Briefly explain why biologists use sampling, rather than counting all members of the population of ...

You're questioning why take samples over census.

A sample is a subset of units in a population, selected to represent all units in a population of interest. It is a partial enumeration because it is a count from part of the population. Information from the sampled units is used to estimate the characteristics for the entire population of interest.

The characteristics of the sample are then used to draw statistical inference about the population from which the sample is drawn.

A census is a study of every unit, everyone or everything, in a population. It is known as a complete enumeration, which means a complete count.

It's easier, cheaper, and faster to administer.