Latitudinal patterns of species richness and range size of ferns along elevational gradients at the transition from tropics to subtropics

Adriana C Hernández‐Rojas,Marcus Lehnert,Jürgen Kluge,Georg Miehe,Thorsten Krömer,César Carvajal‐Hernández,Libertad Silva‐Mijangos,Anna Weigand,Michael Kessler

doi:10.1111/jbi.13841

Abstract

AbstractAimTo assess the range size patterns of ferns and lycophytes along elevational gradients at different latitudes in an ecographical transition zone and search for predictors of range size from a set of environmental factors.LocationMexico, from 15° to 23° N.TaxonFerns and lycophytes.MethodsAll terrestrial and epiphytic species were recorded in 658 plots of 400 m2 along eight elevational gradients. To test whether the range size within assemblages increases with elevation and latitude, we calculated the latitudinal range using the northern and southern limits of each species and averaged the latitudinal range of all species within assemblages weighted by their abundances. We related climatic factors and the changes with latitude and elevation with range size using linear mixed‐effects models.ResultsSpecies richness per plot increased with elevation up to about 1,500–2,000 m, with strong differences in overall species richness between transects and a reduction with increasing latitude. The mean weighted range size of species within assemblages declined with elevation, and increased with latitude, as predicted by theory. However, we also found marked differences between the Atlantic and Pacific slopes of Mexico, as well as low range size in humid regions. The best models described about 76%–80% of the variability in range size and included the seasonality in both temperature and precipitation, and annual cloud cover.Main conclusionLatitudinal and elevational patterns of range size in fern assemblages are driven by an interplay of factors favouring wide‐ranging species (higher latitudes with increasing temperature seasonality; dryer habitat conditions) and those favouring species with restricted ranges (higher elevations; humid habitat conditions), with additional variation introduced by the specific conditions of individual mountain ranges. Climatically stable, humid habitats apparently provide favourable conditions for small‐ranged fern species, and should accordingly be given high priority in regional conservation planning.

Highlights

One of the most striking patterns in nature is the enormous variation of range sizes of species, ranging from species which occur only in a few square meters to others that are found across the entire globe (Brown, Stevens, & Kaufman, 1996; Gaston, 1998)
We explored the patterns of latitudinal range size of ferns along eight elevational gradients located at different latitudes in the Mexican transition zone from the tropics to the subtropics (30 km south of the Tropic Cancer Line), which is considered a global biodiversity hotspot (Myers et al, 2000) and a centre of fern endemism (Brummitt et al, 2016)
Overall latitudinal range size increased with increasing latitude

Summary

| INTRODUCTION

One of the most striking patterns in nature is the enormous variation of range sizes of species, ranging from species which occur only in a few square meters to others that are found across the entire globe (Brown, Stevens, & Kaufman, 1996; Gaston, 1998). Conceived for latitudinal gradients, the idea that range sizes may be determined by climatic seasonality was later extended to elevational gradients as well (Stevens, 1992, climatic variability hypothesis) While these patterns have been documented for a wide range of taxa in many regions (Addo-Bediako, Chown, & Gaston, 2000: insects; Ribas & Schoereder, 2006: many groups; Morin & Lechowicz, 2011: trees; Pintor, Schwarzkopf, & Krockenberger, 2015: lizards; Tomašových et al, 2016: birds and marine bivalves), there are a good number of studies, mainly along elevational gradients in animals and in plants, that do not corroborate the rule or even reporting a reverse pattern or mixed results suggesting that it varies between taxa and continents (Bhattarai & Veetas, 2006; Pintor et al, 2015; Ribas & Schoereder, 2006; Rohde & Heap, 1996; Rohde, Heap, & Heap, 1993; Ruggiero, 1994; Zhou et al, 2019). We predict that mean range sizes decrease with increasing humidity due to the water dependency of the study group, related to the geographical fragmentation of environmentally suitable areas for specialized ferns (H3b)

| MATERIALS AND METHODS

Findings

| DISCUSSION