Occupancy frequency distribution

Definition

In macroecology and community ecology, an occupancy frequency distribution (OFD) refers to the distribution of the numbers of species occupying different numbers of areas. It was first reported in 1918 by the Danish botanist Christen C. Raunkiær in his study on plant communities. The OFD is also known as the species-range size distribution in literature.

Bimodality

A typical form of OFD is a bimodal distribution, indicating the species in a community is either rare or common, known as Raunkiaer's law of distribution of frequencies. With each species assigned to one of five 20%-wide occupancy classes, this law predicted bimodal distributions within homogenous plant formations with modes in the first and last classes. Although Raunkiaer's law has long been discounted as an index of plant community homogeneity, the method of using occupancy classes to construct OFDs is still commonly used for both plant and animal assemblages. Henry A. Gleason commented on this law in his 1929 Ecology article that "In conclusion we may say that Raunkiaer's law is merely an expression of the fact that in any association there are more species with few individuals than with many, that the law is most apparent when quadrats are chosen of the most serviceable size to show frequency, and that it is obscurred or lost if the quadrats are either too large or too small." Evidently, there are different shapes of OFD found in literature. Tokeshi (1992) reported approximately 46% observations have a right-skewed unimodal shape, 27% bimodal and 27% uniform.

Mechanisms

Sampling artefacts
As pointed out by Henry A. Gleason, the variety shapes of OFD can be explained, to a large degree, by the sampling grain, extent and intensity. McGeoch and Gaston (2002)