as the hippopotamus population continues to decline.

Documentation

Last verified: August 7, 2026