Why a wave of hippo poop is great news for predators, but not for fish
We explain how hippos and their poop re-engineers the ecosystems around them, and how that can mean life or death for other species.
Known as 'ecosystem engineers', hippos have a dramatic effect on their environment, and on the lives of the other creatures they share an ecosytem with. Shortly before the Covid-19 pandemic, I visited Zambia to discover how hippo poop has such an effect.
Close encounters of the hippo kind
The hippo glances at us briefly, before continuing its lumber across the track. I can’t help thinking that it doesn’t seem to think much of us. Here in the South Luangwa National Park in eastern Zambia, dusk is fast approaching, the sun sinking inexorably towards the horizon.
My guide, Patrick Njobvu, explains that it’s around this time each day that the hippos leave the pools and rivers in which they’ve spent the day, to graze on the land. They follow the same routes each night, creating paths across the land, which gradually become tracks and then, with repeated footfall and the downpours during rainy seasons, turn into channels and gullies. Patrick adds how, when they become deep enough, these gullies are used by leopards to stalk their prey.
Having been dismissed by the hippo, we carry on towards the viewpoint. It overlooks the river, which is now a brilliant orange in the setting sun, criss-crossed by the shadows of the trees on the opposite bank. A few hippos are still lounging in the waters.
All organisms on earth have an impact on their environment or other species in one way or another, I muse, but some modify their surroundings much more than others. Ecosystem engineers, as they are known, can be found all over the world. Think of ants or termites constructing mounds or the combined efforts of key coral-reef organisms. We humans do it to the extent that there is a proposed geological epoch to describe our impact on the planet – the Anthropocene.
The hippos can’t help but create change wherever they are in the landscape. While they’re hanging out in the river, they release organic matter in the form of urine and faeces. In the rainy season, when there’s a lot of water, the constant flow prevents the build-up of faeces. However, in the dry season, rivers become narrower and shallower, and there’s an increased concentration of hippos, and their dung.
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A paper published in 2018 in the Proceedings of the National Academy of Sciences compared hippo pools in central Tanzania that had become isolated when a river stopped flowing – in this case due to water being drained upstream for farms – to pools that weren’t often visited by hippos. The study found that the isolated pools, which had a lot of hippos, and so a low oxygen content, had only half the diversity of invertebrate and fish species compared to the other pools.
“These results show how human disturbances to natural systems, in this case river flow, can alter the role of ecologically important species,” explains Keenan Stears, a postdoctoral researcher at the University of California, Santa Barbara and lead author on the paper. “Under normal conditions, hippos and their dung are highly beneficial to aquatic systems, however, when river flow is disturbed and rivers stop flowing, hippos become agents of eutrophication and aquatic biodiversity loss. These losses not only influence the ecology of watersheds, but can also influence food security for humans because fish, which are heavily impacted, are an important source of protein for many people in Tanzania.”
When it does rain, the extra water pushes the dung downstream, an event known as a ‘flushing flow’. Biologists working on the Mara River, on the border between Kenya and Tanzania, spotted an interesting correlation with dead fish washing up on the riverbanks, sometimes in their thousands, and the flushing flow events. These die-offs were commonly blamed on pesticides being used upstream on farmland fields, but the scientists decided to investigate further. By creating an artificial reservoir and dam, and using hippo-soiled water, they measured the oxygen levels when water was released downstream. They found that oxygen nosedived to levels that were lethal for aquatic animals, causing the fish to suffocate.
However, their misfortune is good news for other species. “As soon as fish start washing up on the bank of the river, they are immediately taken and consumed by any other wildlife in the area,” says Chris Dutton, an aquatic ecologist at the University of Florida and lead author on the paper, published in 2018 in Nature Communications. “The few times that we were able to actually see a fish kill or hear about it from the game rangers, the dead fish would typically be gone by the next day. We would expect that any of the opportunistic feeders within the Serengeti ecosystem would benefit from these fish kills, including lions, crocodiles, vultures, storks, baboons, as well as multiple invertebrates. Basically, anything with a mouth. These fish-kill events are like a free lunch buffet for the wildlife along the river.”
