How hippos produce over 20 kilograms of “natural fertiliser” a day and keep entire rivers alive

Hippo experts have discovered the very significant part hippos play ecologically in the African aquatic ecosystem. For instance, an average Hippopotamus deposits over 20kg of manure per day, thus depositing large amounts of organic matter into the river and lake. This organic matter is said to be a type of fertiliser for organisms like algae, fish, insects, and microorganisms, which make up the bottom of the aquatic food chain. According to research findings published in the journal "Scientific Reports," hippos carry organic matter from the grassland areas through nighttime grazing activities, depositing it into the rivers as they return to them during the day.

Why hippo poop is vital for natural ecosystems

While hippos live submerged in water bodies throughout their lives, the effects go far beyond the rivers’ banks. Hippos leave the rivers and go out to graze on grasslands every evening and then return to water bodies before sunrise the next morning. As hippos do so, they bring an enormous amount of organic matter to aquatic environments.

According to a study conducted by scientists from the Department of Biological Sciences, Marine Research Institute, University of Cape Town, who investigated hippos' contribution to nutrient recycling in rivers of East Africa, hippos contribute to the enormous amounts of nutrients flowing into water bodies through their droppings. Scientists found that hippos “mediate connectivity and material flows across terrestrial-aquatic corridors” through direct transport of plant material to rivers.

The amount of grass carried by the hippos to the rivers was calculated at about 36 tonnes per day into the Mara River System.

How Hippo dung acts as a natural aquatic fertiliser

In much the same way fertiliser is applied to the farmlands, hippo excreta adds various nutrients, such as carbon, nitrogen and silicon, to river waters. These nutrients promote the growth of algae and microbes, which constitute the very basis of aquatic food webs.

As evidenced by the study titled 'Effects of the hippopotamus on the chemistry and ecology of a changing watershed', conducted at the University of California, Santa Cruz, hippo manure had been used as "a natural source of fertiliser that fuelled aquatic life" for ages.

This process seems quite efficient. The animals eat vast amounts of grass during the night and then excrete partly digested plant material during the daytime hours. It was argued by researchers that the cycle of continuous exchange of nutrients helped ensure biodiversity in numerous African rivers and lakes.

Another independent research showed that hippos deliver about 0.5 tons of silicon into water bodies each day, thus contributing to the balance of nutrients in aquatic ecosystems. This silicon is necessary for the development of microscopic algae known as diatoms.

The environmental downside of too much Hippo waste

While the dung of the hippo is important for ecosystems, too much of this waste material can cause negative impacts as well. With rivers having decreased amounts of water running through them, too much decomposing faecal material can take away oxygen from the water in these rivers.

In an examination of the Mara River, scientists have discovered several occasions where there were incidents of oxygen deficiency which could be traced back directly to the hippo pool. In some instances, fish kills were found further downstream following rainstorms disrupting their waste piles. The scientists concluded that “organic matter loading from hippopotami causes the repeated occurrence of hypoxia in the Mara River”.

Nevertheless, researchers highlight that hippopotamuses are extremely important for ecosystem engineering. While their dung may not look very appealing, it plays a critical role in nutrient recycling in freshwater environments.

Rather than mere river dwellers, hippos make their presence felt as ecosystem engineers every day. One huge dung heap at a time.