Why do biologists measure the number of trophic levels in an ecosystem?

Biologists measure the number of trophic levels in an ecosystem so as to check its stability and health.

Explanation:

Trophic levels are the feeding position in a food chain such as primary producers, consumers, etc. They are positions that an organisms occupies in a food chain (series of steps in which organisms transfer energy by eating or being eaten);- what it eats and what eats it. 

Theoretically there is no any limit to the number of trophic levels in a food chain or number of organisms that live on each level.

to check stability of their health

Explanation:

which fault and or fold did you find most difficult to diagram