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Sampling Techniques Quadrats Sampling • The best way to get information about a particular ecosystem would be to count every individual of every species • This would take unfeasibly long • So we sample a small part of the ecosystem we are studying Quadrats • A square of known size used to sample the ground-living (sessile) organisms in an ecosystem • Traditionally used to count plants in a particular area • Can be used to study changes over distance Counting with quadrats • Best done in reasonably uniform ecosystems, e.g. grassland • Many need to be placed randomly • Averages worked out • Multiplied by area/quadrat size • Works well for common species • Rare species are usually underestimated or occasionally grossly overestimated • Suggest why? (Imagine using quadrats to count trees on the grass outside business and no, it’s not because they wouldn’t fit over the top.) Placing quadrats randomly • Throwing is not random 10 m • Measure out an area using tape measures as axes • Use random number tables to select coordinates at which to place quadrats • Be consistent about how quadrats are positioned in 0m relation to coordinates 0m 10 m Measuring changes • Studying how the environment changes over a distance is done using a transect • A tape measure is placed in a line • Quadrats are placed at standardized distances along the line • Quadrats laid adjacent to each other along the line is called a belt transect • Quadrats at intervals (e.g. 5 m) is called an interrupted transect • Again, placement of the quadrat in relation to the measurement on the tape needs to be standardized beforehand, e.g. bottom left corner of quadrat on tape. Ways of counting what’s in a quadrat 1. Subjective rating 2. Percentage cover 3. Percentage frequency 4. Species density • Increasing accuracy • Decreasing speed Subjective rating • ACFOR • Look at whole quadrat and decide if species is – – – – – Abundant Common Frequent Occasional Rare • If you want to be a bit quantitative you can assign each a score (A=5, R=1) • Quick and easy • Subject to innaccuracy due to subjectivity • May lack reliability between samples/samplers Percentage cover • Estimate, to nearest 5% how much of the quadrat is covered by each species • Doesn’t need to add up to 100% • Still open to problems of subjectivity • Data is more quantitative which is useful for analysis • The average percentage cover of a particular species in all quadrats is called the species cover in the area being sampled Species frequency/percentage frequency • Needs a grid-type quadrat (25 mini squares) • Count how many squares the species appears in • Multiply by 4 to get % • Much more objective and therefore reliable • Tends to produce an overestimate, therefore inaccurate Species density • Count every individual of every species in the quadrat • Objective – Accurate – Reliable • Time consuming • Useful for larger (taller) species with more distinct individuals Point quadtats • Most objective of quick techniques – increases reliability • Frame placed on ground • Pins dropped into holes in frame • The first species the pin touches is the only one counted • May over/underestimate cover of rare/small species – problem with accuracy