Notes on alpha and beta diversity

Biodiversity is an inflated piece of lexicon these days. Mass media use it often for its catchiness, referring as it as something intrinsically good but rarely defined in a rigorous way. The truth is, biodiversity is a loosely defined word even among specialists. It can refer to different biological attributes, such as taxonomic diversity (the diversity of species), genetic diversity, morphological diversity, functional diversity. Moreover, biodiversity is not intrinsically good or bad. For example, introducing allochtonous species to a habitat will increase biodiversity (at least for some time), but this does not inform on whether such action will lead to ‘good’ or ‘bad’ changes to that habitat. As for any ecological process or pattern, its connotation as ‘good’ or ‘bad’ is artificial and anthropocentric.

Bird diversity

Bird diversity (a showcase in the Museum of Natural History of Florence, Italy).

 

As a little contribution of mine to popular ecological rigour, I will hereby review the concept of biodiversity and of its components, and I will produce some examples on how an ecological gradient can affect biodiversity.

Whittaker neatly explained that biological diversity is a consequence of species’ ecological niche, described as the range of values within which a species can thrive for a set of ecological variables: “given a resource gradient (e.g. light intensity, prey size) in a community, species evolve to use different parts of this gradient; competition between them is thereby reduced. Species relationships in the community may be conceived in terms of a multidimensional coordinate system, the axes of which are the various resource gradients (and other aspects of species relationships to space, time, and one another in the community). This coordinate system defines a hyperspace, and the range of the space that a given species occupies is its niche hypervolume, as an abstract characterization of its intra-community position, or niche. Species evolve toward difference in niche, and consequently toward difference in location of their hypervolumes in the niche hyperspace.” The richness of species that can coexist in the niche hyperspace of a defined area or habitat is defined as alpha diversity. Alpha diversity can be measured in a number of ways, the simplest and most intuitive being the number of species.

Whittaker continues: “Given a habitat gradient (e.g. elevation or soil moisture conditions) species evolve to occupy different positions along this gradient. […] Along a particular habitat gradient species populations have scattered centers and usually overlap broadly, forming a community continuum or coenocline. […] The extent of differentiation of communities along habitat gradients is beta diversity.” The total diversity of a landscape, or geographic area, is defined as gamma diversity.

Whittaker intends beta diversity as the turnover of species along an environmental gradient. More recently, two components of beta diversity have been distinguished: a component due to species turnover and one due to species nestedness. Species turnover consists of species replacement between sites, while species nestedness is the result of species loss from site to site (Baselga 2010). It is interesting and important to distinguish between these two components of beta diversity, because they can result from different ecological processes.

Here I will focus on alpha and beta diversity, describing possible scenarios resulting from an ecological gradient. In my example I will refer to a gradient in temperature, an ecological variable of particular interest in the context of ongoing climate change.

  • Scenario 0: null hypothesis. Temperature is an unimportant niche axis, so that species are distributed randomly along the temperature gradient. Consequently, no effect of temperature is observed either on species richness or on the various components of beta diversity (see column 1 in the figure).
  • Scenario 1: species-specific temperature niches result in turnover of species along the temperature gradient, as different sets of species can be found at different temperatures according to their thermal optima. This can lead to high beta diversity, while alpha diversity (species richness) remains constant across the temperature gradient (see column 2 in the figure).
  • Scenario 2: metabolic costs are expected to increase with temperature, thus strengthening competition (Brown et al. 2004). Increasing competition would lead less competitive species to local extinction in warm climates. In this scenario, communities would display a nested pattern across temperature, with communities at high temperatures being composed by subsets of the species found at low temperatures. Alpha diversity would decrease with increasing temperature, while beta diversity between sites at different temperature would be due mostly to its nestedness component (see column 3 in the figure).
Temperature, alpha, and beta diversity

Possible scenarios on how temperature may affect alpha and beta diversity. Note how looking only at the total beta diversity cannot disentangle the actual source of diversity, whether species turnover or community nestedness.

If temperature were not an important ecological variable, then climatic changes would not affect species distributions. This is not the case. In reality, temperature is known to affect the biology, survival, distribution and interactions of species. What is the ultimate outcome for biodiversity? That depends on the species group and on other ecological variables, such as the habitat type. Some species are more sensitive than others to temperature changes, and the effects of temperature are stronger in some habitats than in others. Hence, organism communities from some habitats will be relative unaffected by climate change, while other communities some species will be replaced by others (species turnover), or just lost from the community. Since species differ in their ecological role, differences in community response to climate change may lead to changes in the functioning of some ecosystems, or to function loss in others, with possible consequences on human life.

The trick is understanding what the temperature-driven changes in diversity depend on at the level of community and habitat, to predict (and possibly counteract) these changes before they happen.

References

  • Baselga, A. “Partitioning the Turnover and Nestedness Components of Beta Diversity.” Global Ecology and Biogeography 19, no. 1 (January 2010): 134–43. doi:10.1111/j.1466-8238.2009.00490.x.
  • Brown, J. H., J. F. Gillooly, A. P. Allen, V. M. Savage, and G. B. West. “Toward a Metabolic Theory of Ecology.” Ecology 85 (July 2004): 1771–89. doi:10.1890/03-9000.
  • Whittaker, R. H. “Evolution and Measurement of Species Diversity.” Taxon 21, no. 2/3 (May 1972): 213–51. doi:10.2307/1218190.

WWW resources

http://bit.ly/1D63uC7 provides definitions of diversity components and an example of how to compute them using simple indices (i.e. the number of species, and the number of species by which two habitats differ). The indices used are simpler than those usually used in real studies, that can capture species diversity with more detail; yet they are useful for gaining a practical grasp of the concepts.