How can a physical change to an environment affect the number of organisms in an ecosystem?

It can cause a chain reaction. Every cause has an effect.

Example: Someone throws a cigarette out their window, causing a grass fire

Scenario A: A prairie dog population was killed in the grassfire simply because they had no time to escape.

Scenario B: The grasshoppers don't have grass to eat, so the prairie dogs don't have meat to survive because the grasshoppers are starving. Either way, the person who threw the cigarette caused a decrease in the population.