Characteristics of Populations

CHARACTERISTICS of POPULATIONS Habitat: the place where an organism lives Species: organisms that resemble one another in appearance, behaviour, chemistry and genetic makeup and that have the ability to interbreed with each other under natural conditions (produce viable offspring).

Population Size: the number of individuals of a species occupying a given area at a given time (ex. 700 Wood ducks in Elevator Bay) What is the population of humans in Canada? In Ontario? In Toronto? In this grade 12 biology class?

POPULATION DENSITY Population Density (D) is calculated by dividing the population size or number of individuals (N) by the space occupied by that population (S). D = N S

Population Density: China vs. Canada

Usually smaller organisms have much greater population densities than larger organisms. (Ex. 350 field mice per hectare in Algonquin Park vs. 0.8 moose per hectare)

TYPES of DENSITY D = N S or useable S Crude Density: number of individuals of the same species per total unit area or volume. Ecological Density: number of individuals of the same species per useable unit area or volume. D = N S or useable S

Sample Problem If 480 moose live in a 600 hectare section of Algonquin Park, what is their crude density?

Solution If 480 moose live in a 600 hectare section of Algonquin Park, what is their crude density? # of moose = N = 480 Area of space = 600 ha Crude density = N/S = 480 moose / 600 ha = 0.8 moose / ha Therefore the crude density is 0.8 moose /ha or approximately 1 moose/ha.

What is their ecological density if that section of the park includes 800 000 m2 of open water which the moose do not use? (1ha = 10,000 m2)

Solution What is their ecological density if that section of the park includes 800 000 m2 of open water which the moose do not use? (1ha = 10,000 m2) N = 480 Unuseable Space = Area of open water, convert m2 to ha 800 000 m2 x 1 ha 10 000m2 =80 ha of unusable space Useable Space = 600 ha – 80 ha = 520 ha D = N/S = 480 moose / 520 ha = 0.92 moose / ha Therefore the ecological density is 0.92 moose/ha or approximately 1 moose/ha.

POPULATION DISPERSION Population Dispersion is the pattern in which individuals are dispersed throughout a specific area. There are 3 general types:

POPULATION DISPERSION Clumped Dispersion: organisms are densely populated in areas of the habitat with favourable conditions for survival, usually around resources (most populations exhibit this...cattails growing at the edges of ponds, fish swimming in schools...)

POPULATION DISPERSION continued... Uniform Dispersion: individuals are evenly distributed throughout the habitat. Occurs when individuals set up territories (ie Penguins).

POPULATION DISPERSION continued... Random Dispersion: occurs when the habitat is particularly uniform and interactions with other members of the same species do not affect distribution. This is fairly rare in nature, ex.: some tropical trees.

SAMPLING TECHNIQUES It is rare that biologists can take an exact count of all the members in a population due to the size and/or range over which the population lives. Instead biologists count a sample of the population and estimate the population size from this sample.

QUADRAT SAMPLING A common sampling technique for stationary populations is quadrat sampling. A quadrat, or sampling frame, is placed in a region within the habitat and each member of the population of the species in question that appears in the frame is counted.

The more quadrats used the more accurate the population density and population size measurements.

Equations Average sample density = total number of individuals counted in sample/ total sample area Estimated population size = average sample density X total size of study area. ASD = sample N sample Area Estimated N total = ASD x total area

MARK-RECAPTURE SAMPLING A common sampling technique for mobile or dangerous populations is mark-recapture. Biologists trap/tranquilize/catch a number of individuals of a population, mark them and then release them back into their habitat. A short time later they catch a second sample of the population and use the ratio of the marked recaptures to the unmarked captures to estimate the population size.

Example

MARK-RECAPTURE formula Total # marked (M) = # of recaptures (m) population size (N) 2nd sample size (n) Rearrange to solve for population size N = Mn m M = m N n Solving for

The marking must not : harm the individual, prevent it from going about daily activities or make it more easily caught a second time.

The accuracy of the mark-recapture method depends on certain assumptions: every organism in the population has an equal chance of being captured, during the time between captures the proportion of marked to unmarked remains unchanged, and, the population size does not increase or decrease during the sampling study.