Biology · Organisms and Populations · NEET
Nt is the population density (number) at the present time t. Nt+1 is the density at the next time step (t+1). B = number of Births, I = number of Immigrants (individuals of the SAME species coming into the habitat from elsewhere), D = number of Deaths, and E = number of Emigrants (individuals leaving the habitat and going elsewhere). B and I are added; D and E are subtracted.
Two factors increase density: Natality (Births, B) and Immigration (I). Two factors decrease density: Mortality (Deaths, D) and Emigration (E). NCERT states this directly: 'two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.' Remember: BI in, DE out.
Density increases when (B + I) is greater than (D + E) — that is, when the members added by births and immigration are more than the members lost through deaths and emigration. If (B + I) equals (D + E) the population stays the same; if (D + E) is greater, the population shrinks.
Immigration (I) is individuals of the same species that COME INTO the habitat from elsewhere — it adds to density. Emigration (E) is individuals of the population that LEAVE the habitat and go elsewhere — it removes from density. A simple trick: Immigration = In; Emigration = Exit.
Usually yes. NCERT says 'under normal conditions, births and deaths are the most important factors influencing population density, the other two factors (immigration and emigration) assuming importance only under special conditions.' The special case NCERT gives: when a NEW habitat is just being colonised, immigration may contribute more to growth than birth rates.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. It appears in NCERT Class 12 Biology, Chapter 'Organisms and Populations,' as Fig. 11.2, under population attributes and growth. NEET can test it as 'which factors increase/decrease density' or as a fill-in-the-blank of B, I, D, E.
Nt is the population density at the present time (t). Nt+1 is the predicted density one time unit later. The equation lets you calculate the future size from the present size plus gains (B, I) minus losses (D, E).
Added: Births (B) and Immigration (I). Subtracted: Deaths (D) and Emigration (E). So density rises only when (B + I) is greater than (D + E).
When a new habitat is just being colonised. NCERT gives this as the special condition where immigration can contribute more to population growth than the birth rate does.
Use 'BI in, DE out': Births and Immigrants come IN (added), Deaths and Emigrants go OUT (subtracted). Then the whole equation is just present size plus IN minus OUT.
