The line-intercept technique can be used for estimating the population sizes of smaller organisms such as herbs. A string or tape measure is stretched over the vegetation to be sampled. This transect line should be marked so that it forms several smaller segments. Each sample consists of one segment of the line. The following information is recorded for each individual that is intercepted (touched) by the line:
1. The length of the individual that is intercepted by the line (called intercept length) is measured. The two photographs below are views looking down onto plants from above. The intercept lengths are the horizontal white lines.
2. The maximum width of the individual perpendicular to the transect line is measured. This can be seen as the vertical white lines labeled perpendicular width (W) in the photographs. The perpendicular width of the entire plant is recorded for all plants, even if only a small portion of a leaf is intercepted.
For grasses and clumps of small plants, the base of the clump near ground level can be used as shown below.
Calculations of density, relative density, dominance, relative dominance, frequency, relative frequency, and importance value can be calculated with the data collected.
Larger individuals have a greater probability of intercepting the transect line. This biases the density and frequency estimates for larger species. These biases are corrected by incorporating the term 1/W (reciprocal of the widths) in the formulas below.
| Density = | sum(1/W) ____________________ Total length of all transects | Equation 1 |
where W is the maximum width of the organism perpendicular to the transect line (see diagram above).
The equation above is calculated for each species separately so that each species has one density calculation. To use this equation, each W measurement for each individual of a species is divided into 1. Then, all of the resulting values for a species are added together. This sum is then divided by the total length of all of the transect lines.
The units for density will be the same units that you used for intercept length and perpendicular width. For example, if you measured your plants in centimeters, the calculation for density will be plants per square centimeter. If you measured your plants in millimeters, density will be given as plants per square millimeter. For small plants it is usually convenient to convert these to plants per square meter or plants per hectare. There are 10,000 square centimeters in one square meter and 10,000 square meters in one hectare.
Dominance is a measure of the percent of ground covered by the species. This can be determined using the equation below.
| Dominance = | Total intercept lengths for a species ___________________________ Total length of all transects | X 100 | Equation 2 |
The rationale is that the percent of the transect line covered by a species is the same as the percent of ground covered by the species. The dominance calculation is done for each species separately so that each species has one dominance calculation.
Frequency refers to the number of transects that contain (are intercepted by) at least one individual of the species in question. This method is biased because larger individuals are more likely to be sampled by this technique than smaller individuals. The 1/W term in the equation below corrects for this bias.
| Frequency = | sum(1/W) _____________________ # of individuals of a species | X | # transects with at least
1 individual of the species | Equation 3 |
The values of density, dominance and frequency can be converted to relative values using the equations below.
Relative density is calculated by dividing density by the total density of all species. It's maximum value is therefore 1.0. Similarly, relative dominance and relative frequency each range from 0 to 1.0.
| Relative Density = density/total density of all species | Equation #4 |
| Relative Dominance = dominance/total dominance for all species | Equation #5 |
| Relative Frequency = frequency/total frequency of all species | Equation #6 |
Importance value for a species is the sum of the three relative values above.
| Importance Value = relative density + relative dominance + relative frequency |
Equation #7 |
Measure the intercepted length and perpendicular width for each individual in each of the five transects and record the data in the data table included with this lab. See the photographs at the beginning of this lab for more details on how to perform these measurements. Record your measurements in centimeters (cm) rounded to the nearest 0.1 cm. For example 7.2 cm.
Frequency is measured for each species, by counting the number of sample lines that intercept at least one individual of that species. In this exercise, there are five sample lines.