1. Population and Communities
Chapter 9

2. Studying Populations
Definition: a population is a group of individuals of the same species, living in a shared space at a specific point in time.
They have a lot of interactions and share the same resources:
Water
Food
Sunlight

3. Studying Populations
To describe a population the area that is occupied needs to be specified.
Example: population of Canadian geese in North America.

4. Why do we study populations?
Ecologists study populations of species to discover if the species is thriving or in decline.

5. Population Characteristics
Three main characteristics used to describe populations:
A) Size
B) Density
C) distribution

6. A) Population Size
Definition: population size refers to the number of individuals in a population.
Population size is affected by births, deaths, immigration and emigration.

7. Factors affecting Population Size
Birth: increases population size.
Death: decreases population size.
Immigration: animal joins the population from other region.
Increases population size.
Emigration: animal leaves region and joins different population.
Decreases population size.

8. Stable Population Size
The population size will remain the same if:
_______ + ______ = ________ + _________
Immigration
Death
Emigration
Birth

9. Measuring Population Size
A number of methods exist to measure the size of populations. The most common methods are counting individuals, counting by sample area and mark-recapture.

10. Counting Individuals
If possible, count all the individuals within the area occupied by the population.
Count individuals within the area they occupy. OR
Take an aerial picture of the population.

11. Counting by Sample Area
Provides a good estimate of the population size.
A) Section off sample area into quadrats (rectangular areas marked off with rope or ribbon).
B) The number of individuals are counted in that area and used in the equation below:
Population size = Average number of individuals per section x Total Area
Area of section

12. Counting by Sample Area Example
Example: quadrat is a rectangle (3 m x 4 m = 12 m2), total study area (100 m x 72 m = 7200 m2), average number of individuals (40)
Population size = 40 x 7200 m2/ 12 m2 = individuals.

13. Mark and Recapture
Method used when population moves a lot (sharks, fish etc…)
Catch individuals (nets or cages).
Count the captured animals and mark them.
Release the marked animals back into the population.
Recapture individuals (nets or cages).
Count the individuals with separate count for marked individuals.
Estimate the size of population with the formula (next slide).

14. Mark and Recapture
Population size = Number of marked animalsxnumber captured 2nd time
Number of marked animals recaptured
Example: 30 ducks are captured, marked and released. If on the second capture, 10 of 40 captured ducks are tagged (marked), then the population of ducks is estimated to be:
30 x 40 / 10 = 120 ducks.

15. B) Population Density
Definition: population density refers to the number of individuals per unit area or volume.
Example: there are 21 white-tailed deer per square kilometer on the island of Anticosti.
Population density = Number of Individuals
Space (area or volume) Occupied

16. Population Density Example
Area: A population of maple trees is contained within a 300 km2 forest. The density of the maple tree population is 33.3 maple trees per km2 .
Volume: A pond contains 2500 L of water and 17 lily pads. The density of lily pads is lily pads per liter of water.

17. Population Density Variations
The density of a population can vary depending on the habitat.
The presence of predators, parasites or disease can affect the density.
Increased presence of predators, parasites or disease = lower density
Decreased presence of predators, parasites or disease = higher density
Natural or “man-made” disasters can affect the density.
Deforestation = lower density
The climate and resources within the habitat can affect the density.
More resources = higher density
Fewer resources – lower density

18. C) Population Distribution
Definition: population distribution is the way in which individuals are dispersed within their habitat.
Three types of population distribution
Clumped distribution
Uniform distribution
Random distribution

19. Population Distribution
A) clumped: most common, occurs when certain areas of the habitat offer better living conditions (shelter, food, water …)
Example: fish moving in schools.
B) uniform: due to competition for natural resources.
Example: certain birds are uniformly distributed to allow each bird a certain minimal territory.

20. Population Distribution
C) random: least common
Example: bushes and trees are randomly dispersed because they cannot clump together or spread out uniformly.

21. Ecological Factors
Definition: An ecological factor is an aspect of a habitat that can affect the organisms living there.
Different factors of a habitat can affect the individuals of a population  population density affected.
2 types of ecological factors:
A) abiotic (non-living): factors are ecological factors of physical or chemical origin.
B) biotic (living): factors are ecological factors related to the actions of living organisms.

22. Abiotic and Biotic Factors
Abiotic factors: soil or water pH, amount of light, depth of snow, temperature, air humidity, terrain (land topography).
Biotic factors: predation, birth rate, disease, amount of food, competition, human activity.

23. Limiting Factor
If the effect of the factor is dramatic (reduces the population density or prevents its growth), the ecological factor is called the limiting factor.
Definition: a limiting factor is an ecological factor that causes the density of a population to decrease.
An ecological factor becomes limiting if:
Absent from the habitat (no sunlight)
Insufficient quantities (too cold – not enough heat)
Excessive quantities (too much water causes cactus root to rot)

24. Limiting Factor: Temperature

25. Biological Cycles in Populations
Definition: Biological Cycles are alternating periods of rise and fall in population size.  These periods are of fixed duration and are repeated continually.
Growth Decline

26. The Hare and the Lynx
The population of the hare and the lynx rise and fall together.
The hare population rises  the lynx have more food  reproduce a lot  lynx population increases  more hares are hunted  hare population decreases  less food for the lynx  lynx population decreases  hare population increases etc….

27. Studying Communities
Definition: a community is a set of populations of different species sharing the same habitat.

28. Biodiversity
Definition: biodiversity describes the variety of species living in a community.
We must consider the following to measure biodiversity :
The number of species in the community (species richness).
Relative abundance of each species, meaning the number of individuals of a particular species in relation to the total number of individuals in the community.

29. Biodiversity Example

30. Factors affecting Biodiversity
The biodiversity of a community is high when:
The number of species is high (NOT the number of individuals)
The relative abundance of different species is similar (percentage of each species is similar across species)

31. Interaction between Individuals in a Community
Four main types of interaction between individuals in a community are:
Competition
Predation
Mutualism
Commensalism

32. Competition
Definition: competition is the interaction between living organisms that seek access to the same resource in their habitat.
Example of resources: limited precipitation in a desert allows for competition between plant species.
Competition cannot exist without a limiting factor (resource).

33. 2 Types of Competition
Intraspecific competition occurs between individuals of the same species.
Interspecific competition occurs between individuals of different species.

34. Predation
Definition: predation is the interaction between two living organisms in which one feeds the other.
Predator (the one eating) versus prey (the one being eaten)
Parasitism is a form of predation: parasite versus host. Parasite obtains food from the host. The host is harmed in some way.

35. Mutualism
Definition: mutualism is the interaction between two living organisms that benefit both organisms.
The sea anemone uses the clown fish as bait to attract its prey.
Nemo, the clown fish, is sheltered by the sea anemone.

36. Commensalism
Definition: commensalism is the interaction between two living organisms in which one organism benefits from the relationship, while the other remains unaffected.

37. How Interactions Affect the Population Density
Type of Interaction
Effect on Population A
Effect on Population B
Competition -
Predation and Parasitism +
Mutualism
Commensalism