It is estimated that some 350 000 green plant species (‘green algae’, nonvascular plants, seedless vascular plants, gymnosperms, angiosperms) have been described, of which the vast majority are angiosperms. Confidence in the estimates of species numbers vary from group to group and in some cases are little better than guesswork. The main threats to plant biodiversity are humans, mediated through habitat destruction, introgression and hybridization, human population growth, pollution and overexploitation.