Continuing the search for pattern among rare plants: Are diploid species more likely to be rare?

Abstract

Questions: Are diploid plant species more endangered than polyploids? Does ploidy play any role in rarity and invasiveness of plant species? Data studied: I surveyed and analysed ploidy levels of more than 1000 plant species, including 75 endangered taxa and 43 invasive species, on the Indian subcontinent to test the relationship between ploidy and both rarity and invasiveness. Search method: Null hypothesis: I examined the null hypothesis that diploids and polyploids were equally represented in the sets of endangered and non-endangered/non- invasive species. Logistic regressions; I performed simple logistic regressions (logit model) of the probability of species endangerment and invasiveness on number of chromosomes. Where necessary, plant family was included as a co-factor in the regressions, to partially control for the effects of phylogenetic autocorrelation among species. Conclusions: I show that diploid plant species in the Indian subcontinent are more likely to be rare than are polyploids. The analysis revealed that the diploid-polyploid ratio for non-endangered plants of the subcontinent follows known trends (with 50.5% diploid), but that a significantly larger fraction of the endangered species (64%) are diploid. Clear polyploids comprise only 36% of the endangered species. In contrast, a significantly larger fraction of invasive species studied (72%) are polyploids, with many displaying high levels of polyploidy. The relationship between diploidy and both plant rarity and invasiveness was highly significant. I also find that, in general, plant species with more chromosomes have less likelihood of being endangered than the species with fewer chromosomes. Using a simple logistic regression (logit model), I show that phylogeny has a significant effect on species endangerment and that chromosome number is negatively and significantly correlated to the probability that a species is endangered. © 2006 M.K. Pandit.