Why did the population tend to return to a stable state of about 40 buffalo in many different situations?

Because if you had more room, the buffalo can roam around more for food and space so more can be produced.

what's the question?

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.