Beyond eight forms of rarity: Which species are threatened and which will be next?
We tabulate three measures of rarity: local abundance, breeding range size and elevational extent for the passerine birds of the New World. We determine what fraction of species is threatened with extinction within the combinations of these three measures. Species with smaller ranges, lower abundances and narrower elevational bands suffer higher levels of threat across lowland, montane and island species. For a given range size, lowland species suffer higher levels of threat than island or montane species. (This is counter to the intuition that island species - and those isolated on mountain tops - might be ecologically naïve.) When all three factors are considered together, there is only a slight tendency for lowland species to be disproportionately more threatened. Simply, island and montane species tend to be relatively common within their restricted ranges and their increased abundance reduces their likelihood of being threatened. Elevation is a consistent but relatively unimportant factor in determining threat; abundance and range size are much more important, and have an interactive effect on threatened status. We calculate the number of humans with which each species shares its breeding range, and find that this number does not aid in predicting threat status.
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