How Do We Assess Diversity?

This week (tomorrow) marks the completion of 60/120 microscope slides, and therefore half of the nematode identifications. While I have not checked all of the identifications with the boss, I am pleased to start tackling the second half because many of the nematode genera should appear again, making the process slightly faster or easier. The next few steps in the identification process are i.d. verifications (in progress) and then calculating diversity metrics for the nematode communities. Therefore, I wanted to talk about diversity metrics for this week's blog.

Diversity is a poorly defined term that we often use to describe differences between members of a group. While the word diversity does a poor job of characterizing the unique attributes of groups of a community, in biology we can assess multiple components of diversity to create a better understanding of the complexity of a community. A few prominent diversity metrics we use are Shannon-Wiener's diversity index, Pielou's evenness, and in nematology, Maturity Index (or c-p ratio).

The Shannon-Wiener index is a mathematical way to assess the uncertainty of species in an area. Without getting into the mathematics of the index, the metric describes the disorder and uncertainty of individual species, which helps describe the diversity of the area. I often view the Shannon-Wiener index as a Sparknotes version of diversity, because it provides a simplistic answer to describe a more complex underlying trend. The value of the Shannon-Wiener index, in my opinion, comes when we provide more context by using other diversity metrics in tandem with the Shannon-Wiener index. 

Pielou's evenness is a measure of the abundances of members of taxonomic groups in a community compared to each other. If you have a bag of M&Ms and you sorted the candies by color and compared the numbers of each color, you would be inherently assessing the evenness of that candy community. In biological communities we can use evenness levels to make claims about the status of an environment. For example, if we assess the evenness of bacterial communities and we find predominantly aerobic bacteria, then we can make a claim that there must be an abundance of oxygen since these bacteria thrive in oxygen-rich environments. We can also make general statements about the evenness and its relation to the overall health of an environment. Generally speaking, as in the textbook definition, even environments are healthy environments. Skews in evenness represent some disturbance or factor that promotes some community members over others. 

Maturity Index is a somewhat controversial diversity metric within the nematode community because of disagreements in the classifications the index uses. The maturity index is a system that nematologists use to assess environmental health, by assess the presence or absence of specific taxa within an environment. We assign values to the nematode taxa based on feeding structures and food preferences, and use a similar format to Pielou's evenness to draw conclusions about the health of an environment. Because we have well-documented evidence of nematode responses to environmental conditions (pollution, physical disturbances, etc.), we can use the presence or absence of different taxa as markers of potential disturbance events. 

While I have not calculated any of these metrics yet, I will be using information from these calculations to draw conclusions about the state of the habitats that I sampled and the relationship between wastewater treatment plant sites and nematode communities.

Next week I am hoping to get some pictures of my favorite nematodes from my samples. Although I am not someone who considers nematodes their top animal, I do find some of their features unique and I have been able to pick a few that are my favorite to find because of their almost artistic cuticle patterns. Stay tuned!