Exploring species richness and evenness in biodiversity

In the vast tapestry of life on Earth, the richness and evenness of species within an ecosystem play a pivotal role in maintaining its health and resilience. Understanding these fundamental concepts — species richness and evenness — is crucial in assessing and managing biodiversity.

Species richness

Species richness refers to the total number of distinct species within a specific area or community. From farmland to forests to wetlands, the varied species encountered in each contribute to the richness of that ecosystem. Having many species generally correlates with a diverse and healthy ecosystem.

However, species richness alone doesn’t paint the complete picture. Enter species evenness, an equally critical aspect.

Species evenness

Evenness highlights the equality of the proportion of each species within an ecosystem. Picture an area where one species dominates while others remain scarce: for example, a palm oil monoculture where most rainforest species have been pushed to the unfarmable corners of a landscape or found in small, isolated pockets within. In such instances, even if the total species count is high, the biodiversity may still be lower compared to an area where species are more equally abundant — such as the yet untouched rainforest beyond. These situations can’t last long, and soon these restricted populations start to die off, as we have seen with the orangutan.

Acknowledging the unknown

There is an axiom in biodiversity measurement that is drilled into student ecologists: Absence of evidence is not evidence of absence. That is because, in practice, it is impossible to count every species in an environment.

To account for this, ecologists sample their environment and extrapolate from those samples an estimate of the proportion of species in the environment that they have identified so far. These samples can be collected in a wide variety of ways, ranging from traditional manual methods to sophisticated technological approaches. Techniques like quadrats, transects, point counts, camera traps, and acoustic recordings can all generate samples.

The more samples you take, the fewer new species you find. In other words, if you’re taking more samples and still finding lots of new species, then you probably haven’t sampled enough to estimate the true number of species.

We can plot the relationship between the number of new species we find per sample and the cumulative number of samples taken to create a species accumulation curve that will level off over time. Using statistical analysis, we can then estimate the species richness that would have been recorded when the line levels off, even if that would take many more samples than we have time to collect.

These methodologies aid in capturing a snapshot of biodiversity while acknowledging the presence of rare species that might otherwise go unnoticed.

Tools for comparisons and scaling biodiversity

To make comparisons across different areas or scales, scientists use alpha, beta, and gamma diversity measures. Alpha diversity refers to the number of species within a specific area and beta diversity compares uniqueness between two areas, while gamma diversity encompasses species in multiple areas combined into a region. These measures enable biologists to gauge diversity across spatial scales, providing insights into biodiversity variations. Not every species needs to be everywhere. In fact, it’s a good thing that diverse ecosystems are found across the planet.

In essence, understanding species richness and evenness is akin to understanding the vital signs of ecosystems and the health of our planet’s diverse habitats.

Real-world implications and scientific insights

Scientifically, the relationship between species richness, evenness, and ecosystem health is profound.

Studies, such as Wilsey and Potvin’s experiment in Quebec, showed that increasing levels of evenness in plant species led to linear increases in plant productivity evidenced by higher total and below-ground plant biomass.

These findings suggest that reductions in evenness could indirectly reduce total primary productivity. They emphasize the importance of maintaining balanced species distribution within ecosystems.

Diversity is thought to support the healthy functioning of a range of ecosystem services, including pollination of food crops, water regulation, and the beneficial flows of nutrients into the soil. The value generated by these services can be converted to a monetary value by estimating the cost of replicating these services with technological solutions. For example, we could calculate the expense of constricting a water treatment plant to filter the same quantity of pollutants from our drinking water that is achieved by 10 hectares of reedbeds.

Replicating the free services that are provided by nature would be impossibly expensive for governments. Unfortunately, our best evidence indicates that with every species we lose, we chip away at nature’s capacity to provide free clean air and water, food, and materials. These costs silently are added to our day-to-day expenses in the form of cost inflation to food, products, bills, and taxes.

References:

• Wilsey, B. J., & Potvin, C. (2000). Biodiversity and Ecosystem Functioning: Importance of species evenness in an old field. Ecology, 81(4), 887–892. https://doi.org/10.1890/0012-9658(2000)081[0887:BAEFIO]2.0.CO;2
• Jove Science Education, Measuring Biodiversity, https://www.jove.com/science-education/10596/measuring-biodiversity#