In the context of population ecology, which term describes the number of individuals that leave a population and move out of the habitat during a given time period?

Introduction / Context:
Population size changes not only due to births and deaths but also due to movement of individuals in and out of a habitat. In ecology, specific terms are used to capture each type of change. This question checks your understanding of the term that refers to individuals leaving the population and going elsewhere, which is crucial for analysing population dynamics.

Given Data / Assumptions:

• The definition refers to individuals who leave the habitat and go elsewhere.
• The time period of observation is specified but not quantified.
• Options list natality, mortality, immigration, emigration and dispersion.

Concept / Approach:
Emigration is defined as the movement of individuals out of a population into new areas. It tends to decrease population size because some individuals no longer belong to the original population. The opposite term is immigration, which refers to individuals entering the population from outside. Natality and mortality relate to births and deaths, respectively, and dispersion is a broader term about spatial distribution, not specifically about leaving the habitat.

Step-by-Step Solution:
Step 1: Identify that the key idea in the definition is individuals leaving the habitat and going elsewhere.Step 2: Recall that emigration is the correct term for movement out of a population.Step 3: Note that immigration would instead describe individuals entering the population, which is not what the question states.Step 4: Recognise that natality and mortality concern birth and death, not movement.Step 5: Therefore, emigration is the term that matches the description provided.

Verification / Alternative check:
A useful memory technique is to link immigration with "into" and emigration with "exit". Both start with the letter i or e that can remind you of the direction of movement. Using this simple association, you can quickly verify that emigration is about exiting or leaving the population, confirming it as the right choice for the question.

Why Other Options Are Wrong:
Natality: This is the birth rate or number of individuals added by birth, not individuals leaving.
Mortality: This term describes the number of deaths in the population, reducing size through death rather than movement.
Immigration: This is the entry of individuals into a population from other areas, the opposite of what is described in the question.
Dispersion: This refers to how individuals are spaced within a habitat (for example, clumped or uniform) and does not specifically mean leaving the habitat.

Common Pitfalls:
The main confusion arises between immigration and emigration, which sound similar. To avoid mixing them up, always think of immigration as movement into the population and emigration as movement out of the population. Practising with diagrams and population change equations that include birth, death, immigration and emigration terms can help cement these definitions in memory.

Final Answer:
The term used for individuals leaving the population and moving elsewhere is emigration.