● 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.

● `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.")`

● 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")`.