Then, the 1994 genocide happened. As returnees settled in the park in the following decades, wildlife numbers started to drop dramatically.
"The park was the only spare piece of land in Rwanda. The returnees all settled here with their cows," recalls Jes Gruner, the park's manager.
"The lions took advantage of the cattle and killed them, and the only way the herdsmen knew how to get rid of the lions was through poisoning."
In recent years, the Rwandan government has taken steps to preserve the invaluable natural resource, since employing local villagers as guides, and hence incentive to protect the animals left in the park. So far, it's a tactic that seems to be paying off.
"(In the beginning), we counted, like, 300 buffalos in the whole park. Now there is almost 1,000," says Penninah Kamagaju, a community guide.
"Even the impala, which is a kind of antelope. Those have really increased a lot. We had 500 and now there is 5,000," she estimates.
Attitudes towards wildlife protection have also changed considerably.
"Our grandfathers and our fathers were poachers because they didn't understand why it's important to protect the area," notes Daniel Nishimwe, another community guide.
"But as the young generation, we studied about conservation and we know why it is important to protect the area."