GRAND DESIGNS
BBC Wildlife|November 2021
Thanks to their impressive stature, hippos just can’t help engineering the ecosystems around them
MEGAN SHERSBY

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.

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.

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