NewFor Blog: on biodiversity and its measures.

 Por Rens Brouwer 

In other blogs have discussed the variety of species that are being used in restoration plantations and already noticed that each species has its particularity, shape and function in an ecosystem. This variety of species is what we call biodiversity. But there are many other concepts that fall under the umbrella of biodiversity. In this blog I explain biodiversity and its measures. 

Biodiversity is basically the variety within and among life forms on a site, ecosystem, or landscape. When we hike through a forest we see a diverse range of plants and trees in all shapes and sizes, and we can describe their variation in multiple ways. For example, we can look at the genetic diversity (family, species, varieties, etc.), or in life form diversity (grasses, herbs, trees, lianas, epiphytes, etc.), but also at the diversity of characteristics or functions that life forms and species have (deep rooted, nitrogen-fixing, animal-dispersed seeds, evergreen, etc.). Characteristics that determine how species functions are what ecologists call functional traits.

We can also go from the individual tree level, to the community level and describe how diverse a tree community is (e.g. all the trees of a forest, or those trees that are growing in a plot), not only in terms of diversity in genetics, life forms and functions, but also the diversity in forest structure (many small trees, few large trees, canopy cover etc.). 

In forest ecology, we call these measures of diversity ‘biodiversity attributes’. It is often measured because high biodiversity is perceived as a synonym of ecosystem health.  In general, diverse communities are believed to function better due to increased stability, increased productivity and increased resilience to unwanted disturbances. But we can talk more on Biodiversity and Ecosystem Function relationships in a future blog.

How do we measure diversity?

In the field of biodiversity and ecology, there are a couple of biodiversity attributes that are commonly measured and calculated. I will give a brief overview here, if you are interested in more information please have a look at the supporting literature below

Richness = The number of groups of genetically or functionally related individual trees. In most vegetation surveys, richness is expressed as the number of tree species and is usually called species richness or taxonomic richness. But we can also express richness as the number of functional trait combinations, in this case we talk about functional richness. 

Evenness = Proportions of tree species or functional groups present on a site.  The more equal species are in proportion to each other the greater the evenness of the site.  A site with low evenness indicates that a few species dominate the site, while a site with high evenness indicates that all species occur in more or less equal abundance. The same can be said for functional traits, low functional evenness indicates that a few trait combinations dominate the site, while a site with high functional evenness all trait combinations occur in more or less equal abundance. 

Divergence = The overall divergence between the species within a community; how far apart are the species? E.g. genetically (genetic divergence) or functionally (functional divergence, illustrated by Figure 1).

Figure 1, from Mason et al 2005: Functional divergence. The bell‐shaped curves show the distribution of species abundance (e.g. biomass) in niche space (e.g. leaf nitrogen content) while the histograms show the summed abundance of the species present in each category. The vertical dotted lines indicate the amount of niche space filled by the species. (A) A community with relatively high functional divergence, with the most abundant species occurring at the extremities of the functional character range. (B) A community with relatively low functional divergence, with the most abundant species occurring towards the centre of the functional character range. Functional divergence can change without a change in either functional richness or functional evenness (going between A and B).

As you may understand by now, there are multiple ways to measure and calculate biodiversity. The most straightforward measure is to identify the amount of species that grow in a plot or forest: this is what we call taxonomic diversity. A little bit more abstract is the concept of functional diversity, this is the diversity of functions of trees present in a forest or plot. To summarize, functional diversity is the diversity of species traits in ecosystems. This concept is increasingly used in ecological research. Yet its formal definition and measurements are currently under discussion. Generally, it is assumed that functional richness, evenness and divergence together make up functional diversity. Therefore, functional diversity is a multifaceted concept and if you want to measure all facets of it you need to consider all three components (evenness, richness, and divergence). So rather than looking at species, we look only at the functions and traits found in the trees present in a forest (remember: nitrogen-fixing, evergreen, leaf area, etc.), the more these traits vary, the higher the functional diversity. As you can imagine, some species may have very similar traits while others are very different from each other. So you can have a forest with high taxonomic (species) diversity, but when these species have similar traits and thus similar functions, this will result in a forest with low functional diversity and vice versa. Therefore, only taking into account taxonomic diversity in biodiversity assessments and measurements, may not capture the whole picture of the diversity of species and functions, which is why many ecologists nowadays also include functional diversity and structural diversity to accurately describe biodiversity (figure 2).

Figure 2, from  van der Sande, Poorter, et al., 2017: Four biodiversity attributes (taxonomic diversity, trait diversity, community‐mean traits, and structural attributes), the ecological theories for which they are a proxy and ways to quantify those attributes.

If we go back to the concept of restoration and high diversity native species plantations, and combine this with the knowledge we now have about biodiversity, we can understand better that effective restoration programs rely on reinstating highly diverse native plant populations to ensure the recovery of tree and forest functions. It is widely believed that local species are better adapted (through their traits) and more suitable for restoration than nonlocal species. 

Follow-up blog topic: 

BEF Research - Biodiversity and Ecosystem Functioning! How do the two connect?

Supporting literature:

1- Mason, N. W., Mouillot, D., Lee, W. G., & Wilson, J. B. (2005). Functional richness, functional evenness and functional divergence: the primary components of functional diversity. Oikos, 111(1), 112-118.

2- Perez-Harguindeguy, N., Diaz, S., Garnier, E., Lavorel, S., Poorter, H., Jaureguiberry, P., ... & Urcelay, C. (2016). Corrigendum to: new handbook for standardised measurement of plant functional traits worldwide. Australian Journal of botany, 64(8), 715-716.

3- van der Sande, M. T., Poorter, L., Kooistra, L., Balvanera, P., Thonicke, K., Thompson, J., ... & Mwampamba, T. H. (2017). Biodiversity in species, traits, and structure determines carbon stocks and uptake in tropical forests. Biotropica, 49(5), 593-603.

Bio: Rens Brouwer is PhD Candidate from Wageningen University & Research at the Forest Ecology and Forest Management group. Within the NewFor project he focuses on functional ecology and biodiversity and ecosystem functioning relationships. Contact: rens.brouwer@wur.nl