BIOLOGY Volume 2 - A Guide to General Biology - 2004

11. QUANTITATIVE ECOLOGY

11.2. Biota Analysis

11.2.2. Field Survey Methods

To quantitatively compare the abiotic and biotic components of ecosystems, it is necessary to standardize the units of the surveyed space. Typically, this is done by setting up transects and/or quadrats in the field and conducting research within their boundaries.

LINE TRANSECT. Transects can be used in uniform terrain, but they are particularly useful where an environmental spatial gradient exists or is suspected, such as on a rocky shore (Fig. 11.10) or in a transitional zone known as an ecotone between two communities. In the simplest case, a rope is stretched across the ground between two pegs, and all organisms touched by the rope are counted by species, while noting their positions along the transect. Obviously, using a tape measure is more convenient than a simple rope.

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Fig. 11.10. Line transect on a rocky shore.

BELT TRANSECT. A belt transect is a strip of a specified width (for example, 0.5 or 1 m) laid across the studied habitat, formed by two line transects. All organisms found within this strip are recorded. Naturally, this yields more comprehensive data, particularly in quantitative terms. The drawback of a continuous belt transect is the labor-intensive nature of data collection, which is often compounded by redundant information. Therefore, an interrupted (discontinuous) version is frequently used by setting up plots (e.g., 1 x 1 m) at certain intervals along a straight line (environmental gradient) and recording data only within those plots.

If a transect is laid out along an elevation gradient, it is sometimes referred to as a profile transect (Fig. 11.11).

The choice of transect type depends on the Qualitative and quantitative Nature of the study, the required degree of precision, the CHARACTERISTICS OF THE biota, the size of the area surveyed, and the time allocated for the work. Over a short distance, a continuous line transect accounting for all organisms along it is quite suitable. Over a significant distance, discontinuous sampling—using segments or plots separated by equal intervals—is more convenient.

Fig. 11.11. Aspirator for collecting small invertebrates.

QUADRAT. The quadrat sampling method is widely used, with sizes depending on the organisms under study. Wooden or metal square frames are often used for this purpose (preferably foldable for easier transport), such as those with an area of 0.25 or 1 m² (Fig. 11.12). Sometimes a transparent sheet with a grid of squares drawn on it is suitable, particularly when surveying Lichens covering tree trunks. When investigating large communities, such as a forest, a square plot with 10 or 20 m sides is typically laid out by marking its corners with pegs and stretching a rope between them. Professionals use special kits with spooled ropes of various lengths featuring pre-made loops at intervals corresponding to the sides of the squares (Fig. 11.13).

Fig. 11.12. Quadrat frame (1 m²) divided by a wire mesh into smaller squares, each with an area of 400 cm² (gridded quadrat).

Fig. 11.13. Kit for setting up study quadrats in the field.

As previously mentioned, quadrat sampling is frequently conducted along a line transect. Depending on The Nature of the study, either only the species falling within the frame are recorded, or their population sizes (Abundance) as well (sec. 11.2.3). In any case, the sampling method must be identical across all plots; for example, it should be specified in advance whether or not to include an individual that falls only partially within the quadrat. The frame's design can be varied—in particular, it can be subdivided with strings into smaller squares, counting individuals in just one of them and then extrapolating the data to the entire quadrat, which itself is used for a more thorough Water/305.html">Determination of species composition (Fig. 11.12). This is especially useful when studying vegetation.

In a uniform habitat, a quadrat can also be used without a transect by placing it randomly in the field. The traditional approach involves throwing a sturdy frame over one's shoulder and recording the species it covers. This Procedure is repeated several times to ensure sample representativeness. Of course, a certain "bias" in the results is possible here, caused, say, by the researcher's individual throwing habits. A more scientific approach is to establish sampling plots based on a set of random numbers generated by a pocket calculator. Each pair of numbers can be used as coordinates for a grid laid over the surveyed area, which is marked out using a tape measure and pegs. A pair of random numbers can also determine the distance and direction to the plot from the researcher's current point.

Experience shows that in a uniform habitat using quadrat sampling, new species will sooner or later stop appearing within the frames. This relationship is illustrated by the graph in Fig. 11.14. It is generally accepted that if nothing new is found in five consecutive quadrats, the species COMPOSITION OF THE community has been fully accounted for. However, such assumptions must be noted in the report, as they imply a degree of method inaccuracy.

Fig. 11.14. Graph showing the relationship between the number of species discovered in a given area and the number of survey quadrats examined. Clearly, beyond a certain point, setting up new quadrats is pointless, as no new species are encountered in them.

LEVY POINT QUADRAT APPARATUS. This device consists of a crossbar secured above the ground with holes through which long, thin pins resembling knitting needles are lowered vertically (Fig. 11.15). Such a setup is useful in dense grassy vegetation where species overlap one another. For each hole, all species touched by the pin on its way to the soil are recorded (yielding what is known as a "point quadrat").

Fig. 11.15. Levy hemocytometer.

PERMANENT QUADRAT. For long-term studies of succession, seasonal, or interannual community changes, permanent quadrats or transects are used. Steel survey stakes and nylon ropes are employed to establish them. Within the quadrats or along the transects, abiotic and biotic parameters are periodically recorded, and the results are then grouped to reveal trends in species diversity and, where possible, the underlying factors.