A food web beside a pond

A fictional class draws a small pond food web from observations of algae, insect larvae, small fish and larger fish. In the drawing, an arrow is defined to point from the food source toward the organism receiving matter and energy. After a dry period, fewer small fish are counted in one short observation session. A learner reverses every arrow and claims the count proves a permanent loss caused by the larger fish. The class has no long-term survey, water-quality data or complete record of other influences. The task is to interpret the stated diagram convention and explain what more evidence would be needed for the wider claim.