A keystone species is an organism who has a big impact on the environment. The impact is usually an positive impact. The species greatly affects the health of other species and the ecosystem.
Agouti: An agouti is a small rodent that opens the seed pots of the Brazil nut trees. The Brazil nut trees depend on this species to open the seed pots. By doing this the Brazil nut trees can continue to grow and be home to other species. The Agouti collects and buries fruit and nuts. Those fruits and nuts will grow into healthy trees away from the parent tree.
Orangutan: The orangutans live in the Southeast Asian tropical rainforests of Borneo and Sumatra. They are important because they keep the biodiversity. When they eat the fruit and travel, they disperse the seeds. They are keeping the diversity of the rainforest's woody plants.