Why can the removal of one important species affect an entire ecosystem?

In southern Africa, the hippo was a keystone species; local fishermen killed the animals because the hippos would attack them during the early evening fishing season.

Reintroduced after the fishermen adjusted their fishing schedules, hippos kept the grasses under control and fertilized. When the hippos perished, the grasses perished, small fish perished, large fish perished, and the lake became sterile.

The removal of one important species can affect an entire ecosystem because species are interconnected through various ecological relationships, such as predation, competition, and mutualism. When a key species is removed, it can disrupt these relationships, leading to cascading effects throughout the ecosystem, potentially causing population declines, changes in species composition, and alterations in ecosystem function.