Submitted to: Biodiversity and Conservation Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/31/2011
Publication Date: 2/17/2011
Publication URL: hdl.handle.net/10113/50274
Citation: Espeland, E.K., Emam, T. 2011. The value of structuring rarity: the seven types and links to reproductive ecology. Biodiversity and Conservation Journal. 20(5): 963-985. Interpretive Summary: Describing species distributions is one of the primary foci of ecology. How does one species occur here, another there? The answer to the question of what limits (or enforces) plant species distributions is very important to agriculture, weed ecology, and conservation. Often, species are described as “common” or “rare”, but the word “rare” could be applied to species with dense local populations and small geographic range as easily as it could be applied to species with sparse local populations and large geographic range. In 1981, Deborah Rabinowitz suggested separating different kinds of rarity into seven types. She suggested three axes: geographic range size (large vs. small), habitat specificity (wide vs. narrow), and local abundance (dense vs. sparse). This matrix consists of eight types, with the large geographic range, wide habitat specificity, and dense local abundance type being “common”, and all other types being “rare”. This matrix has been heavily cited in the plant conservation literature (365 citations since 1981). We conducted a review and analysis to determine if separating rarity into different types would be useful. For example, do species with narrow habitat requirements (one type of rarity) share characteristics that species of small geographic range (another type of rarity) do not? We found that species with small geographic range size are more likely to have abiotic dispersal mechanisms and did not tend to depend on insect- or animal-mediated dispersal. Species with narrow habitat requirements are more likely to have biotically-mediated pollination (biotic:abiotic ratio 5:1) compared to generalist species. Rare species with narrow habitat requirements thus are disproportionately dependent on pollinators for reproduction, thus conserving pollinator communities in areas of endemism (e.g. areas with unusual soil types or hydrology) is important for conserving plant communities in these environments. Although the Rabinowitz matrix is primarily utilized within the plant conservation literature, we suggest that separating types of rarity and commonness may be useful for investigating questions regarding plant species distributions.
Technical Abstract: Since 1981, the plant conservation ecology literature has heavily referenced a rarity matrix organized along three axes: geographic range size (large vs. small), habitat specificity (wide vs. narrow), and local abundance (dense vs. sparse). Here we examine the utility of the matrix using data on 101 rare plant species that have been classified on at least two of the three rarity axes. We examined correlations between the axes and the reproductive ecology of the species in the dataset, including pollination syndrome, mating system, and seed dispersal mechanism. Species with small geographic range size are more likely to have ballistic or wind dispersal than insect- or animal-mediated dispersal (abiotic:biotic ratio 3:1). Species with narrow habitat requirements are more likely to have biotically-mediated pollination syndromes (biotic:abiotic ratio 5:1) compared to generalist species. At the common end of each rarity axis, species were equally likely to have abiotic or biotic pollination syndromes and equally likely to have abiotic or biotic dispersal vectors. These results show that separating types of rarity (e.g. treating sparse local abundance differently from small geographic range) is informative and can aid in the examination of basic mechanisms of plant distribution and abundance.