Students are studying the effects of beach pollution by counting populations of seagulls at two different beach locations. One location is a beach near a large industrial marina where boats are serviced; the second location is an isolated beach surrounded by a state park. The students plan to count all the visible seagulls at the beaches at specific times of day. They will repeat the bird count for 10 days and then analyze the data. What is the outcome variable (dependent variable) in this study?