Measuring Biodiversity

Biodiversity. The word evokes the splendor of a great forest, or the teeming richness of the ocean, and is simply defined as the variety of organisms in an ecosystem of interest. To protect biodiversity, scientists must be able to measure it. This means figuring out how many different species are living together in a particular space. What is a convenient way to count species?

Trying to count everything in an entire ecosystem would be impossible, so scientists use a tool called the quadrat, which is a frame of fixed size placed randomly in the environment in which to do the counting. After cataloging the species and individuals found in this small section, the process is repeated, placing more quadrats at random, or alternatively, at set positions along a line through the environment, referred to as a transect.

In order to then estimate the total number of species in an area, species accumulation curves are used. If the cumulative number of species found in a quadrat are plotted against the number of quadrats sampled, a curve will emerge. For example, in this data set, when four quadrats were investigated, it was found that there were 10 unique species. Six contained 17 and so on. The asymptote of this type of curve represents an estimate of the number of species supported by an environment. In this case, it's about 30. But while measuring diversity at a single site is incredibly useful, comparing sites over a greater area can give us an even larger scale indication of diversity.

In 1972, the ecologist Robert Whittaker described three major kinds of biodiversity, alpha, beta, and gamma. Alpha diversity refers simply to the number of species in an area and is often referred to as species richness. For example, at this site there are seven different species, so the alpha score is seven. A second site, site B, has five species, and a third, site C, has seven. But by comparing between sites, we can determine what is called the beta diversity, the sum of species unique to each area. So if we compare site A with site B, we see three species in common between the two. Counting the remaining species, we find that there are six. This means that there is a beta diversity between site A and site B of six. Sites A and C also have three species in common, leaving eight unique ones. This is a beta diversity of eight. Sites B and C have two common species between them, or a beta diversity value of eight. Finally, gamma diversity is the number of different species in all sites combined. In this example, there is a gamma diversity of 12. So to summarize the three kinds of biodiversity, we can look at them this way, alpha, beta, and gamma. As well as recording diversity, scientists often refer to species evenness, meaning how many individuals of each type are present. For example, these two sites have the same richness, or alpha diversity, as they both have seven species. But site A is relatively overrun by rabbits with low numbers of the other species, whereas site B has a pretty even distribution of species, so it is considered to have greater evenness compared to site A. Scientists generally considered ecosystems with higher richness and evenness, i.e. many evenly distributed species, to be the healthiest. Disturbed habitats, often due to the actions of humans, like farming or pollution, often have poor richness and evenness. Being able to compare sites is critical because it allows researchers to determine the relative health of ecosystems.
In this laboratory, you will carry out quadrat and transect sampling at three different environmental sites, as well as carrying out a laboratory simulation, and then analyze the data collected to describe the observed biodiversity.