In population ecology and demography, _____________ is defined as the number of deaths in a population during a given period of time, usually expressed per unit population per year.

Introduction / Context:
Population ecology and human demography use specific terms to describe how populations change over time. Births, deaths, and movements in and out of a population all affect its size and structure. To interpret population growth rates, health statistics, and ecological dynamics, it is essential to know the precise meanings of natality, mortality, immigration, and emigration. This question focuses on identifying the term used for the number of deaths in a population over a certain period.

Given Data / Assumptions:
- The question clearly mentions “number of deaths in the population during a given period.”
- It refers to standard demographic and ecological terminology.
- Options include natality, mortality, immigration, and emigration.
- We assume usual textbook definitions where these four processes determine changes in population size.

Concept / Approach:
Mortality is defined as the incidence of death in a population, often expressed as a mortality rate (for example, deaths per 1000 individuals per year). Natality is the number of births in a population during a given period and contributes positively to population growth. Immigration and emigration describe the movement of individuals into and out of a population respectively and are also important components of population change. Because the question specifically asks for the number of deaths, mortality is the correct term to choose from the list.

Step-by-Step Solution:
Step 1: Identify the key phrase “number of deaths in the population during a given period.” Step 2: Recall that mortality relates to death, while natality relates to births. Step 3: Recognise that immigration and emigration are about movement into and out of a population, not about deaths or births. Step 4: Match the phrase in the question with the definition of mortality as the number of deaths per unit time. Step 5: Conclude that mortality is the correct term.

Verification / Alternative check:
Ecology and demography textbooks usually present a simple equation for population change: population size increases due to natality and immigration and decreases due to mortality and emigration. Diagrams often show arrows labelled “births (natality)” and “deaths (mortality)” alongside “immigration” and “emigration.” Government statistics also use phrases like “infant mortality rate” to refer to the number of infant deaths per thousand live births. These consistent usages confirm that mortality is the term for the number of deaths in a population during a given period.

Why Other Options Are Wrong:
Natality, the number of births, describes the production of new individuals and increases population size rather than reducing it.
Immigration, the arrival of individuals into a population from outside, adds members but does not measure deaths.
Emigration, the departure of individuals from a population, reduces numbers but refers to movement, not to death events.

Common Pitfalls:
Students sometimes confuse mortality with morbidity. Morbidity refers to the incidence of disease, while mortality refers specifically to death. Another common error is mixing up immigration and emigration because their spellings are similar. A helpful memory trick is to link “M” in mortality with “M” in “minus,” indicating loss of individuals through death. Remembering that mortality counts deaths will help you correctly interpret population questions and choose the right term in exams.

Final Answer:
The number of deaths in the population during a given period is called Mortality, the number of deaths in a population during a given period.