sampling techniques Flashcards
sampling animals
The following techniques can be used to collect living animals for study later. Remember, all living organisms must be handled carefully and for as short a time period as possible. As soon as any sample animals have been identified, counted and measured if required, they must be released back into the habitat at the point they were collected.
• A pooter is used to catch small insects. By sucking on a mouthpiece, insects are drawn into the holding chamber via the inlet tube. A filter before the mouthpiece prevents them from being sucked into the mouth.
• Sweep nets are used to catch insects in areas of long grass.
-Pitfall traps are used to catch small, crawling invertebrates such as beetles, spiders and slugs. A hole is dug in the ground, which insects fall into. It must be deep enough that they cannot crawl out and covered with a roof-structure propped above so that the trap does not fill with rainwater. The traps are normally left overnight, so that that nocturnal •
Tree beating is used to take samples of the invertebrates living in a tree or bush. A large white cloth is stretched out under the tree.
The tree is shaken or beaten to dislodge the invertebrates. The animals will fall onto the sheet where they can be collected and studied.
Kick sampling is used to study the organisms living in a river. The river bank and bed is ‘kicked’ for a period of time to disturb the substrate. A net is held just downstream for a set period of time in order to capture any organisms released into the flowing water.
sampling plants
Plants are normally sampled using a quadrat, which can also be used to pinpoint an area in which the sample of plants should be collected.
Quadrats can also be used to sample slow-moving animals such as limpets, barnacles, mussels, and sea anemones.
There are two main types of quadrat:
• Point quadrat - this consists of a frame containing a horizontal bar. At set intervals along the bar, long pins can be pushed through the bar to reach the ground. Each species of plant the pin touches is recorded.
• Frame quadrat - this consists of a square frame divided into a grid of equal sections. The type and number of species within each section of the quadrat is recorded (Figure 4). Further details are given below.
To collect the most valid representative sample of an area, quadrats should be used following a random sampling technique. To study how the presence and distribution of organisms across an area of land varies, the quadrats can be placed systematically along a line or belt transect.
measuring species richness
As you learnt in Topic 11.1, Biodiversity species richness is a measure of the number of different species living in a specific area.
You should use a combination of the techniques described above to try to identify all the species present in a habitat. A list should be compiled of each species identified. The total number of species can then be calculated.
To enable scientists to accurately identify organisms, identification keys are often used. These may contain images to identify the organism, or a series of questions, which classify an organism into a particular species based on the presence of a number of identifiable characteristics.
measuring species evenness
As you learnt in Topic 11.1, Biodiversity, species evenness refers to how close in numbers the populations of each species in an environment are. For example, 50 organisms are found living under a decaying log. Of these, 20 are woodlice, 15 are spiders, and 15 are centipedes - the community is quite evenly distributed between species. However, if the 50 insects comprised just 45 woodlice and 5 spiders, the community would be described as uneven.
using frame quadrats
A frame quadrat is used to sample the population of plants living in a habitat. There are three main ways of doing this:
• Density - if individual large plants can be seen clearly, count the number of them in a Im by Im square quadrat. This will give you the density per square metre. This is an absolute measure, not an estimate as the following two methods.
• Frequency - this is used where individual members of a species are hard to count, like grass or moss. Using the small grids within a quadrat, count the number of squares a particular species is present in. For example, if clover is present in 65 out of 100 squares, the frequency of its occurrence is 65%. (Each square represents 1%.)
Another commonly used quadrat contains 25 squares - in this case, each square represents 4% of the study area. Therefore if, during a sampling of grassland, eight quadrat squares contained buttercups, the frequency of occurrence would be 32%.
-Percentage cover - this is used for speed as lots of data can be collected quickly. It is useful when a particular species is abundant or difficult to count. It is an estimate by eye of the area within a quadrat that a particular plant species covers.
For each approach, samples should be taken at a number of different points. The larger the number of samples taken, the more reliable your results. You should then calculate the mean of the individual quadrat results to get an average value for a particular organism per m?
(To calculate the mean value, sum the individual quadrat results, then divide by the number of samples taken). To work out the total population of an organism in an area that has been sampled, multiply the mean value per m? by the total area.
estimating animal population size
As animals are constantly moving through a habitat and others may be hidden, it can be difficult to accurately determine their population size. A technique known as capture-mark-release-recapture is often used to estimate a population size. This involves capturing as many individuals of a species in an area as possible.
The organisms are marked and then released back into the community. Time is allowed for the organisms to redistribute themselves throughout the habitat before another sample of animals is collected. By comparing the number of marked individuals with the number of unmarked individuals in the second sample, scientists can estimate population size. The greater the number of marked individuals recaptured, the smaller the population. The species evenness in an area can then be calculated by comparing the total number of each organism present. Populations of plants or animals that are similar in size or density represent an even community and hence a high species evenness. Species evenness can also be expressed as a ratio between the numbers of each organism present.
measuring abiotic factors
Abiotic factors are the non-living conditions in a habitat. They have a direct effect on the living organisms that reside there. Examples are the amount of light and water available. To enable them to draw conclusions about the organisms present and the conditions they need for survival, scientists normally measure these conditions at every sampling point.
Table 1 summarises the ways in which common abiotic factors can be measured.
wind speed-anemometer-metres/second-1
light intensity-light meter- lx
relative humidity-humidity sensor-mg dm-3
pH-pH probe-pH
temperature-temperature probe- degree celsius
oxygen content in water-dissolved oxygen probe-mg dm-3
Many abiotic factors can be measured quickly and accurately using a range of sensors, which are advantageous for a number of reasons:
-Rapid changes can be detected.
-Human error in taking a reading is reduced.
-A high degree of precision can often be achieved.
-Data can be stored and tracked on a computer.
