Maybe the decrease in plants they eat Otherwise known as deforestation
The Hippopotamuses lives in the river and lakes of Africa. They are herbivores and co exist with other herbivores and carnivores.
There can be two possible reasons for the decrease in the number of hippos.
The carnivores lion and hyena can eat the hippo(young hippos) which will reduce the number of hippos.
The other reason could be reduction in the number of plants that hippos eat.
hope this helps
