An ecology research project aims to understand how reducing habitat size impacts extinction rates of a beetle species. Relevant data include the number of beetles that are captured over time in habitats of different sizes. This question focuses on what subfield of ecology?

population ecology.

Explanation:

Population ecology is a subfield of ecology in which the ecologists study various aspects of ecology such as habitat, temperature or other environmental factors which are responsible for the increase or decrease in the population of particular specie. In this subfield of ecology, we study that how is the population changes over time and why the population changes with the passage of time or what factors are responsible for this change.