Population census: calculating the size of the population
Determining the population size is essential when characterizing CWR populations as it is a good indicator of its conservation status as well as of its genetic diversity and availability of propagating material for use in plant breeding.
The objetive of the census is to determine the number of reproductive or mature individuals of each target CWR population so the best time to do it will be during flowering season. All flowering or fruiting individuals will be considered as potentially reproductive individuals. However, those individuals without flowers or fruits but showing evidence of having flowered before or likely to flower later in the season should also be included.
Before carrying out a population census it isimportant to study the architecture of the plant to define what an individual plant is going to be. This is usually not a problem in plants that grow a single shoot from their roots. However, it can be confusing for vegetatively propagated adjacent plants when several shoots growing from underground rhizomes, tubers or bulbs intermingle difficulting the definition of a single individual plant. Here, the best approach could be to define an individual plant as a set of shoots (ramets) that grow together below certain distance from each other. Once the surveyors get their eye in, such distance will not have to be constantly measured for each plant.
Carrying out multiple observer sampling trials to determine the rates of observer recording variation and acting to reduce the error will help obtain a more accurate census. The idea is to adopt a pragmatic approach that will be systematically followed through the management of the population.
Methods
When the number of reproductive individuals is manageable the best way to proceed is to carry out a direct censusby counting all individuals one by one. For larger population sizes it is better to replace it for an estimation of the population size through sampling. Find here what to do and how:
A) Direct population census
Carefully scan the population. All reproductive plants must be marked using flag stakes which will then be counted as they are removed. Note that counting plants without previous marking can lead to error by omission or double counting. If possible, use also click counters.
It is essential to be very systematic when doing the counting and also collect all data in an appropiate data sheet.
B) Population size estimation through sampling
There are many ways of estimating the population size through sampling. Find below how to do it by setting a transect, one of the simplest methods:
- Strech a string from one end of the widest side of the population to the other and attach it to steel pegs, for example, at either end. You can use some more steel pegs in between to keep the string in place.
- Then, set a threshold distance along both sides of the string and mark/count all reproductive individuals. The threshold distance will depend on the type of plant (e.g. 1-2 m for herbaceous plants, 3-5 m for small shrubs, 5-10 (20) m for large shrubs)
An alternative option is to set a rectangular plot straightaway and count all individuals within.
Regardless of the option chosen, it is important to georeference the four corners so the same transect or plot can be set up in the future for monitoring purposes.
Finally, calculate the population size in two simple steps. First, estimate the population density. Divide the number of individuals counted by the transect/plot area, and second, multiply the population density by the area covered by the population.
If the population occurs in a heterogeneous habitat with great differences in plant density, you can set one or more rectangular plots in each type of habitat and estimate the density in each of them. Calculate the total population size by adding up all individual habitat census, which at the same time are the result of multiplying density by the area covered by the population in each specific habitat.
Find in Iriondo et al., 2008 or Elzinga et al., 2009 or Iriondo, 2011 (in spanish) some additional easy-to-apply approaches to obtain demographic information of plant populations.