Population

Population: a group of organisms of the same species living in an area at one time who can interbreed

If two populations live in different area they are unlikely to interbreed with each other

If two populations of the same species are geographically isolated for a longer period of time, they may develop differences in their characteristics and eventually become different species

Estimating Population Size

Count a sample from a small area and multiply this by the total area of the habitat

--> estimate of the total population

Two common sampling methods for sessile (an organism in a fixed place) organism:

• Quadrat sampling: a method used to take a measurement of an area without systematically deciding where to take it

--> eliminates bias which may influence your measurements

Used to work out population size of a species of stationary animals or plants

Only suitable to sample sessile organisms like plants or very, small moving animals

• Line or Belt transects

Random Quadrat Sampling

Procedure:

1. Base line along the edge of the habitat using a measuring tape

2. Generation of random numbers using a table or a random number generator

3. The first random number determines the distance along the tape, the second random number is used to determine the distance out across the habitat at right angles to the tape

4. The quadrat is placed precisely at distances determined by the two random numbers

Percentage Error = [(estimated value - actual number)/actual number] x 100

Capture-Recapture

Used to estimate the size of motile (an organism that moves around) organisms

Methods:

• Pitfall traps

• Pouters (aspirators)

• Mammal traps

• Nets

1. Capture as many individuals as possible in the area occupied by the animal population using netting, trapping or careful searching

2. Mark each individual, without making them more visible to predators

3. Release all the marked individuals and allow them to settle back into their habitat

Assumptions made about the period of time between capture and recapture:

• There's no migration into or out of the population

• No deaths or births

• Marked individuals mix back into the population and have the same chance of being captured on the second occasion

• The marks remain visible

• The marks don't increase the chance of predation or other threats to survival

1. Recapture as many individuals as possible and count how many are marked and how many are unmarked

2. Calculate the estimated population size by using the Lincoln index:

Population size = (M x N)/R

M = the number of individuals caught and marked initially

N = the total number of individuals recaptured

R = the total number of individuals recaptured with marks