Faculty of SITE Northern Territory University Flora & Fauna Survey Techniques
what is flora and fauna survey?
why survey?
factors to consider
preparing to sample
flora survey techniques
vegetation descriptions
sampling concepts
fauna survey techniques
analysing data
presenting data

Sampling concepts - sampling methods

There are two major methods of sampling flora:
using sample units (plots), or
using plotless sampling methods

Sample units (plots)top of page
Sample units or plots comprise a subset of the total population from which measurements are taken during sampling. Sample units are distinct, non-overlapping entities, such as quadrats or transects, individual plants, branches within a plant, etc.

  • Quadrat: a 2-D shape (e.g. square or rectangle, or other shape) used as a sampling unit. The choice of dimensions and shape of the quadrat will affect the precision and accuracy of the parameter estimates subsequently computed (see below). Sometimes a tape is laid on the ground, more often a frame is used to define the quadrat boundaries.
  • Transect: Long, thin quadrats that are used to sample along narrow ecotones or to sample across ecotones to get better averages.
    Line transects
    feature only a length dimension, usually defined by a tape stretched across the area to be sampled.
    Belt transects
    have a width as well as length.
    Pace-transects are established when the observer strides along an imaginary line across the sample site, and uses their foot placement to determine specific sampling points.

    Transect sampling in areas with broad-scale vegetation patterns increases the likelihood of encountering species that occur at a very low density (Krebs 1989, Caughley, 1977).

    Line intersects can be used in aerial surveys. In difficult terrain, line transects allow estimates without having to locate quadrats. In open terrain, line transects allow sampling over large areas to provide more precise estimates.


    1. Individual vegetation patches are randomly oriented with no preferred orientation of the major axis of the patch.
    2. Sample lines are randomly oriented across the study area.
  • Gradsect: This is the deliberate selection of transects which contain the steepest environmental gradients present in an area to ensure sampling of the greatest range of vegetation (Gillison & Brewer, 1985). This is a cost-effective method for sampling large areas (Austin & Heylinger 1991).



The gradsect method was used in this study at Bateman's Bay to describe the floristic variation of the forest environment. Climatic, lithological and topographic characteristics were used.



5.2 (i)
Please describe why and when a gradsect would be used. Give an example of the application of this method, either from your reading or work experience.
What are the limitations of this method?

We will discuss this topic in our next online tutorial.

Plotless sampling methods top of page
Due to the fixed nature of plants, plotless sampling methods can be used to estimate density, species composition, growth and environmental factors. Plotless sampling methods are based on the random selection of points within a particular survey area and can be summarised into four main techniques (from Goldsmith et al. 1986):

  • closest individual method
    Distance is measured from each random point to the nearest individual.
  • nearest neighbour method
    Distance is measured from an individual to its nearest neighbour.
  • random pairs method
    Distance is measured from one individual to another on the opposite side of the sample point.
  • point-centred quarter (PCQ) method
    Distance is measured from the sampling point to the nearest individual in each quadrat.
    • commonly used in conducting forest inventories in moderate to dense stands of woody plants that are shrub size or larger; also applied in bunch-style grassland
    • will underestimate density when plants are aggregated.

From the mean value of each of these measurements the mean area and thus the density of the species can be calculated: Density = square root (mean area/2)

Sample plants are identified, and some measure of dominance (e.g. basal area) is taken for each, providing estimates of total and relative dominance and frequency, by species, for the sample area.


5.2 (ii)

Please describe when and why a plotless (or distance) method of sampling would be used. Describe an example and the use of this method, either from your reading or work experience.
What are the limitations of this method?

We will discuss this during out next online tutorial.

back to sampling concepts

Tropical Savannas CRC top of page

Tropical Environmental Management Course Homepage
last updated by lrp@cdu.edu.au 6 August, 2004
© Copyright