C H A RAC T E R I S T I C S O F P O P U L AT I O N S

SBI4U POPULATION DYNAMICS

WHAT IS A POPULATION?

• Population: a group consisting of members of the same species that live together in a certain area at the same time

• Populations live in a particular habitat, that provides them with the resources they need

• Community: consists of all populations of all species that live and interact in the same area

• Ecosystem: A community plus it’s abiotic environment

• Population Dynamics: the study of changes in populations

TERMINOLOGY

• Brainstorm with your lab partner to define the following terms in your own words AND provide an example. • Population size (N)• The number of individuals of the same

species living within a specific geographical area.

• Population density (Dp)• Number of individuals of the same species

that occur per unit of area or volume.

How many daisies in the field?

You have 15 seconds…

How did you estimate the number of daisies?Did you try to count them all?Or did you use another method?

We need a quantitative estimate for the number of daisies – it doesn’t have to be perfect but it should be as close as possible to the real number.

Write your first estimate down, then try again, seeing if this will help…

How many did you count?

How did you use the grid to estimate the number of daisies? Did it help?There are 78 daisies

If you were asked to count the number of daisies in the school field, it would be impractical to count each one.How could you use the grid method to get an

accurate, reproducible estimate? Make a rough plan.

Use your plan to make an estimate for the next field:

Is your estimate the same?

There were 103 daisies in the field. How close were you?

Your plan should have involved the following steps:Select at least three quadrats and count how

many daisies are in each (eg 4, 8, 3)Add them to get a total and divide by the

number of samples to find the mean number of daisies per quadrat (4 + 8 + 3 = 15. 15/3 = 5 daisies per quadrat)

Multiply the mean by the number of quadrats that would fit into the field to get your estimated total number of daisies. (5 x 20 = 100 daisies estimated in the field)

How many daisies were there?

CAN YOU DETERMINE THE POPULATION SIZE?

Characteristics of Populations – Population Density

• Who cares how big or dense a population is?

• The density of a population can be related to how healthy the population is. We will talk about this and what influences population size/density in more detail later.....

MEASURING POPULATION SIZE AND DENSITY

• How do we determine the human population?• Can we use the same methods with other

species? Why or why not?• Instead of counting individuals in a populations

inefficiently, ecologists estimate population size by studying smaller samples extrapolation• 3 methods:• Transects• Quadrats• Mark-Recapture

TRANSECTSTransect – a long line or narrow area used for population sampling1. Researchers choose a line of

specific length (i.e. 100m)2. Walk along the line and count

number of individuals of each species being monitored within the area• For sessile organisms – within

1m• Mobile organisms – within

50mExample: http://www.countrysideinfo.co.uk/wetland_survey/transec1.htm

TRANSECTS

Useful for:1. low population densities

or 2. larger organisms like

trees

QUADRATSQuadrat – a box used to sample a population1. Location is chosen at

random2. Quadrat is drawn

area can be large or small depending on size of species under study (A)

3. Number of individuals are counted within quadrat boundaries (N)

QUADRATS• Information can be used to calculate population

density:DP = N/A

Sample Problem:• Ecologists are sampling the distribution of flowering

white trillium plants in a section of forest covering 100m x 100m (so what is the area)?• They place four 1m by 1m quadrats randomly • They find 5, 4, 3, and 1 individuals in the four

quadrats Calculate the population density

QUADRATS

The calculated population density can be used to determine the estimated population size

Estimated Population Size = DP x total study area

What is the estimated population of trilliums?

QUADRATS

What type of species do you think this method would be most useful for?Useful for:1. Sessile organisms or 2. Organisms with low motility

MARK-RECAPTURE

1. Animals are captured2. Then, marked with a tag/collar/band3. Released back into their habitat4. Recaptured at later time (days/weeks) Watch this captureData is used to estimate the population size according to the following formula:

Population size (N) = M (number originally marked) x n (total individuals in

recapture)/m (marked individuals in recapture)

MARK-RECAPTURE

Sample Problem 1A group of researchers captured 20 warblers, which were marked with leg bands. They were released soon after. One week later, 50 warblers were captured. Of the 50 birds, 10 had bands on them. What would be their estimated population size?

MARK-RECAPTURE

What type of species do you think this method would be most useful for?Useful for:

Highly mobile populations

Population Dispersion

• Dispersion:The population dispersion for a species is the pattern in which individuals in a population are distributed or spaced throughout an area.• Often related to density

• Clumped Dispersion: occurs when individuals are concentrated in specific parts of the habitat• Often due to patchy resource distribution• May be social benefits ie. schooling fish

Population Dispersion

• Uniform Dispersion: Individuals are spaced evenly throughout the habitat• Often due to intense competition. Eg. Nesting, feeding or mating

territoriality, toxins released by plant roots

Population Dispersion

• Random Dispersion: The population is spread out in an unpredictable manner that is unrelated to the presence of others• Least common and hardest to

observe• Resources are not often distributed

randomly

DONE!

HOLD UP! DISPERSAL PATTERNS

1. Resource distribution is uniform2. Resource distribution is clumped3. Resource abundance varies4. Resource abundance is scarce5. Resource abundance is abundant6. Interaction between members of a population is

positive7. Interaction between members of a population is

negative8. Interaction between members of a population is

random.