The population numbers look small because this information was taken from only one part of Africa.
The hippo population has increased because of reproduction. It reached its peak in 1983. It then started decreasing from there. The reasons for the population decrease could be overpopulation, the hippo's had too many babies and they died because ther was not enough food in the area. Or simply natural selection; some of the hippos could have been born with variations and they died because they were not fit to live in that area. Based on the graph, we can predict that the hippo population (for this area) will grow through out the years and become a lot larger.