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Raunkiærís law of frequency reaffirmed

In 1934, the Danish botanist Christen C. Raunkiær presented his law of frequency, portraying a bimodal occupancy frequency distribution (OFD) in plant communities. This law suggests that species in a community are either rare or common, with only few species having intermediate occupancies. Specifically, when the occupancy is divided into five classes (0~20%, 21~40%, 41~60%, 61~80% and 81~100%), the two modes (a mode is defined as the class that has more species than its adjacent classes) appear in the first and last classes, with the valley (the least frequent occupancy) representing the class of 61~80% of all sites. Although the form of OFD can be diverse, the bimodality is still common (27% of the 48 matrices reviewed by McGeoch and Gaston (2002 Biol. Rev. 77: 311-331)).

Three explanations of the bimodality in OFDs include: first, an artefact from sampling highly skewed relative abundance distributions (Papp and Izsák 1997 Oikos 79: 191-194); second, the core-satellite hypothesis that if local extinction is subject to a strong rescue effect in a meta-community, the occupancy dynamics that are balanced by the colonization of new species and the extinction of existing species will lead to a bimodal OFD (Hanski 1982 Oikos 38: 210-221); finally, the bimodality could represent a transient pattern due to the effect of spatial scales on species occupancy (Hui and McGeoch 2007 Oikos 116: 2097-2107).

Using a representative dataset of 289 species-by-site matrices, C·I·B researcher Cang Hui presented the mathematical relationship between rank curves of species occupancy and OFDs, reaffirmed the estimate of about 24% bimodality in OFDs, and identified the truncated power law as the dominant form of occupancy rank curves. This paper outlined the use of occupancy-based null models in identifying underlying ecological processes in communities. This work further suggests the potential of using OFD forms as signals for community functions and structures; for example, work currently underway includes investigating whether we can use OFD forms as an indicator of community invasibility. Specifically, when compared with a right-skewed unimodal OFD, a bimodal OFD could signify a more deterministic-process-driven community that has a low invasibility for any further invasive aliens.

Raunkiærís law of frequency

Raunkiærís law suggests that species in a community are either rare or common, with only few species having intermediate occupancies. When the occupancy is divided into five classes (0~20%, 21~40%, 41~60%, 61~80% and 81~100%), the two modes appear in the first and last classes, with the valley (the least frequent occupancy) representing the class of 61~80% of all sites. An illustration of Raunkiærís original law of frequency, portraying a bimodal occupancy frequency distribution in plant communities.




Read the paper:

Hui, C. 2012. Scale effect and bimodality in the frequency distribution of species occupancy. Community Ecology 13: 30-35.

For more information, contact Dr Hui at chui@sun.ac.za