Gause's experiments on two species of paramecium with very similar niches allowed him to formulate the principle of competitive exclusion. however, he also performed a similar experiment with a third species of paramecium that spends more time swimming on the bottom of the jars, and this species was able to coexist with the first which spent more time in the middle of the jars. what ecological principle does this second experiment illustrate?

The given example suggests the principle of resource partitioning.

The resource partitioning is a method of division of resources present in a particular niche to avoid the competition. In this case, the organism divide the regions having the abundant resources and they do not interfere in each others region.

In this case, the resource partition takes place by dividing the regions in the flask. one species use he resources present in the middle region and other uses the resource in the bottom.