The fish are dung for: hippo poo threatens African river life
Combination of climate change and overfertilising is turning hippo pools into fetid black cesspools
Hippo poo is the latest threat to Africa’s biodiversity.
The effects of climate change on rivers mean accumulations of dung are killing off aquatic life.
“Everything we thought we knew about how African ecosystems work appears to be changing,” said US ecologist Douglas McCauley. “Global change has turned once-productive hippo pools, once teeming with fish and life, into fetid black cesspools.”
McCauley was a member of a team from the University of California and Sokoine University of Agriculture in Tanzania, which studied water flow and hippo density in the Great Ruaha River in the 20,226km² Ruaha National Park, Tanzania’s largest.Reporting their findings in the Proceedings of the National Academy of Sciences, the team said the river was the “backbone of life” in the region, but it had stopped flowing every dry season since 1993.
“During the dry season, when there was no flow, the [hippo] pools were completely separated,” said study leader Keenan Stears. “We found a huge build-up of hippo dung, and therefore nutrient concentrations within high-density hippo pools.”
For millennia, hippo dung was a source of fertiliser for aquatic food webs, but the amount of oxygen dissolved in the pools’ water was so low that most fish species could not survive in it, said Stears.
Deforestation and water-intensive agriculture had been altering water cycles for decades, but climate change was a new factor changing their behaviour.“This work ... illustrates that the net impact of hippos on river ecosystems is dynamically controlled by river hydrology and reveals the capacity of human disturbances on river flow to drastically alter the role of ecosystem-linking species,” said Stears.The researchers tested nearly a dozen attributes of water quality and measured the diversity and abundance of aquatic life in hippo pools over multiple years, both when river flow was high and during dry periods.
McCauley said the results were an alarm bell for African wildlife. “Hippos are to Africa what polar bears are to the Arctic.”When the rains returned and the river resumed its flow, the researchers saw a revival of biodiversity. “This suggests some kind of resilience within the system that allows it to recover after the hydrological disturbance every dry season,” said Stears.“This resilience signifies that there is hope for this system, but without intervention soon the chronic stress caused by river drying and overfertilising of hippo dung may cause long-term species loss in this river system.”Stears said the study’s findings highlighted the value of water-management policies and land-management practices to ensure the sustained health and functioning of African watersheds.“A lot of our results directly assess how changing river flow alters the hippos’ influence on the ecological diversity and functioning of watersheds,” he said.
“However, these findings also call attention to the profound ways in which the dry-season impacts of hippos may influence local communities that rely on rivers as a food source.
“Tilapia are a commonly consumed fish throughout Africa and, during the dry season, we found that the presence of hippos reduced tilapia abundance by 41% across the watershed. That’s not only bound to have ecological consequences but will also impact the human populations that rely on these rivers.”