Removing the Guesswork: Quadrats and Transects

Photo credit: flickr

Photo credit: flickr

Remember the last time you were at a carnival or work party and they had that jar full of jelly beans, and all you had to do was be the best at estimating the actual contents and you won TICKETS TO THE RED SOX?!  And you stared at it and stared at it (maybe you count the bottom layer) and then picked a random number?  If you’re random guessing is anything like mine, then you’ve never been close to the right number of jellybeans and never won those tickets.  Luckily, for science, we’ve developed strategies that take the random guesswork out of the process.

Quadrat, quite often made of PVC, for measuring snails and crabs in the rocky intertidal. Photo credit: Sara Edquist

Quadrat, quite often made of PVC, for measuring snails and crabs in the rocky intertidal. Photo credit: Sara Edquist

As an ecologist, I am often going out into the field (that’s what we say when we go to a site that we’re studying) and trying to describe a habitat or population. I may want to know how many snails live on a mudflat or how many fish are active above a coral reef or what proportion of a forest is made up of a particular type of tree.  The problem of course, is that unlike the jellybeans in the jar, we can’t actually count all the snails, fish, or trees to figure this out.  We have to estimate.  We have two simple yet elegant strategies to do this.

Transect for measuring mudflat organisms.  Photo credit: Flickr

Transect for measuring mudflat organisms.  Photo credit: Flickr

Recall the mighty ruler that Claire mentioned.  In field biology and ecology, we use what is essentially a super ruler, more commonly known as a transect line.  A transect is essentially, a line of known length (often something resembling a really long tape measure).  You can then walk, or swim, along the line counting whatever it is you want to count.  Another technique is to use a quadrat.  A quadrat is basically a two-dimensional area, usually a square, of known size.  You lay the quadrat down, and count whatever is inside the square.  The key to both of these tools is that they allow you to count something within a known space.  Repeat that several times at different locations within a larger area and we have the information we need to estimate, scientifically and accurately, the number of snails, fish, or trees in the whole area.

Stay tuned to learn more tools of the trade!