Populations

IB Topic 5.3

Populations

How do populations grow and maintain themselves?

Recap:A population is a group of individuals of the same

species that have the potential to breed with each other

Some populations are stable, some have rapid growth, some crashPopulations can vary greatlyWhy? What factors in an ecosystem control population

size?

Population growth

Think of the situation where a small number of young rabbits enter a large, well stocked meadowWhat would happen to the population size? What would eventually happen to the amount of

available resources? How might population growth be affected?

Population growth curve

If we were to graph this situation (# of individuals vs. time), the shape of the graph would be sigmoidal (S-shaped curve)

Figure 5.6 in your book

Lag phase

Stages of population growth

1. Lag phase Little or no growthPeriod of adaptability

2. Exponential growth or log phase Number of individuals

increases at a faster and faster rate

Lag phase

Stages of population growth

3. Transitional phase or linear growth Growth rates slows

down considerably Population is still

increasing

4. Plateau phaseNumber of individuals

has stabilizedNo more growth

Causes for exponential growth

Plentiful resources Food, space, light

Little or no competition from other inhabitants

Favorable abiotic factors Temperature, dissolved oxygen

Little or no predation or disease

Causes for the transitional phase

Increasing competitionPredators, attracted by a growing food

supply, start to move into the areaLimited space leads to opportunities for

disease to spread

Causes for the plateau phase

Less spaceLimited food supply

Results in less offspring

Predators and disease In this phase, the number of births = the

number of deaths No growth

Exponential growth cannot be maintained

Populations cannot continue to grow and grow forever

As a population increases, it begins to experience environmental resistance Space and resources are reducedCompetition increases

There comes a time in growth when its numbers stabilize

Carrying Capacity

The maximum number of individuals that a particular habitat can support is called the carrying capacity Represented by the letter K

If numbers start to increase above K, shortage of resources reduces the numbers of offspring producedPopulation regulates itself at the carrying capacityPopulations tend to be naturally self-regulating

Population Fluctuation

Most populations do not show significant fluctuations, but show varying degrees with time

Why?

4 main factors that affect population size

The birth rate (natality) variesThe death rate (mortality) variesMobile members move away (emigration)New members move in (immigration)

Which two factors cause population growth?Which two factors cause population decline?

Limiting factors can affect population size

Limiting factors define the carrying capacity of a habitat

Examples: Availability of resources (water, sunlight,

shelter, space, oxygen)Build up of waste (excrement or excess carbon

dioxide)PredationDisease

Two categories of limiting factors

Density-dependent limiting factorsRelated to population size

Competition for resourcesPredationDisease

Density-independent limiting factorsUnrelated to population sizeDrought, intense cold

What’s our carrying capacity?

Many biologists, environmental groups, economists, and governments wonder what is the carrying capacity of Earth for human population

We are in exponential growth phaseWill climate change, competition, and

disease lead to a transitional phase or plateau phase?

In conclusion

The size of a population depends on births, deaths, immigration, & emigration

Populations tend to follow sigmoidal-shaped growthLag phaseExponential phaseTransitional or log phasePlateau phase

The carrying capacity (K) is the max number of individuals that the region can support

Exit Slip

You need a sheet of notebook paperProper headingTitle: Populations Exit Slip

Exit Slip

1. Draw and label a population growth curve (also be sure to label the axis).

2. Suggest what factors might lead to an increase in a population of songbirds.

3. Suggest what factors might eventually limit the growth of that population.