1 4 H 11 Quadrat sampling is a method by which organisms in a certain proportion (sample) of the habitat are counted directly. As with all sampling methods, it is used to estimate population parameters when the organisms present are too numerous to count in total. It can be used to estimate population abundance (number), density, frequency of occurrence, and distribution. Quadrats may be used without a transect when studying a relatively uniform habitat. In this case, the quadrat positions are chosen randomly using a random number table. The general procedure is to count all the individuals (or estimate their percentage cover) in a number of quadrats of known size and to use this information to work out the abundance or percentage cover value for the whole area. The number of quadrats used and their size should be appropriate to the type of organism involved (e.g. grass vs tree). Estimated average density Total number of individuals counted Number of quadrats X area of each quadrat Guidelines for Quadrat Use: 1. The area of each quadrat must be known exactly and ideally quadrats should be the same shape. The quadrat does not have to be square (it may be rectangular, hexagonal etc.). 2. Enough quadrat samples must be taken to provide results that are representative of the total population. 3. The population of each quadrat must be known exactly. Species must be distinguishable from each other, even if they have to be identified at a later date. It has to be decided beforehand what the count procedure will be and how organisms over the quadrat boundary will be counted. 4. The size of the quadrat should be appropriate to the organisms and habitat, e.g. a large size quadrat for trees. 5. The quadrats must be representative of the whole area. This is usually achieved by random sampling (right). Sampling a centipede population A researcher by the name of Lloyd (1967) sampled centipedes in Wytham Woods, near Oxford in England. A total of 37 hexagon-shaped quadrats were used, each with a diameter of 30 cm (see diagram on right). These were arranged in a pattern so that they were all touching each other. Use the data in the diagram to answer the following questions. 1. Determine the average number of centipedes captured per quadrat: 2. Calculate the estimated average density of centipedes per square meter (remember that each quadrat is 0.08 square meters in area): 3. Looking at the data for individual quadrats, describe in general terms the distribution of the centipedes in the sample area: 4. Describe one factor that might account for the distribution pattern: 1 2 3 4 Area being sampled 5 6 Quadrat 7 8 9 10 The area to be sampled is divided up into a grid pattern with indexed coordinates 1 2 3 4 5 6 7 8 9 10 AS 0 Quadrats are applied to the predetermined grid on a random basis. This can be achieved by using a random number table. 2 0 0 10202 0 0 0 0 0 2 0 1 1 11 Each quadrat was a hexagon with a diameter of 30 cm and an area of 0.08 square meters. 0 0 0 3 1 0 3 1 2 0 1 0 1 0 2 1 2 The number in each hexagon indicates how many centipedes were caught in that quadrat. Sunline 552 Centipede Investigating Ecosystems