Immigration and Emigration: Their Effect on Population Density
Biology · Organisms and Populations · NEET
Immigration is the number of individuals of the same species that come INTO a habitat from elsewhere, so it increases population density. Emigration is the number that LEAVE the habitat, so it decreases density. Memory hook: "I" in Immigration = "In" (density up); "E" in Emigration = "Exit" (density down).
The four processes that change population density: natality (B) and immigration (I) add individuals (green, density up); mortality (D) and emigration (E) remove them (red, density down).
Your doubts, answered
What is the difference between immigration and emigration?
Both involve individuals of the SAME species moving across a habitat's boundary during a given period. Immigration = individuals coming INTO the habitat from elsewhere, so it ADDS to density. Emigration = individuals LEAVING the habitat and going elsewhere, so it REMOVES from density. Simple hook: Immigration = In, Emigration = Exit.
Which two processes increase population density and which two decrease it?
Four processes change density. Two INCREASE it: Natality (births, B) and Immigration (I). Two DECREASE it: Mortality (deaths, D) and Emigration (E). NCERT groups them exactly this way, so learn them as two pairs: (B + I) go up, (D + E) go down.
Are immigration and emigration attributes of a population?
No. They are PROCESSES that change density, not attributes. The attributes of a population are birth rate (natality), death rate (mortality), sex ratio, age distribution and population density itself. NEET has directly asked 'which is NOT an attribute of a population', so do not confuse the two.
Is immigration the same as migration?
No. Migration is a two-way seasonal to-and-fro movement (an animal leaves and later returns), and in NCERT it is a RESPONSE to abiotic stress. Immigration is a one-directional entry of new individuals into a habitat that changes that habitat's density. Do not mix migration (a stress response) with immigration/emigration (density processes).
When can immigration matter more than birth rate?
NCERT says that under normal conditions births and deaths are the most important factors. But when a NEW habitat is just being colonised, immigration can contribute MORE to population growth than birth rate, because there are few or no residents breeding yet. This 'special condition' line is a common one-liner in exams.
⚠️ The NEET trap ✗ Immigration, emigration, natality and mortality are all attributes of a population. ✓ They are the four PROCESSES that change density. The attributes are natality (birth rate), mortality (death rate), sex ratio, age distribution and population density. Immigration and emigration are movement processes, not attributes. 🧠 Attribute = a property the population HAS (like a birth rate). Process = an event that CHANGES it (individuals moving in or out).
Real NEET questions
2018
Natality refers to
A · Number of individuals leaving the habitat
B · Birth rate ✓
C · Death rate
D · Number of individuals entering a habitat
Solution: Natality is the number of births added to the initial density, i.e. the birth rate. The distractors describe the other three processes: 'individuals leaving' = emigration, 'death rate' = mortality, and 'individuals entering' = immigration. Sorting these four apart is the whole point of this topic.
2020
Which of the following is not an attribute of a population?
A · Mortality
B · Species interaction ✓
C · Sex ratio
D · Natality
Solution: A population's attributes are natality, mortality, sex ratio, age distribution and density. Species interaction is a community-level event between two different species, not an attribute. Note that even mortality (a decrease process) counts as an attribute expressed as a rate, while immigration/emigration are pure movement processes.
Solved Organisms and Populations NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Do immigration and emigration involve the same species?
Yes. NCERT defines both strictly for 'individuals of the same species'. Movement of a different species into a habitat is not immigration for the population being studied.
What is the density change equation that uses immigration and emigration?
If N is density at time t, then density at time t+1 is Nt+1 = Nt + [(B + I) − (D + E)], where B = births, I = immigrants, D = deaths, E = emigrants. Density rises when (B + I) is greater than (D + E).
Under normal conditions, which two factors matter most?
Births and deaths (natality and mortality) are usually the most important. Immigration and emigration become important only under special conditions, such as when a new habitat is being colonised.
Can a population's density stay the same even with movement?
Yes. If additions (B + I) exactly equal losses (D + E), density stays constant. Immigration can balance emigration, and natality can balance mortality, giving zero net change.
