Population DynamicsPopulation dynamics - the study of the long term changes in population sizes and the factors that cause a change

Current focus is on human populations (ageing studies, population booms or declines), but biologists use population dynamics to study competing species and predator-prey relationships as well

Examples

Examples

Population Dynamics220 year-old discipline

Thomas Malthus proposed the 1st equation to characterize human population growth

This led to equations for exponential and logistic growth which we will look at later

Factors affecting population growthThere are 4 factors affecting population growth:

1) Births (natality)2) Deaths (mortality)3) Immigration4) Emigration

They can all be put into a neat little equation…

N1 = N0 + (B – D) + (I – E)

If you simply want to look at the change in population from one time interval to the next simply look at:

(B – D) + (I – E)

NX = population size in generation X

Population DispersionPopulation Dispersion is how individuals are arranged

in their habitat.

There are 3 types of population dispersion:1. Uniform dispersion

(usually due to competition between individuals)

Population DispersionPopulation Dispersion is how individuals are arranged

in their habitat.

There are 3 types of population dispersion:2. Clumped dispersion

(usually due to uneven distribution of resources)

Population DispersionPopulation Dispersion is how individuals are arranged

in their habitat.

There are 3 types of population dispersion:3. Random dispersion

(usually due to an even distribution of resources)

Population DensityPopulation Density - a measure of how many individuals of a species are found in a given area

Two types of density:1. Crude density

# of individuals divided by the total area

2. Ecological density # of individuals divided by the total useable area

D = N/S D = densityN = population sizeS = area

Density Sample Questions1) A backyard measuring 3.0 m by 4.0 m contains 215

dandelions. Determine the population density of the plants.

2) A small field having an area of 1.5 ha contains a pond with a surface area of 0.3 ha and is home to 300 field mice. Calculate their ecological density.

3) Researchers want to relocate some nuisance black bears into a forest in Northern Quebec that is 750 km2. If each bear requires 40 km2 of forest to live successfully, how many bears can be successfully relocated here?

Population DensityDensity calculations are

done differently if the species being studied is mobile or stationary

Quadrat studies are used for non-mobile populations like plants

Mark-Recapture studies are used for mobile populations like animals

Quadrat StudiesUsed to study non-mobile

populations

Quadrat - a small frame of known area that is placed, at random, in a larger ecosystem

All of the species in question that are found in that quadrat are counted

That number is scaled up according to the ecosystem’s true size

More practical than counting every individual in a given ecosystem!

Quadrat StudiesEstimated Population Density

(EPD)

(basically a density equation)

Total # of sampled individuals

Sampled areaEPD =

Quadrat Study Example4) Scientists are studying the distribution of Trilliums in a

section of Lemoine Point covering 100m by 100m. They place four 1.0m X 1.0m quadrats randomly in this area and count the number of trilliums in each to be 5, 2, 1 and 3

What is the estimated trillium population density?

What is the estimated trillium population size?

What is one source of error in this method?

Mark-Recapture StudiesUsed for mobile

populations

Individuals are trapped/captured, marked with a ‘tag’ and returned to the environment

A Longworth trap: standard small mammal trap

Mark-Recapture StudiesOver time, more

individuals are trapped/captured in subsequent rounds

Every round it’s noted how many individuals are tagged

This data can be plugged into an equation to figure out population density

Mark-Recapture StudyM/N = m/n

M = # of individuals marked in first captureN = estimated population sizem = # of marked individuals recaptured in

2nd roundn = # of individuals recaptured in 2nd round

Mark-Recapture Example5) On day one, 20 warblers are captured in mist

nets. The birds are then marked with leg bands and released. One week later, the nets are reset and 50 warblers are captured. Of these 50, 10 of them were banded from the week before. Estimate the warbler population size.

Transect StudiesA transect is a straight line (or rectangle) chosen by

researchers and along which they will travel, counting the species being monitored

Both mobile and stationary species can be counted this way

Size of the transect depends on the species monitored

Works best when the expected density is low or when the individuals are very large (eg. Douglas Firs)