In population ecology, __________ is defined as the number of individuals of the same species that have entered a population and its habitat from elsewhere during a specified time period.

Introduction / Context:
Population size in a habitat changes over time due to births, deaths and movements of individuals in and out of the area. Ecologists use specific terms to describe these processes, and understanding them is essential for analysing population growth and decline. This question focuses on the process in which individuals move into a population from outside and asks you to choose the correct term that describes this movement.

Given Data / Assumptions:

• The definition mentions individuals of the same species.
• These individuals have come into a habitat or population from elsewhere.
• The movement is considered during a certain time period.
• The options are natality, mortality, immigration and emigration.
• We assume standard ecological definitions used in textbooks.

Concept / Approach:
Natality refers to the birth rate, the number of individuals added to a population through reproduction during a given time period. Mortality refers to the death rate, the number of individuals removed from a population due to death. Immigration is the process by which individuals of a species move into a population from outside, increasing the population size. Emigration is the opposite: individuals leave a population and move out to other areas, decreasing the population size. Since the question clearly describes individuals entering the habitat from elsewhere, the correct term is immigration.

Step-by-Step Solution:
Step 1: Focus on the key phrase in the definition: individuals that have come into the habitat from elsewhere. Step 2: Recall that immigration is movement into a population, while emigration is movement out of a population. Step 3: Recognise that natality deals with births within the population and does not involve movement between habitats. Step 4: Recognise that mortality deals with deaths within the population and also does not involve movement. Step 5: Conclude that immigration is the term that exactly matches the described process of individuals entering a population from outside.

Verification / Alternative check:
Population ecology formulas often express change in population size as a combination of natality, mortality, immigration and emigration. For example, population size at a later time can be calculated by adding births and immigrants and subtracting deaths and emigrants. Diagrams in textbooks show arrows pointing into the population labelled immigration and arrows pointing out labelled emigration. These diagrams and definitions consistently use immigration for inflow and emigration for outflow, confirming that immigration is the correct answer for individuals entering a habitat from elsewhere.

Why Other Options Are Wrong:
Option a, natality, refers to the birth rate within the population, not to individuals moving in from outside.

Option b, mortality, refers to the death rate within the population and removes individuals through death, not movement.

Option d, emigration, is the opposite of immigration and describes individuals leaving the population to settle elsewhere.

Common Pitfalls:
Learners often confuse immigration and emigration because the words sound similar. A good memory aid is to link immigration with coming in and emigration with exiting out. Another pitfall is to confuse births with immigration, because both processes increase population size. However, natality refers specifically to new individuals born in the habitat, whereas immigration refers to individuals that were already born elsewhere and then move in. Keeping these distinctions clear helps you choose the correct term in questions like this one.

Final Answer:
The process described is Immigration, the entry of individuals of the same species into a population from outside.