Because gorillas are a predator. They eat food and rarely ever get killed for food. But if we take away their habitat by making buildings and houses then they will die for other causes like hunger. If the gorillas go extinct then the food change will get all messed up. The food they normally eat will over populate meaning the food gorillas eat (like fruit) will over populates and take over the area. That means it will start to take up the space that other plants need to grow. The other plants that other animals will need to eat will slowly start moving away because their food is dying. They will start to go somewhere else and mess up that habitat too. Those animals will eat the food needed by the animals that lived there originally and food will become scarce for animals that all need the same type of resources.