In scientific literature, you will often find these two terms; “species richness” and “species diversity” being used. While they sound quite similar (and mistakenly used interchangeably by many), they are two different concepts. I’d like to dedicate this blog to explaining what they mean, and why each of them are equally important in sustaining the ecosystems on Earth.
Species richness is a term used to define the number of species in an area. It’s merely a numerical characteristic of an ecosystem.
Species diversity is a term used to define the different number of species in an area (Species richness) and the distribution of these species in that ecosystem. It’s a measure of the variety in the ecosystem.
So, having one is not a guarantee that the other is also present in the ecosystem. You could have high species richness but low species diversity in an ecosystem. A forest dominated by Oak trees is an example of this (Other trees are also present, but the distribution is skewed). Likewise, you could have an ecosystem with high diversity and low species richness. An aquarium with one individual of 5 different fishes, is an example (Even distribution, but less number of species). Such conditions (high-low) are commonly found in man-made ecosystems.
Having either of the above conditions is not ideal in a natural ecosystem. And here, we need to look at the importance of high species richness and species diversity.
Diversity is something I have been talking about (read Earth’s Paradox: Greater complexity, more stability and Diversity on Earth: How is it created and maintained?).
Diversity is important because first and foremost, it helps buffer environmental stresses on an ecosystem. A diverse ecosystem has a better chance of surviving rapid changes with minimal losses. Also, diversity plays an important role in providing a variety of diets for the organisms in the ecosystem. It is diversity that leads to a food web; this is decidedly a better situation to have than a real food chain. It is also important aesthetically. As a person, you like to see a forest with different kinds of trees and animals, rather than rows and rows of the same tree with nothing in between. Diverse ecosystems can sustain complex ecological interactions and can survive on their own.
While species diversity is extremely important, it will not fulfill its full role in the ecosystem without species richness. Diversity in large numbers are important because it enables large scale interactions among organisms and the environmental factors (also called Biotic and Abiotic factors). It also ensures that the ecosystem will not collapse after a short while. For example-
This is an example of a food web which clearly shows high diversity (so many different organisms). Now imagine an ecosystem where only a maximum of 5 individuals of each organism is present. With all the eating they will be doing, how long do you think it’ll take for this ecosystem to die?
All conservation efforts focus on both the species richness and species diversity. You must have heard that the Bengal Tiger is an endangered species. These terms; endangered, critically endangered, vulnerable are all used to explain the low species richness of these organisms in their ecosystems. And the fact that so many organisms are being studied and given these classes show that species diversity is also being considered.
The crux is: species richness and species diversity are both vital for the survival and effective functioning of the ecosystem.