Plant Rarity

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Rarity is the opposite of richness; it is a measure of species infrequency. Scientists calculate rarity by taking the inverse of richness, a conversion called a rarity-weighted richness index (RWRI). To calculate this index, each plant species found in a study area receives a numerical value. This value comes from the inverse of the number of areas in which the plant occurs by completing the following steps:

1. Record the number of grids where a species is observed by using a range-incidence map.
2. Divide 1 by the number of grids where the species was observed. (A species inhabiting only one grid receives a score of 1.0; a species inhabiting 20 grids receives a score of 0.05.)
3. Calculate RWRI for each grid by summing the scores of each species in the grid. As the RWRI number increases, so does a plant species’ rarity. Therefore, the plant rarity value may be thought of as a measure of irreplaceability, because the plants scoring the highest are considered the rarest. Conservation biology currently focuses on plants with high RWRI scores.

The world contains plant rarity hotspots the same way it houses plant biodiversity hotspots. Surprisingly, rarity occurs not in places where biodiversity is poor, but in places with high diversity. The largest number of rare plants on Earth occurs in places with rich biodiversity and also in places of large human population size.

The United States contains about 8,840 rare plants that make up onethird of the country’s total flora. Some plants are naturally rare and may not be in danger of extinction. Others become rare due to fragmented habitat, loss of compatible soils, seeds that do not disperse well in the environment, or seeds that depend on certain insects or pollinators that are themselves endangered. The ecosystem approach to biodiversity loss focuses on the entire plant-animal population in any given area. In this approach, the plant richness and rarity composition plays a critical role.