 Other factors that have been proposed to be able to affect the shape of OFD include the degree of habitat heterogeneity, species specificity, landscape productivity, position in the geographic range, species dispersal ability and the extinction–colonization dynamics.Three basic models have been proposed to explain the bimodality found in occupancy frequency distributions.Random sampling of individuals from either lognormal or log-series rank abundance distributions (where random choice of an individual from a given species was proportional to its frequency) may produce bimodal occupancy distributions.