Given a scientific question, generate a correct answer to it.

Input: Consider Input: What do you call a predator species that affects other species' populations when its own population size changes?

Output: keystone species

Input: Consider Input: Any sound with a frequency above the highest audible frequency is defined as what, a phenomenon useful in medical diagnosis and therapy?

Output: ultrasound

Input: Consider Input: The location of an object in a frame of reference is called what?
Output: position