The shape of occupancy distributions in plant communities: the importance of artefactual effects

Occupancy frequency distributions are commonly used as an approach to describe and analyse interspecific distribution patterns. However, the relative importance of biological versus artefactual mechanisms in shaping occupancy distributions is still largely undetermined. We evaluated the importance of different and interacting artefactual effects on the shape of occupancy distributions in local plant communities. The effects of sampling protocol parameters (i.e. size and number of sample units, sample extent, coverage, and intensity) on the shape of the occupancy distributions were examined separately. We identified the mechanisms that cause the effects by tracking the shifts of individual species between occupancy classes with varying parameters. Furthermore, the impact of different species abundance distributions and increasing levels of intraspecific aggregation on occupancy distributions was investigated by means of artificial assemblages. We show the following results: 1) increases in the number of sample units, sample extent, coverage, and intensity all result in a unimodal occupancy distribution with the mode in the lowest occupancy class; 2) an increase in sample unit size leads to a bimodal distribution; 3) changes that occur in the shape of the occupancy distributions with varying sampling protocol parameters can be explained by the movements of the species between occupancy classes; 4) different species abundance distributions may cause occupancy distributions with a left-hand mode or a bimodal distribution; and 5) the number of species in the highest occupancy class decreases with increasing degree of aggregation. The mode that almost always occurs in the lowest occupancy class is most likely due to the high number of rare species existing in most communities; the mode in the highest class emerges as a pure artefact that occurs when the sample unit size is relatively large compared to the sample extent. Consequently, the exclusion or separation of concurrent artefactual mechanisms is crucial when investigating the biological causes for the shape of occupancy distributions.