Answer:
the carrying capacity
Step-by-step explanation:
The forty-five individuals is a figure that represents the carrying capacity of Michigan for wrens.
The carrying capacity of an environment or habitat is the maximum number of individuals of a particular species that the environment/habitat can support at a time based on the resources that the environment/habitat has.
The species whose population is below the environment's carrying capacity still has the potential to experience growth in their population while those whose population is already at the carrying capacity can no longer experience significant population growth.
The population is kept oscillating around the carrying capacity by environmental factors such as competition, diseases, predation, etc.