The study of how and why population size change over time is

1. Analyze the question The question asks for the name of the study of how and why population size changes over time. The key words are "change over time", which suggests movement or variation.

2. Explain the concept

• Population estimation is the methods used to count or estimate the number of organisms in an area, such as quadrats and capture-recapture. It does not study causes of change.
• Population dynamics is the study of how and why population sizes change over time. It looks at birth rate (natality), death rate (mortality), immigration, emigration, and the factors that influence them, such as food, predation, disease and competition.
• Population ecology is a broader field. It studies populations and their interactions with the environment, including distribution, density, structure and growth. Population dynamics is a part of it.
• Population cycle is a specific pattern in which populations rise and fall regularly, such as predator-prey cycles. It is a phenomenon, not the name of a field of study.

3. Step-by-step solution

1. The phrase is "change over time", which points to dynamics, meaning the study of change.
2. The phrase "how and why" refers to the causes of change: births, deaths, immigration and emigration.
3. Eliminate A (it only counts), D (it is one pattern, not a study), and C (it is too broad, since it also covers distribution and density).
4. That leaves population dynamics.

4. Answer The correct option is B. Population dynamics.

5. Exam tip Think of "dynamics" as "movement or change". In physics, dynamics deals with forces and motion. In biology, population dynamics deals with the forces (birth, death, migration) that make population size go up or down. If you see "change over time" in a population question, look for "dynamics".