If you want to study an ecosystem such as a sand dune, lithosere, hydrosere or a saltmarsh, you probably won’t be able to study the entire area due to time / size constraints (or the high probability of complete boredom setting in). Therefore you will need to sample from the ecosystem in order to collect data that is accurate and representative of the area as a whole. There are many different types of sampling; here we shall consider methods of random and systematic sampling as these are the two of most practical use for geographical fieldwork in these environments.
is used to study a homogenous area; when the study area is the same throughout (for example a flat meadow). As it is reasonable to assume that the environmental conditions do not change within the meadow, it doesn’t matter whereabouts within the area you take your samples from. However one thing of vital importance is that you do not choose sample sites yourself, as this will introduce bias. Random sampling is achieved by generating two random numbers (from a random number table or a scientific calculator) and using them as co-ordinates for the placement of a quadrat. This is illustrated below:
is used when the sampling area includes an environmental gradient; where physical conditions change within the locality (for example a sloped hillside where factors like wind speed may increase as you move up the hill). Sampling of hydroseres, saltmarshes and sand dunes requires this approach, as the environmental gradient in these habitats is very distinct. Systematic sampling involves having structure to your method in order to obtain data in a series, rather than at random.
A transect is required to systematically sample through an environmental gradient. Transects can be of many types, the two most common being line transects (sampling along a line, e.g. a series of points along a tape measure) and belt transects (sampling within an area, e.g. a series of quadrats).
|line transect||belt transect|
Belt transects can be of two main distinctions. Continuous belt transects sample in an unbroken manner through the entire environmental gradient (for example by turning a quadrat end-over-end). Interrupted belt transects leave some areas un-sampled (e.g. placing a quadrat at 10 metre intervals). Of these two types of transects, the interrupted belt transect is the one most frequently used for conducting fieldwork within large areas e.g. saltmarshes or sand dunes.
|continuous belt transect||interrupted belt transect|
When carrying out any form of vegetation sampling, one piece of equipment you will almost certainly require is a quadrat. Quadrats are defined simply as sampling areas, and can therefore be of almost any shape, size and type.
Frame quadrats are 2-dimensional empty frames of any known area. These days they are usually square as this makes for easy calculation of the sample area; however they can be any shape – the Victorians favoured circular ones. Frame quadrats allow you to obtain data as direct counts (exactly how many of each species there are inside the quadrat); or as percentage cover (an estimate of how much of the quadrat area is taken up by each species).
Grid quadrats are also 2-dimensional quadrats, this time divided into a known number of small squares (often 100). By doing this it is possible to generate percentage frequency data (how many times a species occurs; present in 1 square = 1% frequent), as well as direct counts and more accurate percentage cover data (if a species fills 4 squares the coverage is therefore 4% - there is less estimation involved).
Point quadrats are 3-dimensional quadrats (see photograph right) and therefore ideal for sampling vegetation, which tends to grow in layers or canopies. A point quadrat consists of a frame with 10 holes which is inserted into the ground by a leg. A pin is then dropped through each of the holes in turn, and the species that the pin touches are recorded. In this way the total number of pins touching each species can be converted to percentage frequency data (if a species touched 6 out of the 10 pins it is 60% frequent).
Any style of quadrat can be used to sample ecosystems such as saltmarshes or sand dunes; the final decision will depend on a number of factors including availability, cost, time, practicality and ease of use, as well as the type of data required. There are both advantages and disadvantages to all quadrats (grid quadrats are robust and easy to use, yet can be time-consuming and destructive to vegetation; point quadrats are generally quicker and less destructive, however rare or small species can be missed). Ultimately the type of quadrat you choose to collect your data does not matter, provided you can justify why you chose it.
Environmental conditions are important to consider when conducting fieldwork, as changes in vegetation are often as a response to changes in the physical and chemical environment. N on-living environmental factors are known as abiotic factors, and can include light, temperature, water, atmospheric gases, wind, humidity, and soil conditions to name but a few. Abiotic factors cannot be controlled, however can (and should) be monitored. Methods for measuring abiotic factors vary greatly according to the time and equipment available. Some of the more common methods are given below:
Temperature can be recorded using a simple thermometer or more precisely with digital probes or data-loggers if they are available. Light levels can be monitored with a lux-meter to record the amount of light reaching the vegetation canopy or the soil surface. Wind direction can be determined with a compass, and wind speed recorded with an anemometer. Humidity can be monitored with the use of a whirling hygrometer or digital probes. Soil characteristics can be analysed in all manner of ways; soil moisture can be recorded with soil probes in the field, or can be determined from the analysis of soil samples in an oven, as can soil organic and inorganic matter. Simple chemical tests can determine nitrogen, oxygen, phosporous, and sodium content; and pH can be determined with soil probes, indicator solution or a Litmus test. The comapction of the soil substrate can recorded with a soil penetrometer if required.
Regardless of which abiotic factors are to be measured, or indeed the technique employed to do so, there are two imprtant rules when measuring abiotics. The first is that repeat readings must be taken (one sample is not enough), and the second that all samples must be taken during the same time period (as close together as can realistically be achieved).
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