Archive for March, 2010

The quadrat

March 25, 2010

The quadrat is sampling shape. It is often thought of as a square or rectangular frame used in the field to sample an area. This is the way the quadrat appears in geographical, ecological, or forestry studies. But the fact is that the quadrat is not necessarily a 2-dimensional sampling shape.

Before looking into the more general definition of a quadrat lets consider the 2-dimensional quadrat. That an area. Typically, an area is a square or rectangle. There’s a simple reason for that. It’s easy to make a quadrat of that shape. A few boards and fasteners and the quadrat is done.  Early on it was realized that there existed a problem with quadrats. The problem was realized as early as probably the late 1890s when Clements and Pounds published there article on the quadrat. One of the early solutions was the proposal to use disc shaped quadrats. This meant that the quadrat minimized the edge effect. In fact, it didn’t. The problem was that the sampling rules associated with curved quadrats were too complex to use effectively.

You’d think that the square quadrat would have been the choice. It enclosed a large area for the perimeter. That didn’t happen. It was learned that the long thin quadrats such as the belt or strip quadrats were more effective sampling methods.

The shapes of quadrats was a hot topic in a number of disciplines from range management, to forestry, to ecology, to grassland studies. In all of this of course the assumption was that a quadrat was a 2-dimensional shape.

A quadrat is geometric sampling shape. It can be any dimension from 0 to 3 dimensions. These are points, lines, areas, and volumes. Even though you might think of a quadrat as a square frame, a quadrat can be any sampling geometric shape that is needed to get the work done and done efficiently.


Measures of abundance

March 8, 2010

One of the basic concerns of field work is to develop a sense of how much is out there. In a recent presentation I encountered a fascinating study in which the researchers were studying a shore bird called the Wilson’s plover. This bird is a beach nester that is not too common, but not endangered.

One of the foods of choice for the bird is the hermit crabs. The food source lives in burrows along the shore in muddy zones. The crabs dig burrows. A knowledge of the number of crabs in the nesting are used by the Wilson’s plover provides information about the available food supply. The measure of abundance in this case is the number of crabs and not the weight of the crabs.

The present sampling strategy is to toss a quadrat, a wooden square in this case, onto the mud flat where the crabs live and to count the number of crabs in the frame. Crab counting is hard. They move. They hide. The strategy used today is count multiple times and use the average number of crabs as the estimate for the selected quadrat.

A short discussion with the researchers demonstrated their knowledge of sampling and its issues, but they were not aware of 2 things.

  1. They were not aware of counting frames
  2. They were not aware of the efficiency of SRS

This isn’t surprising since stereology is a relatively unknown science.

The researchers will be doing more studies next year. They know that it is better to count burrows than it is to count crabs that run and hide. They are likely to use the counting frame and SRS in next year’s study.