Answer: Because it can change size based on the needs.
Step-by-step explanation:
The term niche defines an organism´s role in an ecosystem and it also encompass what the organism eat or how it interacts with nonliving elements of the environent.
The realized niche could be regarded as flexible because it is able to change its size based on what it needs. For example, it could increases or decreases its size in competition between resources with other species since they are part of the same ecosystem.