Sampling Estimates Abundance

1. Abundance is the number of organisms of a species in an area, or how common that species is.
2. Sampling measures part of a habitat because counting every organism is usually too slow, impractical or damaging.
3. A representative sample has the same general pattern as the whole area, so the estimate is less affected by selection bias.
4. Raw data are the original counts or percentage-cover readings recorded during fieldwork before a mean or estimate is calculated.

Quadrats Estimate Population Size

1. A quadrat is a square frame of known area used to sample plants and slow-moving organisms.
2. Mark out the area with two tape measures at right angles, then use a random number generator to choose coordinate pairs.
3. Place the quadrat at each coordinate and count one named species, or estimate its percentage cover.
4. Use a boundary rule, such as counting organisms touching the top and left edges but not the bottom and right edges, and apply it every time.
5. Repeat at many coordinates and calculate the mean so one unusually dense or sparse quadrat has less effect on the estimate.
6. Estimate population size with $estimated\; population=\frac{total\; area}{area\; of\; one\; quadrat}\times mean\; count\; per\; quadrat$.

Raw Data Must Be Processed Carefully

1. Calculate a mean with $mean=\frac{sum\; of\; all\; counts}{number\; of\; quadrats}$.
2. Population density can be calculated with $population\; density=\frac{number\; of\; organisms}{area\; sampled}$.
3. Percentage cover is estimated with $percentage\; cover=\frac{occupied\; grid\; squares}{total\; grid\; squares}\times 100$.
4. Keep units consistent because the total area and quadrat area must use the same area unit before scaling.

Transects Show Change Across a Gradient

1. A belt transect uses quadrats placed at regular intervals along a line to measure abundance across an environmental gradient.
2. A line transect records organisms touching the line, while a belt transect samples a strip and gives abundance as well as presence.
3. Systematic sampling uses fixed intervals because the purpose is to reveal a change with distance, rather than estimate one area's mean abundance randomly.
4. Measure an abiotic factor at each position with suitable equipment, such as temperature with a thermometer or light intensity with a light meter.
5. A relationship in the data supports an association, but it does not prove that the measured factor is the only cause because other conditions may also change along the transect.

Reliability Depends on Sampling

1. Increase reliability by using many quadrats, repeating transects and calculating means.
2. Reduce bias by using random coordinates when estimating abundance in one area and fixed intervals when testing a gradient.
3. Improve validity by measuring the intended factor accurately and keeping other conditions as consistent as fieldwork allows.
4. Report estimates honestly because clumped organisms, identification errors, seasonal change and movement can make a sample differ from the whole habitat.