Population density at time \( t+1 \) is typically expressed as:
\( N_{t+1} = N_t + [(B + I) - (D + E)] \), where \( N_{t+1} \) is,
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Inversely proportional to D + E
Four processes change population density. Births (B) and immigration (I) add individuals; deaths (D) and emigration (E) remove them. So the density next time, Nt+1, rises when B + I rise and falls when D + E rise.
Option (1) Inversely proportional to B + I — Incorrect. B + I are added, so more births and arrivals raise the density.
Option (2) Inversely proportional to D + E — Correct. D + E are subtracted, so the more deaths and departures, the lower Nt+1.
Option (3) Directly proportional to E — Incorrect. Emigration means individuals leave, so it lowers the density.
Option (4) Directly proportional to D — Incorrect. Deaths lower the density, the same way as emigration.
NCERT Extract:
Population Growth – “(i) Natality refers to the number of births during a given period in the population that are added to the initial density. (ii) Mortality is the number of deaths in the population during a given period. (iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. (iv) Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration. So, if N is the population density at time t, then its density at time t +1 is Nt+1 = Nt + [(B + I) – (D + E)]
You can see from the above equation (Fig. 11.2) that population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E).”