Biology POPULATION GROWTH

### KEY TOPICS

star Factors Affecting Population Size
star Effect on Population Size
star Growth Models

### FACTORS AFFECTING POPULATION SIZE

● The color{violet}("size of a population") for any species is not a color{violet}{"static parameter").

● It keeps changing in time, depending on various factors including color{violet}("food availability, predation pressure") and color{violet}("reduce weather").

● In fact, it is these changes in color{violet}("population density") that give us some idea of what is happening to the population – whether it is color{violet}("flourishing or declining.")

● Whatever might be the ultimate reasons, the color{violet}("density of a population") in a given habitat during a given color{violet}("period, fluctuates") due to changes in four basic processes:

● Two of which color{violet}("(natality and immigration)") contribute an color{violet}("increase in population density") and two color{violet}("(mortality and emigration)") to a decrease.

### EFFECT ON POPULATION SIZE

● color{brown}("Natality") refers to the color{violet}("number of births") during a given color{violet}("period in the population") that are added to the color{violet}("initial density.")

● color{brown}("Mortality") is the color{violet}("number of deaths") in the color{violet}("population") during a given color{violet}("period.")

● color{brown}("Immigration") is the number of individuals of the same species that have come into the habitat from elsewhere during the color{violet}("time period under consideration").

●color{brown}("Emigration") is the number of individuals of the color{violet}("population") who left the habitat and gone elsewhere during the color{violet}("time period under consideration.")

● So, if N is the color{violet}("population density") at time t, then its color{violet}("density") at time t +1 is
color{violet}(Nt+1 = Nt + [(B + I) – (D + E)])

● You can see from the above equation that color{violet}("population density") will color{violet}("increase") if the color{violet}("number of births plus the number of immigrants (B + I)") is more than the color{violet}("number of deaths plus the number of emigrants (D + E)"), otherwise it will color{violet}("decrease.")

● Under normal conditions, color{violet}("births and deaths") are the most important factors color{violet}("influencing population density"), the other two factors assuming importance only under color{violet}("special conditions.")

● For instance, if a new habitat is just being colonised, color{violet}("immigration may contribute") more significantly to color{violet}("population growth") than color{violet}("birth rates.")

### GROWTH MODELS

● Does the color{violet}("growth of a population") with time show any specific and color{violet}("predictable pattern?")

● We have been concerned about color{violet}("unbridled human population growth") and problems created by it in our country and it is therefore color{violet}("natural") for us to be curious if different color{violet}("animal populations") in nature behave the same way or show some color{violet}("restraints on growth.")

● Perhaps we can learn a lesson or two from color{violet}("nature") on how to color{violet}("control population growth.")

● Environmental scientists use two models to describe how populations grow over time: the color{brown}("Exponential growth model") and the color{brown}("Logistic growth model"). Two important concepts underlie both color{brown}("Models of population growth:")

star color{brown}("Carrying capacity") : Carrying color{violet}("capacity") is the number of individuals that the available resources of an environment can successfully support. In color{violet}("equations") and color{violet}("models"), the symbol K represents carrying color{violet}("capacity.")

star color{brown}("Limiting resource") : A color{violet}("limiting resource") is a resource that color{violet}("organisms") must have in order to color{violet}("survive") and that is available only in color{violet}("limited quantity") in their environment. Therefore, a color{violet}("limiting resource") functions to color{violet}("limit population growth. Food") and color{violet}("water") are common color{violet}("limiting resources for animals").