How hippos redesign their habitat
Not all hippos spend their days in rivers. Some of them find wet mud to wallow in, which helps cool them down while providing an additional layer of sunscreen. The hippos’ frequent mud baths churn up the ground, and eventually create pools. As the pools become more established and grow larger, a new habitat is created, supporting a range of plants, invertebrates and fish.
During the heat of the afternoon, Patrick and I venture out to one of the larger pools, where a photographic hide has been built by the water’s edge. A few bushes grow on either side of the hide, one of them filled with blue waxbills. A kingfisher appears on a branch, before diving into the waters in search of prey. Through it all, a hippo’s eyes, nostrils and the large hump of its back have been jutting just above the surface, the huge ungulate perfectly still, save for the occasional flick of its tiny ears.
Ecosystem engineers in the UK
Of course, hippos aren’t the only ecosystem engineers. In the UK, our most obvious example is the European beaver, a native species that went extinct in this country 400 years ago after it was hunted for its meat, fur and glands. It has now been reintroduced, both legally and through escapes from private collections, in a number of locations across Britain.
Beavers cut down trees to build a series of dams on rivers, which in turn create deep pools. This habitat creation benefits many other species including water voles, aquatic invertebrates, trout, amphibians and wetland birds such as teal and snipe. There are benefits for humans too, as the dams and pools improve water quality, slow the speed of water and prevent flooding downstream. They also capture carbon through holding back silt and causing new plant growth.
And it’s not just animals that can have a profound effect on their environment. Named for the rattling noise its seed capsules create when shaken (a sound traditionally seen by farmers as a signal to cut their hay) the beautiful yellow rattle (Rhinanthus minor) is well known to anyone who has tried to create a wildflower meadow from scratch.
It is a hemi-parasite on grasses, causing growths in the host’s roots, called haustoria. The haustoria enable the yellow rattle to draw water and nutrients from the grasses, thereby weakening them and suppressing their growth. This means there is then less competition between the grasses and other wildflowers, allowing the latter to grow and thrive. As a result, yellow rattle is sometimes called ‘the meadow maker’.
European native oysters
Ecosystem engineers can be found underwater too. Around the coasts of the UK, there are conservation projects in place to restore the European native oyster (Ostrea edulis). This is the only true oyster species native to UK waters and was once a key feature of London’s food trade.
However, native oyster populations have declined by 95 per cent in the UK since the mid-19th century, and oyster beds are one of the most threatened marine habitats in Europe, due to many different factors, including disease, pollution, competition with invasive non-native species, and historic over-harvesting. This humble mollusc is important because it has the remarkable ability to improve water quality while it feeds. Seawater is pumped across its gill structures, with a single oyster filtering an estimated 100-200 litres of seawater every day.
“Oysters are unsung heroes of the sea, keeping our coastal environment clean and healthy for fish, other marine habitats and people,” says Celine Gamble, the Zoological Society of London’s (ZSL) Wild Oysters project manager. “Although oysters are small, they are mighty, and can make some huge changes in our marine environment.”
Sleeping with the hippos
Back in Zambia, on my last night in South Luangwa National Park, I have an unforgettable new experience, when I sleep in what’s called the ‘elephant hide star bed’, on a platform up an ebony tree above an ancient elephant crossing of the Luangwa river. It’s open to the skies except for the thin layer of a mosquito net. I have to admit it takes me a while to fall asleep, my ears picking up every rustle and wondering if it’s a leopard making use of the gullies left by the hippos.
However, my focus soon returns to the hippos below. Once I’m attuned to their grunting chatter, it becomes an odd sort of lullaby to help me drift off. In the morning, I rise before the sun and sit on the terrace in expectation of the spectacle to come. With my morning cuppa in hand, I watch in awe as the rising sun lights up the waters below me and reveals the dark, humped backs of the hippos.
And so the cycle continues, just as it has done since these leviathan mammals diverged from their cetacean cousins some 55 million years ago. Every movement of the hippo sends out ripples, not just in the water, but in the ecosystem they shape, and in the biodiversity that their morning wallow supports.
Main image: Two common hippopotamus in the water. © USO/Getty