5.1+How+Populations+Grow

__ Characteristics of Populations __
There are three main characteristics of populations: georgraphic distribution, density, and growth. These characteristics make up all the other characteristics of populations.

Geographic Distribution
Also called range, geographic distribution describes the area inhabited by a population. It can be measured in measurements as small as cubic centimeters, to as large as square kilometers by saying 300 people to every square mile, for example. Populations may inhabit many different types of ecosystems, like Brown Bears, or they may inhabit only a single type of ecosystem, like Kiwi Birds, which only live on the island of New Zealand.

Population Density
Population density is the number of individuals per unit area, like in geographic distribution. Depending on an ecosystem and the type of speicies, the densioty of species can change a lot. The number of cacti in Massachusetts, for example, is very low, and therfore the density is very low. However the number of cacti in somewhere in the desert would most likely be much higher. The ecosystems are different and make a large difference in the density of the population. The type of species also affects it. Cacti in Massachusetts may be rare, but the number of Coniferous trees in the area would be much higher. In terms of the human population, there are population density maps, like the one to the right, that measure the amount of people in one area. Using measurements of geographic distribution, the display how many people per square mile, or another measurement.

Population Growth
A population can grow or decrease, sometimes rapidly. There are three factors that can affect the growth of a population. These are number of births, number of deaths & the number of individuals that leave or enter a population. The birthrate is referred to as ** natality **. If the natality of a population is equal to the death-rate or ** mortality **, then the population remains the same. If the natality of the population is greater then the mortality then there is a population increase. If the mortality is greater then the natality then the population shrinks. The third factor is the number of individuals that enter or leave a population. **Immigration** is the movement of individuals into an area, or into a new population. Animals from outside the population come in to find food, or mates. Immigration increases the size of a population. **Emigration** is the movement of individuals out of an area. Emigration causes a population to decrease in size. Animals sometimes emigrate if there is a shortage of food in one area, or if young animals reach their maturity and leave the area to find their own territory.

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