Populations

A population is a group of individuals of the same species in the same place at the same time

The amount of individuals making up a population is called the population size.

The human population of Muncie is 68,750. The world human population is 7.674 billion

Population size can describe all species, not just humans.

The squirrel population in my neighborhood might be 236

The black walnut tree population in Muncie might be 670

Population density is the number of individuals of a population in a specific area

You can calculate population density (D) of squirrels in your yard by dividing the number of individuals (n) by the area that they inhabit  $\left(D=n\div area\right)$  

For example, if there were 140 earthworms (n) in a  $4m^2$  area of your yard, the density (D) of earthworms would be 140 divided by  $4m^2$ .

 $D=n\div area$  

 $D=140\ earthworms\div4m^2$  

 $D=35\ earthworms\ per\ m^2$  

With all of this info ecologists can predict how populations will change .

If a population could grow without anything stopping it, it would have exponential growth. The graph shows what an exponential growth curve looks like.

If a bacteria began at noon dividing every hour, by one there would be two of them.

By 2:00 there would be four. At 3:00 there would be 8 bacteria.

Every hour the number would double.

By noon the next day there would be 16,777,216 bacteria (now that's an infection!)

Exponential growth is rare in nature for many reasons.

Predators, space, available food, all of these factors limit the growth of populations

The greatest population a particular environment can support is called the carrying capacity

A graph of data showing the carrying capacity of a population is different than an exponential growth graph curve.

This graph shows the carrying capacity of a population. The population will grow until it reaches the carrying capacity, then it will level off.

In nature all sorts of factors impact population sizes - forest fires, hunting, drought.

As a result, population growth rarely is a smooth curve.

How would different factors impact different populations?

The population of Seals in Alaska grew over the past century due to limits on seal hunting.

Early in the century the population was low because of hunting. With the limits in place the population has grown.

In normal natural ecosystems populations go through cycles

When conditions are good, populations grow, often these times are followed by population decrease.

In the chart, when the population of wolves was down the moose population increased, when the wolf population went up, the moose population went down.

All of the interacting populations in an area make up a community

For example, a city park community may be made up of grasses, and weeds, trees and bushes, birds, squirrels, racoons, hawks and maybe even cayotes.

A community and its abiotic (nonliving) environment make up an ecosystem.

Scientists group all of the ecosystems of the Earth into large groups called biomes.

A biome is a very big ecosystem characterized by it's dominant plants, animals and features.

There are two major types of biomes: aquatic and land biomes.

The major aquatic biomes are shown in this picture.

The organisms that live in each biome are determined by the amount of saltiness, the movement, and the depth of the water.

Land biomes are categorized as tropical forest, savanna, desert, polar and high-mountain ice, chaparral, temperate grassland, temperate deciduous forest, coniferous forest, and tundra. 

Temperature and rainfall are the main abiotic factors that determine the kinds of organisms that live in each land biome.