- Slides: 12
Investigating Populations Why do we use sampling to estimate population size? List as many methods of sampling as you can.
Basic ecological questions • What organisms live there? • Where do these organisms live? • How many of them are there? • Numerical data must be collected • Not necessary to count every organism – use sampling techniques • Results must be representative of the habitat
Frame Quadrats • 0. 25 m 2 • Each square represents 4% of total area
Point Quadrats • Can be used to estimate percentage cover in relatively short vegetation • Randomly select where to place quadrat, drop the first needle • Record what it touches on its way to the ground • The point may touch more than one species • Repeat with other needles • Repeat whole process in other parts of the habitat
You may get a % cover greater than 100%
Data • Species frequency – how likely is it that an organism appears in a quadrat • Species density – how many organisms per unit area • Percentage cover – how much of the ground is covered by a species (particularly useful if you can’t tell one individual from the next e. g. grass)
Random Sampling • Do not make a conscious decision about where to sample • Every part of the habitat has an equal chance of being sampled • Used when distribution is uniform • Could use random number generator to select coordinates to sample
What is a transect? • This a method of carrying out systematic sampling along a linear strip of a habitat. • It is useful for showing changes in the vegetation as you progress along the transect. • At regular intervals along the transect you would place down a quadrat and record the species present and possibly a measure of their abundance. • You can carry out a line transect or a belt transect.
Transect sampling (a) a line transect (b) a continuous belt transect (c) an interrupted belt transect In the line transect, all individuals touching the line are recorded. In the belt transect, all individuals occurring within the quadrats placed in the numbered locations are recorded.
Mark release recapture • Sampling animals • A known number of individuals is caught and marked, then released back into the habitat. • A second sample is then captured a short time later. • It is assumed the proportion of marked individuals to non-marked individuals in the second sample is the same as marked to un-marked in the first sample, so the population size is calculated as; • Population = total in 1 st sample x total in 2 nd number of marked in 2 nd sample
Assumptions • Marked individuals distribute themselves evenly. • There is no immigration or emigration during the test. • There are no births or deaths during the test. • The mark is not lost or affects the survival chances of individuals.
Ethical fieldwork • Studies do not take precedence over the lives of animals and plants that you are investigating • Avoid disturbing a habitat as much as possible