Species Diversity

What does Species Diversity Mean?

Let's start by looking at the definition of species diversity.

Species diversity has two major components:

• Species richness: The number of different species that live in an area.

• Species evenness (or relative abundance): The representation of each species relative to the total number of individuals in an area (Fig. 1).

It is important to note that two areas with similar species richness do not necessarily have the same species evenness.

Species Diversity Calculation

Let’s say there are two forest communities each with four tree species. We’ll call them species A, B, C, and D. The distribution of the tree species in our hypothetical forest communities are as follows:

In this example, species richness is equal for both communities because they both have four tree species, but their relative abundance differs. Imagine what these two communities would look like. It would be easy to notice that there are four different species of trees in Community 1 because they are all well-represented.

On the other hand, it would be harder to notice the different species in Community 2 because of how abundant species A is relative to the other species. Just by visualizing these communities, we can intuitively say that Community 1 is more diverse than Community 2.

Species Diversity Calculation Using the Shannon diversity (H) Index

While we can intuitively describe the species diversity of a community, there are tools used to calculate diversity using species richness and relative abundance. One of these tools is called the Shannon diversity (H) index.

The Shannon diversity index can be calculated using the following equation:

We can determine the ln of each value of p using the “ln” function in a scientific calculator. The higher the value of H, the more diverse the community is.

Let’s try calculating the Shannon diversity index of the two forest communities in the previous example.

These calculations show that–as we had thought intuitively–Community 1 is more diverse than Community 2.

Species Diversity Calculation using Simpson's diversity (D) Index

Another tool used to describe species diversity is Simpson's diversity index.

Simpson's diversity index can be calculated using the following equation:

Let's try calculating Simpson's diversity index of the two forest communities in the previous example. Note that the lower the value of D, the more diverse the community is.

Again, as we have intuited, Community 1 is more diverse than Community 2.

The two indices can be used to calculate species diversity but are slightly different: the Shannon diversity index measures species diversity with the assumption that all species are represented in the sample and that they are randomly sampled, while Simpson's diversity index gives more weight to dominant or common species.

Limitations and Challenges in Calculating species diversity

It can be challenging to determine the number and relative abundance of species in a community for several reasons:

1. There are many species that are quite rare, making it hard to come up with a sample big enough to represent them.

2. Some species are difficult to identify based only on morphology; scientists may compare its DNA sequence to other DNA sequences in a database, but it is a more expensive procedure.

3. Species that are more mobile or less visible–for example, nocturnal species, deep-sea creatures, and microorganisms–may also be difficult to census.

Examples of Species Diversity

The glaciers of Antarctica have a harsh, inhospitable environment, making it low in species diversity. The Lesser Sunda Islands in Indonesia is relatively new, so there are not many species that have colonized it, also making it species-poor.

But, as with other species-poor areas, the few species that are able to inhabit it can proliferate because it does not have many other species to compete with for resources like food.

On the other hand, areas near the equator–such as the Amazon Rainforest–tend to have higher species diversity. There are many explanations as to why this is the case. One explanation is that there are more diverse habitats and ecological niches towards the equator. Another explanation points to the higher amount of energy at the equator, this is known as the latitudinal diversity gradient (Fig. 2).

Highest Species Diversity

High species diversity can be found in a variety of ecosystems around the world. Here are some examples:

1. Tropical rainforests: These forests are home to a wide variety of plant and animal species, including a large number of endemic species that are found nowhere else on Earth. For example, the Amazon rainforest is estimated to contain around 10% of the world's known species.

2. Coral reefs: Coral reefs are incredibly diverse marine ecosystems, with a vast array of fish, invertebrates, and other organisms living in and around the reef. The Great Barrier Reef in Australia is home to over 1,500 species of fish and 600 species of coral.

3. Grasslands: Grasslands are often overlooked for their diversity, but they are home to a wide range of plant and animal species. The African savanna, for example, is home to large herbivores like elephants and giraffes, as well as predators like lions and hyenas.

4. Wetlands: Wetlands are important habitats for a variety of species, including birds, fish, amphibians, and reptiles. The Florida Everglades, for example, is home to over 400 species of birds and is considered one of the most biodiverse areas in North America.

5. Coastal forests: Coastal forests are rich in biodiversity, with a wide variety of plant and animal species adapted to the unique conditions of the coast. The Pacific Northwest rainforest in North America is home to a diverse array of species, including bears, wolves, and bald eagles.

How is Species Diversity Different from Genetic Diversity and Ecosystem Diversity?

Species diversity is one out of three levels of biodiversity, the total variety of life on Earth. The two other levels of diversity are genetic diversity and ecosystem diversity.

Genetic diversity is the number of different inherited traits of a species. It can be observed within a species: for example, human populations have different inherited traits (e.g., eye color, height, complexion, and even diseases) that reflect their genetic diversity.

On the other hand, ecosystem diversity refers to the number of different ecosystems in a particular area. For example, a marine ecosystem contains other subgroups including coral reefs, mangrove systems, saltwater estuaries, and the ocean floor.

Species Diversity and Stability

There are multiple relationships between species diversity and stability.

If we are talking about stability at the ecosystem level, then species diversity can stabilize ecosystem processes provided that the species have different responses to changes in the environment such that when one species increases in number it can compensate for the decrease of another.

Higher species and genetic diversity can also translate to a higher chance of individuals having traits that would enable them to adapt to changes in the environment.

On the other hand, if we are talking about stability at the species level, then higher species diversity can actually lead to less species-level stability. This is because the number of individuals that can be packed into a community has a limit, therefore as the number of species in the community increases, the average population sizes of the species in the community decrease. With the decrease in population size, there is a higher risk of local extinction.

Why is Species Diversity Important?

Species diversity is important for biological, economic, and cultural reasons.

Healthy ecosystems have a diverse range of species, each of which plays a part in the functioning of the ecosystem. Species interact in ways that affect each other’s survival and reproduction.

Species diversity is also important for economic and cultural reasons. The food we eat, the clothes we wear, and even the houses we live in–much of what we use and consume in our everyday lives are derived from nature. Even many medications come from compounds naturally produced by a diverse group of organisms.

Unfortunately, because of their value, species diversity is threatened by habitat loss and overexploitation (including hunting, fishing, and extraction) by humans. This is why it is essential for natural resources to be managed and protected by individuals and institutions alike.