Rarity and commonness in New Zealand ferns

Abstract

A macroecological approach was used to look for patterns in, and correlates of, rarity and commonness in the New Zealand fern flora, which is comprised of 250 species. Herbarium records and expert knowledge were used to map fern species distributions from which range sizes were calculated, and used as a measure of rarity and commonness. Fern species frequency distributions showed a pattern of many small and fewer large range sizes, with bimodal peaks at very small and medium range sizes. Latitudinal differences in range sizes showed evidence of a geometric constraint and the mid-domain effect, and did not directly support Rapoport’s Rule which suggests larger range sizes at higher latitudes and vice versa. Trait data for each fern species, including morphological characteristics, were compiled from the literature, as were habitat preferences, elevational range, and biostatus,. Both univariate and multivariate linear mixed models were used to determine relationships between species traits and range sizes for 211 fern species for which data were available. Habitat specialists had smaller range sizes than generalists, and species occurring in forest and/or montane environments had larger range sizes than those that did not, as did epiphytic species. The variation in range size explained by phylogeny (taxonomic family and genus) was lower than that explained by traits. Patterns of rarity and commonness differed markedly between indigenous and introduced species. This study provides new knowledge of patterns in the diversity of the New Zealand fern flora.