Hierarchies of Cause: Toward an Understanding of Rarity in Vascular Plant Species

Abstract

Four classes of naturally rare vascular plant species are described and classified, based on parameters of spatial distribution and longevity. Properties intrinsic to these time/space parameters are explored and an importance hierarchy of causes of rarity is proposed for each class. These hierarchies serve as the basis for a predictibe classification. Human causes of rarity such as habitat destruction and taxonomic difficulties are not considered in detail here but are discussed as confounding factors in the elucidation of rarity in vascular plants. Several examples are provided to illustrate this classification and provide testable hypotheses concerning the origins of natural rarity in plant species.