Population DynamicsDay 4

Population Characteristics

• Population Density: – The number of organisms per unit area

• Dispersion:– The pattern of spacing a population within an area– 3 main types of dispersion

• Clumped• Uniform• Random

– Resource availability determines dispersion patterns

Population Limiting Factors

1) Density-independent factors– Factors that limit population size, regardless of

population density.– usually abiotic– include natural phenomena

• Ex. drought, flood, fire extreme temperature, tornadoes, etc.

Population Limiting Factors (cont.)

2) Density-dependent factors– Any environmental factor that depends on the

density of the population– Usually biotic – For ex.

• Predation• Disease• Parasites• Competition

3) Population growth rate– How fast a population grows– Factors that influence this are:

• Natality (____ rate)• Mortality (_____ rate)• Emigration (the number of individuals moving _________

a population)• Immigration (the number of individuals _________ a

population)

birthdeath

away frommoving to

Population Limiting Factors (cont.)

Understanding Exponentials

• Put your pens down for a minute & think about this:– An employer offers you two equal jobs for one hour

each day for fourteen days. – The first pays $10 an hour.– The second pays only 1 cent a day, but the rate

doubles each day.– Which job will you accept?

Understanding Exponentials

0102030405060708090

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Job 1 Job 2

Job 2 lags for a long time before

exponential growth kicks in!

Now, how much would your employer owe you if you stayed at this job for another 2 weeks?

What would happen if this type of growth took place

within a population?

1) Exponential growth model• Also called geometric growth or J-shaped growth.• First growth phase is slow and called the lag phase• Second growth phase is rapid and called the exponential

growth phase• Bacteria can grow at this rate, so why aren’t we up to our

ears in bacterial cells?

Population Growth Models

• Limits to exponential growth– As population density increases, competition

follows as nutrients and resources are used up– The limit to population size that a particular

environment can support is called carrying capacity (k)

– When you’re done writing, put your pens down…

Population Growth Models (cont.)

What population do you think this is?

So, what do you think is going to happen to the human population?

• We will probably reach our carrying capacity.• Our growth rate will start to look like most

organisms, which is the Logistic Growth Model

Carrying Capacity (k)

What letter does this curve kind of look like?

2) Logistical growth model– Often called the S-shaped growth curve– Growth slows or stops following exponential growth at the

population’s carrying capacity– Populations stop increasing when:

Birth rate < Death rate Emigration > Immigration

– Carrying capacity can be raised or lowered. How? Ex. 1: Artificial fertilizers have raised Ex. 2: Decreased habitat can lower k

Population Growth Models (cont.)

Community Dynamics

Day 5

Communities

• Review:– A community is a group of interacting populations

that occupy the same area at the same time.

Limiting Factors of Communities

Any abiotic or biotic factor that restricts the numbers, reproduction, or distribution of organisms.

Range of Tolerance

Upper and lower limit of conditions in which an organism can survive.

Ex. Temperature, Salinity, pH

Ecological SuccessionThe change in an ecosystem that happens when

one community replaces another as a result of changing biotic and abiotic factors

Ecological Succession (cont.)2 types:10 Primary Succession20 Secondary Succession

Primary Succession

– In primary succession (10), the community establishes in an area of exposed rock (no soil)

It occurs very slowly at first

Primary Succession (cont.)– Pioneer species, or the first organisms to arrive,

are usually lichens or mosses– They secrete acids that can break down rock,

creating soil.

Primary Succession (cont.)

– Small weedy plants and other organisms become established.

– As they die, more soil is made

Primary Succession (cont.)

– Animals, water and wind bring seeds that sprout– Eventually there is enough soil for shrubs and

trees to grow.

– The stable, mature community that eventually develops from bare rock is called a climax community.

Primary Succession (cont.)

Secondary (20) Succession– Disturbances (fire, flood, windstorms) can disrupt

a community.– After a disturbance,

new species of plants and animals might occupy the habitat.

Secondary Succession (cont.)

– Pioneer species in 20 succession are usually plants that grow in disturbed area.

– Much fasterthan 10

succession

Succession End point?

– Cannot be predicted– Different rates of growth &

human involvementmake it impossible toknow if a true climaxcommunity has beenreached.