 A population consists of interbreeding members of one species living in a specific area, more or less isolated from other members of their species.

 A population changes over time when some members leave and new members join.  A natural population is affected by birth rate, death rate, immigration and emigration.

 A community consists of a collection of populations of different species interacting within a specific area, functioning more or less as a unit with certain identifiable characteristics

 A population explosion may result when there is a rapid growth of a species.  The causes could include: introduction of a new species that eliminates the natural predators a increase in food supply or increase in space available.

 Following a population explosion, the abundant species may take over the ecosystem and change the dynamics of the ecosystem.

 A population extinction occurs when more members of the population leave then enter.  If circumstances within the ecosystem have caused an increase in death rate but not an increase birth rate. .

 This decrease in population can further lead remaining population to produce offspring poorly equipped to withstand environmental changes

 Each population of an ecosystem has its own carrying capacity.  Together a variety of populations of species form a community.  A grouping of communities and the abiotic components of the environment form an ecosystem that exists within the biosphere.

Carrying Capacity  The largest population of a species that can be maintained within an ecosystem is called its carrying capacity.  The main factors that determine the carrying capacity of an environment are:

1. Amount of material and energy available  This not only includes the amount of energy from the sun, but also the water, carbon, nitrogen and phosphorous and any other required nutrients for the species.

2. The amount of food available  The population of a species is limited by the number of organisms in the trophic level below it.  If there is an incident that causes a decrease in the number of producers or primary consumers, a third level organism will not have enough food to survive.  The population of the species is also affected by the population of the species in the trophic level above it. The third level organism is prey to the fourth level organisms. This will also affect the carrying capacity of the species

3. Competition for food, water etc.  The population of a species is also limited by the competition in the region for the food, water, space and mates available to it.  Each member of the population will be competing for the same substances for survival. When the members of the same species compete for food and space it is called intraspecific competition. Interspecific competition is when members of different species compete for the same food, or space.

4. Amount of Space  Once the density of a population reaches its upper limits, nature provides a method for decreasing the population. Overcrowding can cause increased aggression of species, which in turn causes an increase in death rate, thus decreasing population size.

Limiting Factors  The basic components necessary to sustain survival of an organism are food, water, shelter and space.  If there is enough of these components available, the populations of the species will continue to grow. However, uncontrolled population growth cannot continue.

 Ecosystems have environmental controls that keep population growth in check. These controls are called limiting factors.  There are two groups of limiting factors: density-independent factors and density- dependent factors.

Density Independent limiting factors  environmental stresses that limit the growth of the population regardless of the density or numbers of the population  include things such as natural disasters (forest fires caused by lightening, tornadoes, floods or drought), or changes in temperature, the amount of sunlight, or human interference.

 The physical characteristics of some species allow them to cope with environmental changes.  The ability to grow fur when colder weather comes, the ability to adapt to new environmental pressures are all helpful in allowing the survival and prosperity of a population.

Density Dependent Limiting Factors  factors that inhibit the overgrowth of a population when the numbers of that population are large  These are things such as competition for survival requirements (food, overcrowding), predation and disease  Disease and parasitism is more devastating to a population when its numbers are high.