What’s the measure of species diversity of a habitat? Is it the number of inhabitants? Is it the number of species? Or is it the presence of a rare species? In this article, Abhijeet Bayani, a field biologist from the Indian Institute of Science, throws light on how he approaches this question of species ‘diversity’ in his classroom (a.k.a nature), while ensuring that his students do not confuse it with a very related concept of species ‘richness’.
The beginning of ecological research in a new geographical area usually begins with understanding what flora and fauna are present in it. In the same way, I like to begin teaching ecology or organismal biology to my undergraduate students by introducing the term species diversity to them. Since many sub-fields of ecological research revolve around why we have so many species around us, students also show a great interest in discussing the topic at various levels.
To make the concept clearer, I conduct frequent introductory field trips, during which I teach methods of species identification, some basics of taxonomy and ethical ways of animal handling, among other things. I also show them a wide variety of species and teach them how to catalogue the species systematically. This usually involves writing the names of observed species, the number of individuals of each observed species and the habitat in which it was found.
Students, in the beginning, do not understand the rationale behind the whole exercise; especially recording the number of individuals or the habitat. While the joy of watching creatures in the natural set-up is unbeatable, such field trips are expected to go beyond leisure.
After such sessions, we discuss what we observed and look for any patterns that may emerge out of the collected data, at least qualitatively. The field trips include covering multiple habitats that are fairly distinguishable into broad categories such as open habitat, closed canopy, scrub and so on. The rearrangement of data according to this rough classification then leads to discussing the ‘most diverse habitat’ visited.
Students go through their species checklists and most of them indicate that habitat X has a greater number of species than Y, so X is more diverse. Some other students also have an opinion that habitat X has a higher population of some species than Y, so it is more diverse. A small subset further claims to see a particular “rare” species in only one habitat and hence calls it more diverse. Here, students use the terms ‘species diversity’ and ‘species richness’ interchangeably, perhaps because they do not know the actual difference between these two terms. But… is there any difference?
Having observed a higher number of species in an area does not mean it is more diverse. It rather means that it is more ‘species rich’. If one compares two habitats based on only the number of species, then such a comparison would not provide any conclusive evidence of their difference in diversity. To know if the habitat is diverse, one needs data on the relative abundance of each species. There are several quantitative measures to know if and how much a habitat is diverse, but Shannon’s index is the most popular one. It essentially calculates the uncertainty in the outcome of a sampling process or uncertainty in predicting what the next species in a given area would be. There is a well-defined mathematical expression that can be found in any ecological textbook.
Students’ answers to ‘which habitat is more diverse’, as mentioned before, are all valid but they are incomplete if treated independently. They either compare the highest number of species observed in an area, the abundance of certain (not all) species across two areas, or the mere occurrence of a rare species in a particular area. A diversity index considers ALL of these components at the same time, which species richness does not. The index is, however, only a measure of diversity, and not the diversity itself. It is just like the radius of a sphere being an index of its volume but not the volume itself! Therefore, such indices need to be treated with caution.
The confusion between species diversity and richness is more common than what I had earlier imagined, and I know this being editor and reviewer of some ecology journals. It is not a confusion only among undergrads but occurs almost equally frequently at various education levels (postgraduates, PhD and even among some scientists). Many researchers, authors, teachers, popular science writers, and educators in different sectors use these two terms interchangeably.
I have found out that this confusion occurs mainly due to the term ‘alpha diversity’, which is essentially taught, used and defined very loosely as the ‘total number of species in an observed area’. Whittaker (who coined the term alpha diversity) himself has used this term in different and incomparable contexts. There is no common consensus over how and when to use this term to quantify species richness or species diversity. Some texts even equate ‘alpha diversity’ to species richness referring to it as ‘alpha richness’.
If students have this confusion, it is best to clarify the difference immediately. It is best to keep the categories simpler i.e., ‘species diversity’ when it includes data on population, otherwise call it ‘species richness’. If this distinction is not clear enough, the interpretation of data may mislead the conservationists and policymakers in deciding the priority areas for biodiversity conservation, or restoration ecologists in deciding what biodiversity to be focused on for the restoration.
I attempt to get rid of the misconception by providing them with hypothetical data that has the same number of species in two habitats but has different abundance values and ask them to judge. In such cases, if one goes just by the number of species, both are equally species-rich but not equally diverse. Teaching diversity indices in such a manner turns out to be more useful and impactful.