Population

Population Members of the same species living in a specific

geographic area.

Species – A group of similar organisms that can

breed and produce fertile offspring

Factors that effect population size:

• Availability of Food

• Weather conditions

• Space

• Breeding Patterns

Population• When studying a population boundaries are set to

limit or make a study easier

Population Density – The number of individuals of a

particular species per unit

area or volume

• Can be expressed as a formula:– Individuals / Unit Area– 1000 Trees / 50 km² = 20 Trees/km²

• 20 Trees per km²

Limits to Growth “Factors”Exponential Growth – Population multiplies by a constant factor at constant time intervals• A populations ability to grow depends partly on the

rate at which an organism can reproduce.Reproduction – the formation of new individuals.

– Bacteria reproduce very fast– Rabbits fast– Panda – S L O W – one reason they are critically

endangered – fewer than 1000 in the wild

Exponential Growth

• Exponential growth is usually represented by a J – shaped curve

Limiting FactorsA condition that can restrict a population growth

• Space – “Normal” cells stop dividing when they run out of

room to grow• Cancer cells do not; they loose this restriction and continue

to grow beyond available space.

Lung Cancer Cells

Limiting Factors• Food• Disease• Carrying Capacity – The number of organisms in a population that the environment can maintain or “carry” with no net increase or decrease.• As you near the Carrying Capacity death rate may

increase or birth rate may decrease or BOTH– Continues till about equal

Carrying Capacity

• It is usually represented by an S – shaped curve.

Factors Affecting Population GrowthDensity – Dependent Factors – A factor that limits

a population more

as population

density increases.

• Availability of food

• Disease that spreads easier among organisms in a dense population; than one that is less dense.

Factors Affecting Population GrowthDensity – Independent Factors – Factors that limit

populations but are

unrelated to population

density.• Extreme Weather: Hurricanes, blizzards, droughts• These conditions have the same effect on population

regardless of its size• Fires, floods, habitat changes like deforestation

Factors Affecting Population Growth

Growth Cycles

“boom-and-bust” Growth Cycles• A rapid increase in population for a period of

time; followed by a rapid decrease in number– An uncontrolled growth cause a hormonal secretion

making animals sterile (rodent)• A limit to food

Human Population: Birth Rate vs. Death Rate

History

• As with all other species, the human population growth depends on birth and death rates.– Birth rates go up and death rates go down population

goes up

Human Population: Immigration & Emigration

Immigration – individuals move into an area

from elsewhere

• Increase the population

Emigration - individuals move out of the

population

• Decrease a population

Human Population

• 10,000 years ago farming replaced hunting and gathering– Provided a consistent food supply– More food = increase population

• Death rates over the last few centuries have decreased.

Predicting Population Growth

Age Structure – the proportion of people in

different age groups• In the U.S. age groups are balanced• Some nations (Sweden) have a declining growth rate

– Low birth rates and a top heavy age structure

Sweden’s Growth RatesBlue – Certified Birth Rates

Red – Certified Birth Rates

Age Population Pyramids

Predicting Population Growth

Species interact in Biological Communities Populations do not live in isolation; they interact

with other populationsThey do not live in isolationIn some cases they compete for resources such as food,

water, or shelter.

Competition

Community – all inhabitants living in a particular area.• From the organisms in the soil all the way up the food

chain

Interspecific Competition – competition between species• This competition within a single species limits the

growth of the population• This happens when two or more species rely on the

same limited resource

Disturbances and Communities• Communities change continuously

– Natural disturbances:• Fires• Volcanic Eruption• Floods• Storms• Droughts

– Changes not always negative– Some plants require extremely hot temperatures (in a

forest fire) to sprout

• Destroy Organisms

•Change resources such as shelter and water

Disturbances and Communities

• Man made disturbances:

Disturbances and Communities

• Positive “man made” natural disturbances:

Human Activities / Species Diversity

Humans have the greatest impact on Humans have the greatest impact on communities world widecommunities world wide

Land ClearingLand Clearing

• Areas cut down for lumber, planting crops, building (homes)