Rare species do not disproportionately contribute to phylogenetic diversity in a subalpine plant community

Abstract

Premise of the study: Worldwide, 36% of all plant species are exceedingly rare, and are at the highest risk of extinction. Extinction risk has also been observed to be phylogenetically clustered, threatening the loss of lineages that contribute unique evolutionary diversity to communities. Rare plants' contributions to phylogenetic diversity are largely unknown, however. We investigate whether rare species (as measured by local abundance and range size) contribute disproportionately to phylogenetic diversity in a subalpine plant community. Methods: We collected abundance data at three sites in Washington Gulch near the Rocky Mountain Biological Laboratory (RMBL, Gothic, Colorado, USA) in 2021 and 2022. We then calculated range size for each species. We calculated phylogenetic signal for abundance and range size, compared community phylogenetic metrics weighted by range size and abundance to unweighted metrics, and quantified the change in phylogenetic diversity when removing single species and groups of species ranked by rarity. Key results: We found phylogenetic signal for abundance, but not range size. There was no difference between rarity-weighted and unweighted phylogenetic diversity metrics. Finally, phylogenetic diversity did not decline more when we removed single rare species and groups of rare species than when we removed single common species and groups of common species. Conclusions: Overall, we found that rare species do not disproportionately contribute to phylogenetic diversity. This suggests that rare species likely provide phylogenetic redundancy with common species, and losing rare species may not cause disproportionate drops in phylogenetic diversity in this system.