Here's why:

* Independent Variable: This is the factor that the biologist changes or controls in the experiment. It's the "cause" in a cause-and-effect relationship.

* Dependent Variable: This is the factor that is measured or observed in response to changes in the independent variable. It's the "effect" in a cause-and-effect relationship.

Example:

A biologist wants to study the effect of different fertilizers on plant growth.

* Independent variable: The type of fertilizer used.

* Dependent variable: The height of the plants.

The biologist would manipulate the type of fertilizer used (independent variable) and observe how it affects the height of the plants (dependent variable).