The richness of diverse meanings…

In sustainability science, it is easy to get confused. One good example is the word “diversity”. It occurs in many research articles, and people working in sustainability often aim to maintain “biodiversity”. But what exactly is the meaning of “diversity”?

In a biological context, “diversity” expresses the variety of life on the levels of species, genes and biotopes. The number of species is often taken as the currency to measure diversity in ecology (Jurasinski and Koch 2011). Diversity can be quantified whenever a dataset consists of units of observation (e.g. individuals), that can be classified into types (e.g. species) (Tuomisto 2010).
In many cases in research, indices of species diversity are presented as a surrogate for true diversity, even though true diversity would be as easy to measure (Tuomisto 2010). It is very important to see that diversity indices are not the same as diversity itself. The variety of different indices is often focused on conceptually different phenomena.
Species richness is an indicator of diversity, but it is not equal to diversity. Species richness is based on binary (presence/ absence) data and does not take proportional abundances into account. It reflects the actual number of species by an unweighted arithmetic mean, whereas species diversity reflects the effective number of a region or a locality by a weighted mean by the proportion of data it contributed to the dataset (Tuomisto 2010). Even in recent publications in high-ranked journals, authors state they measured diversity, but often they only present a count of the number of species. When only species richness is taken into account, it is be better to talk about α richness. Furthermore, species density has to be separated from diversity, too. The density of a species depends on species richness and the number of individuals per area. (Gotelli and Colwell 2001).

In its core meaning, the measurement of diversity accounts for species richness and the evenness of abundance across species (Whittaker, Willis et al. 2001). Diversity is the inverse of mean species proportional abundance (Hill 1973).
The phrase first came up in ecological context by Whittaker (1960), who distinguished three levels of species diversity in natural communities: α-, β-, and γ-diversity. These levels were based on consideration of spatial scale: the local (α) and the regional (γ).
The local scale was considered the smallest spatial units in which diversity was measured. The total species diversity observed in a dataset or in a landscape of interest is called γ-diversity. The β-scale was defined by Whittaker as a measure in between the smallest spatial units, which would show the heterogeneity in a dataset.

Whittaker´s definitions were not very precise, and they have been used in a broad meaning of the concepts. Especially the term “β-diversity” has been discussed, calculated and changed during the last half century, which led to confusion in the usage of this index until today. It is important to distinguish between the strict and the broad meaning of β-diversity, since in the literature both definitions are commonly used. Among others, β-diversity is considered to reflect the changes in species composition with distance. This is also called “species turnover” (Jurasinski and Koch 2011). In its strict sense, β diversity shows the number of effective compositional units that is necessary to accommodate the effective number of species found on aggregation scale when it only refers to two sampling units.

Basically, the γ-diversity of a region gets decomposed into α & β components to account for internal heterogeneity. The way these two compounds are calculated to contribute to the total diversity can be by additive partitioning (=cut into 2 parts) or multiplicative partitioning. Jost (2007) recognized that these components should be defined in a way so that they are independent of each other and therefore promotes the multiplicative approach, which was also introduced by Whittaker in 1977. The main problem in this approach is that there is no upper boundary or limitation for γ-diversity (Whittaker, Willis et al. 2001). However, there are some suggestions for the different scales of diversity levels of plants: α: 102-104m, γ: 106-108m, β: species turnover between patches α (Shmida and Wilson 1985). It is important that all sample sites or datasets have the same scale and are similar to be comparable.
With this entry I would like to emphasize that as scientists we should be precise about what is exactly presented by the analysis of the dataset. The word diversity is very often used, but in many cases the research investigates a related concept that is not the same as the actual diversity.

Literature:
Gotelli, N. J. and R. K. Colwell (2001). “Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness.” Ecology Letters 4(4): 379-391.
Hill, M. O. (1973). “Diversity and evenness: a unifying notation and its consequences.” Ecology 54(2): 427-432.
Jost, L. (2007). “Partitioning diversity into independent alpha and beta components.” Ecology 88(10): 2427-2439.
Jurasinski, G. and M. Koch (2011). “Commentary: do we have a consistent terminology for species diversity? We are on the way.” Oecologia 167(4): 893-902.
Shmida, A. and M. V. Wilson (1985). “Biological Determinants of Species-Diversity.” Journal of Biogeography 12(1): 1-20.
Tuomisto, H. (2010). “A consistent terminology for quantifying species diversity? Yes, it does exist.” Oecologia 164(4): 853-860.
Whittaker, R. H. (1960). “Vegetation of the Siskiyou mountains, Oregon and California.” Ecological Monographs 30(2): 279-338.
Whittaker, R. J., K. J. Willis, et al. (2001). “Scale and species richness: towards a general, hierarchical theory of species diversity.” Journal of Biogeography 28(4): 453-470.

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