Quadrat Method

The quadrat method orginated with Frederick Edward Clements (1874-1945) (Pound & Clements, 1898). A quadrat is a frame of any shape that can be placed over vegetation so that cover can be estimated, plant counted or species listed. Quadrats are used to define sample areas within the study area and are usually made from strips of wood, metal or rigid plastic which are tied, glued, welded or bolted together to form the quarat. It can be helpful to use bolts so that the quadrat can be dismantled for storage or transport. For aquatic macrophytes a wood or plastic frame will float and can be used to sample floating or emergent vegetation on the water surface. For large quadrats over 4 sq. m folding rulers or string can be used.

Three factors need to be considered in relation to the use of quadrats.

1. Distribution of plants
2. Shape and size of the quadrat
3. Number of observations needed to obtain an adequate estimate of density

Shape

Quadrat are not natural sampling units, and one must always decide what size and shape of quadrat to use. The spatial pattern obtained, and the resulting index of dispersion, depends on quadrat size and shape. The term "Quadrat" strictly means a four sided figure but in practice this term is used to mean any sampling unit, whether square, rectangular, circular, hexagonal or even irregular in outline. Although a square is often used, the quadrat shape is unimportant as long as you know its area.

There are two conflicting problems regarding shape

1. Edge effect: Minimal in a circular quadrat and maximal in a rectangular one. The ratio of length of edge to the area inside a quadrat changes as

CIRCULAR < SQUARE < RECTANGULAR

Edge effect is important because it leads to possible counting error. A decision must be made every time an animal or plant is at the edge. For plants or animals that are partly in and partly out of the quadrat, you have to adopt some reasonable convention, e.g.

• Include plants that are more half in and exclude ones less than half in
• Include all the edge plants on the north and east side and exclude all those on the south and west side.

2. Long thin quadrats are better than circular or square ones of the same area. The reason for this is habitat heterogeneity: long quadrat cross more patches.

Size

Different vegetation types requires different quadrat sizes. Vegetation with smaller plants, greater plant density or greater species diversity need smaller quadrats. Increasing the size of quadrat generally results in a lower variance in your observations, but more work in getting them. Numerous studies have evaluated quadrat size, and no consistent recommendation has been made about the sizes to use. The sizes most often used are

• 0.01-0.25sq. m - bryophyte, lichens and algae
• 0.25-16 sq. m - grassland, tall herb, short shrub or aquatic macrophyte
• 25-100 sq. m - tall shrub community
• 400-2500 sq. m - trees

A better approach, if time and resources are available, is to determine for your particular study the optimal quadrat size and shape. To do this, you first need to decide what you mean by "Best" or "Optimal" quadrat size and shape. Best may be defined in the following two ways:

• Statistically, as the quadrat size and shape giving the highest statistical precision for a given total sample area samples or for a given total amount of time or money.
• Ecologically, as the quadrat size and shape that are best to answer the question being asked. If you are investigating questions of ecological scale, the processes you are studying will dictate quadrat size.

But in many cases the statistical criterion is the major concern.

Number of Samples

The number of samples to take varies from community to community. Because many species are not distributed randomly, variation normally is quite high and the number of samples required is quite large. Frequently the less common species require a larger number of samples than do the more common species.

Choosing Samples

Random Sample

A random sample is one in which every member of the population i.e., every individual animal or every point of ground has an equal and independent probability of being included. Quadrats can be located randomly by constructing two imaginary axes along the edges of the area, dividing the axes into units, and picking pairs of units from a random number table or other ways. The easiest method of random sampling is one you throw the quadrat with closed eyes, from above the shoulder level in any direction on the study plot. We can also usually achieve the same effect more simply by using the random numbers generated by a calculator or a computer, or included in books of statistical tables. A random number table consists of a long series of digits that has been checked for non-randomness. To use it, simply enter the table in some way that prevents your exercising choice in the first sample then simply take the number is same predetermined order such as left to right. Other appropriate ways of making random decisions are drawing numbers from hats, flipping coins, rolling dice, etc.,

Stratified Random Sample

All the random quadrats might be clustered by chance in one section of the area. To avoid this possibility, the area can be subdivided into roughly equal areas and each section randomly sampled with fewer quadrats. This is called stratified random sampling, or restricted random sampling.

