Pokémon : S3 | E3 – A Sappy Ending

While on their way to Violet City, Ash and his friends stop in the middle of a forest and notice several of the trees are missing their leaves without showing any other signs of damage. Suddenly, Pikachu spots a horde of unknown Pokémon clamped onto a healthy tree trunk. Ash's Pokédex identifies them as Heracross and mentions their docile nature, great strength, and preferred food: tree sap. Thinking he can catch some new Pokémon and save the tree at the same time, Ash is about to throw a Poké Ball when a voice behind him tells him to stop. The trio turns around to see a man in a red uniform, who warns Ash that catching the Heracross will only cause more damage to the forest.

The man introduces himself as Woodruff, a forest ranger. After the group introduces themselves, Woodruff explains that the Heracross are important to the forest's natural balance. They share a commensalistic relationship with the local Butterfree population, who also feed on the tree sap but are too weak to break into the tree trunk to get to it. The more powerful Heracross dig into the trunk to start the sap flow, eat their fill, and then allow the Butterfree to come and feed. When Misty becomes worried about both the Heracross and Butterfree hurting the forest, Woodruff explains that invaders broke the balance of nature in the forest. Just then, a large herd of Pinsir stampede toward the tree. Woodruff identifies the Pinsir as the "invaders" as the Bug Pokémon leap onto the tree trunk and scare away the Butterfree. Ash yells at the Pinsir to leave the other Pokémon alone, while Misty demands that the Heracross do something about it. In unison, the Heracross fly up and attack the Pinsir, while Ash and Misty cheer them on. However, the Heracross are easily scared off by the Pinsir's massive pincers and fly away. Woodruff tells the group that the Heracross are too peaceful to have any desire to take on the Pinsir; this worries Brock, who says that the forest will surely be destroyed if the Heracross don't do something to prevent it.
(Source: Bulbapedia)