First, ask what the change took away, or added. Draining took away the water and added dry ground.

Next, ask which population needed that. The frogs and the water plants needed the water, so their populations got smaller.

Then, follow who eats whom. The herons ate the frogs, so with fewer frogs the heron population got smaller: the herons left.

The insects ate the water plants, so the insect population got smaller too.

Last, ask which population gets what it needs now. Dry-ground grasses need dry soil, so their population grew.

After the draining: the water plants, insects, frogs and herons get smaller, and the dry-ground grasses grow.

Drained land beside a ditch: dry soil and dry-ground grasses, no standing water.