Final answer:
Ecologists use a quadrat, a square frame, to measure and estimate population size and density of small, sessile organisms in a habitat, by sampling representative areas systematically.
Step-by-step explanation:
For small sessile organisms, ecologists frequently use a quadrat, which is a small square frame that is used to standardize the area being measured. A quadrat can be made of wood, plastic, or metal, and functions as a systematic way to count the number of organisms within its boundaries.
The accurate use of quadrats involves placing them randomly at multiple locations within a habitat to get a representative sample, which then allows researchers to estimate population size and density. This method is particularly effective for slow-moving or stationary organisms where direct counting of the entire population is not practical. Depending on the organisms and their distribution, the number and size of quadrat samples may vary, ensuring enough individuals are counted for a reliable estimate.