SNC 1D Estimation of Plant Biodiversity at Holy Cross

Introduction : http://bio150.chass.utoronto.ca/sampling/book/sampling.html How to Count Populations – one sampling method – a Quadrat study

The censusing of plant and animal populations is central to population ecology, which is basically the study of the patterns of organism abundance and distribution. Ecologists seek explanations for why species such as Black Rat (Rattus rattus) can live almost anywhere and are abundant, when the Karner Blue butterfly (Lycaeides melissa samuelis) is rare and only found in oak savannahs with Lupin bushes (Lupinus perennis).

Censusing a population is usually just the start of broader studies, but it is nevertheless important to do it right. Since total and direct counts of populations are rarely possible (except for the rarest of species), a census entails using sampling methods. This means that we are estimating population size from what we hope are representative samples. By using a method to count a population the observer is influencing the very individuals they are trying to count. How the method used influences the estimates obtained should be understood by the biologist. Sampling Methods Biologists use a variety of sampling methods depending on the type of organism and its natural abundance and distribution. In this exercise we use a variety of methods which fall into two broad categories:

 Plot-based (quadrat) methods, where individuals are counted in a known area. This type of sampling is widely used, especially for plants.  Capture-based methods, which includes mark-recapture, and removal. This type of sampling is used for very mobile or elusive species. Quadrat Sampling Method The quadrat method has been widely used in plant studies. A quadrat is a four-sided figure which delimits the boundaries of a sample plot. The term quadrat is used more widely to include circular plots and other shapes.

A common shape used in studies of birds and large mammals is a long, skinny quadrat in which the observer travels a transect and counts individuals within a known area either side of the line. These types of quadrats can be called transect strips. (There are various distance-based methods which use transects and the distance off the line that individuals are seen, but these methods are not dealt with here.)

Quadrat sampling involves counting all individuals within a known area (or volume). Since density (D) and population size (N) are related, as N = D x area, we can estimate the density for the sample and from this compute the total population. This assumes that the area the population occupies is finite and known. In the case of the Algonquin Park species, we take the Park boundaries (in the case of Moose), the Lake Opeongo area (Lake Trout), or the study area boundaries (White-throated Sparrow, Sugar Maple) as the total area.

Normally, a series of samples (quadrats) are counted. We do not know which, if any, of the sample estimates give the true estimate. Instead, we can calculate the average of these samples. The average or mean estimate, while not likely to be the same as the true population size, is nevertheless unbiased. Associated with the mean, we can compute a measure of the variability of the samples (the standard deviation), and this is a measure of the reliability of our population estimate.

Assumptions of Quadrat Sampling

The quadrat method has the following assumptions:

1. The number of individuals in each quadrat is counted. 2. The size of the quadrats is known. 3. The quadrat samples are representative of the study area as a whole.

HC study

Biodiversity Percentage of Location of Number of index Population Population of grass plants 10 cm by 10 cm different of grass all other square plot types of # of species blades plants pop. of grass (2 straws cut in 2) plants total # of Total pop. organsims

Football field

High Traffic Area

Undisturbed grassy area