Systematic Sample

Taking samples that have some sort of systematic or regular arrangements. For example sample plots may be located every 100 meters along a line. Thisis simpler than random sampling but bias will be present if the pattern of sampling is picking up some pattern in the distribution.

Estimation of Cover, Frequency and Height

Cover or Coverage: Defined as the percentage of quadrat are beneath the canopy of given species or group. Percentage cover can be estimated by eye. Visual estimates can be made of the cover of the species either in the sample plots or in the whole study area. Different measures can be used. The simplest is the classification Dominant, Abundance, Frequent, Occasional, and Rare (DAFOR). These classes have no strict definition and you must decide on your own interpretation. Vegetation is often layered, so percentage cover values can sum to more than 100%. So it is useful to divide the vegetation into layers and present the cover estimates separately each layer.

Frequency: Defined as the percentage of total quadrats which contains at least one rooted individual of a give group or species.

Height: The height of a plant is defined as the length of the main stem above the ground level.

Lundy Island Grassland Classifications

It is difficult to identify the Lundy grassland plants down to species level in the field. We can identify some of the grasses by using our identification manual. The samples of plants can be collected from the field and it can be identified here by consulting the experts. However, the plants are classified into following groups. They are Grasses & Sedges, Broad leafed herbs (forbs), and Shrubs.

Grass: Generally have long, narrow parallel-veined leaves inserted distichously on a round hollow stem. Open grasslands fall conveniently into three height groups viz., high (or tall) (6-8ft or more), medium (2-5ft) and short (less than 2). Grasses can be divided into four groups. They are

Fine leaved grass: Grasses which have thin needle like leaves mostly deep green

Blade leaved grass: Grasses with leaves that look like a blade i.e., broad and flat.

Tussocks: A tuft of grass looking like a grass heap.

Rushes: Grasses which have hollow stems that look like a pipe, with a white inner coating.

Broad leafed herb or Forb: A non-woody plant, or at least one which is not woody above the ground level. It is classified into two categories i.e.,

Rosette: Any plant with its leaves radiating outwards as a circular cluster from a short stem at soil level. Further these can be classified into short and tall rosettes.

Prostrate: Describing a plant that grows flat upon or closely along the ground.

Shrubs: A woody plant not having a main trunk but several main branches. In general shrubs are smaller than trees. Two types will be seen on Lundy. They are

Gorse: Shrubs which have needle like leaves and it is spiny.

Heather: Shrubs which have needle like leaves but are not spiny.

Identifying the impact of grazing

Over grazing can be identified by a low number of tussocks, short grass height and more rushes, rosette, and prostrate herbs, and less heather. Gorse is thick and looks like a cushion.

In less grazed areas, more tussocks can be seen. The height will be high and there will be fewer rushes and more heather. Gorse is not thick and gaps can be seen between plants.

References

• Barbour, M.G., Burk, J.H., and Pitts, W.D. (1980). Terrestrial plant ecology. London: Benjamin/Cummings Publishers. 604pp
• Bookhout, T.A. (Ed). (1996). Research and management techniques for wildlife and habitats. Maryland: The Wildlife Society. 740pp
• Brewer, R., and McCann, M.T. (1982). Laboratory and field manual of ecology. London: Saunders College Publishers. 269pp
• Krebs, C.J. (1989). Ecological methodology. London: Harper & Row Publishers. 654pp
• Pound, R., and Clements, F.E. (1898). A method of determining the abundance of secondary species. Minn. Bot. Studies 2: 19-24.
• Sutherland, W.J. (1997). Ecological census techniques a handbook. Cambridge: Cambridge University Press. 336pp