Lecture 10 Population Determination Methods- Plants -2 PDF

BDB 211 POPULATION AND COMMUNITY ECOLOGY Lecture 9 Population determination methods- plants 17 September 2024 Prof S W Makhabu Learning Objectives To learn about the methods used in determining plant populations Methods to determine...

BDB 211 POPULATION AND COMMUNITY ECOLOGY Lecture 9 Population determination methods- plants 17 September 2024 Prof S W Makhabu Learning Objectives To learn about the methods used in determining plant populations Methods to determine Aerial cover of plants  Aerial cover of a plant species is commonly estimated to describe vegetation (Goodall 1952, Sykes et al. 1983).  Numerous techniques for quantifying cover have been developed, including a framepoint quadrat (Goodall 1952).  Point-intercept using points along a tape measure or line, point-intercept using a cross-hair sighting tube (Winkworth et al. 1962), step-point using a mark on an assessor’s footwear (Evans and Love 1957), and wheel-point apparatus (Tidmarsh and Havenga 1955, van Broembsen 1965). The wheel-point Method  The wheel-point was developed primarily for assessing aerial cuver of the herbaceous layer in vegetation with a sparse shrub and tree cover and is now the most widely used of the point cover techniques (eg., Mentis 1981, Holm et al. 1984, Friedel and Shaw 1987).  A wheel-point apparatus is a rimless wheel that rolls over the ground on its spokes. The position where a spoke (point) touches the ground or a plant vertically above a point on the ground is considered an intercept point for data recording. Aerial cover is the sum of strikes on plant species as a percentage of the total number of points sampled on a transect.  The frequent choice of the wheel-point over other sampling devices is because of a combination of preferred attributes.  It provides an acceptable level of accuracy and repeatability, particularly if a single observer is used (Walker 1970, Sykes et al. 1983). It enables rapid assessment over a large area and is relatively uncomplicated in its operation.  Its drawback is in recognising what constitutes a strike of the point on a plant. This is overcome by using the consistency of a single observer, but implies that sampled sites can only be compared relatively. The wheel-point Method  The acquisition of data with a wheel-point apparatus usually requires 2 people. Walker (1970) suggested that 3 are need& one to operate the wheel-point, one to observe the strikes, and another to record by writing on to data sheets.  The Quadrat method  A quadrat is a frame that is laid down to mark out a specific area of the community to be sampled.  Within the quadrat frame, the occurrence of plants is recorded using an appropriate measure of abundance.  Quadrats may be square, rectangular or circular and they may be of any appropriate size.  The quadrat method can be used in virtually any vegetation type to quantify the plant community.  However, some vegetation types are best sampled using other techniques (e.g., a point‐frame for grasslands, or point‐quarter method for forests).  Because a single quadrat cannot be expected to sample a community adequately, repeated quadrat samples are taken.  Typically, the community is divided up into sub‐areas dependent on topography, aspect, other physical features – and apparent floristic differences – and these are sampled separately; within sub‐areas, quadrats are located randomly.  This type of sampling approach ensures a representative sample of the different physical and floristic features of the community. This type of sampling is called stratified random sampling. Once collected, the sample data from all quadrats are added together and are considered to constitute an adequate sample of the community. The Quadrat method  When sampling vegetation using quadrats, different measures of abundance can be quantified to assess the influence or “importance” of each species in that quadrat. For example:  Counts – a simple tally of the number of individuals of a species  Cover – the percent (%) area of the quadrat occupied by a plant species.  Density – estimated by quantifying the number of individuals of a species per unit area.  Frequency – the proportion of quadrats sampled in which the species is represented.  Overall cover, density and frequency estimates are then calculated for each species from the entire data set by combining all of the quadrats together, as indicated on the left side of the table below. To determine the proportional representation of each species relative to the entire plant community, relative cover, relative density and relative frequency values can be computed.  For example, relative cover is the proportional cover of an individual species as a percentage of total plant cover; hence, it is expressed as a percentage, ranging from 0 – 100%.  “Importance” is a measure of overall influence of a plant species in the community.  An Importance Value (IV) for each species is derived from the combined contribution of the relative cover, relative  density and relative frequency of each species in the community.  Because it combines relative cover, density and frequency, importance values range from 0 – 300. Formulas for calculating the key quantitative community measures derived from the quadrat method Size and shape of Quadrat  The shape of a quadrat can be square, rectangular or circular.  Each one has advantages and disadvantages. Two main considerations must be taken into account when deciding on which shape to use.  The first has to do with edge effect, which occurs when researchers must make subjective decisions on when a species is considered “in” or “out” of the quadrat. This bias reduces the accuracy of the sample. Circular quadrats have the least edge to interior ratio and so have the least bias. They are also easy to define in the field.  However, this shape may not be advantageous in dense plant communities. Square and rectangular quadrats are sometimes easier to define, since tape measures can be strung through dense vegetation stands.  Rectangular quadrats are considered a good compromise because they have a lower perimeter to interior area than a square and also can capture more linear distance along the ground. This distance property can more effectively capture environmental variation than square quadrats. Size and shape of Quadrat