DOI: 10.5061/dryad.fqz612jz0
title: Data for: Biodiversity and elevation gradients across worldwide Mountains

description: Aim: Identifying macroecological patterns and biases in species
 distribution is a challenging but essential task in biodiversity-oriented
 studies. Despite the extensive attempts to find consistent species
 richness elevation (SRE) patterns, the topic remains controversial owing
 to widespread conflicting, idiosyncratic, and non-generalizable underlying
 mechanisms. We compiled a database to perform a meta-analytical review to
 answer why patterns of species-richness in elevation gradients remain
 elusive, a long-standing, central but contentious macroecological and
 biogeographical question. Location: Global elevation gradients. Taxon:
 Major terrestrial taxa (invertebrates, vertebrates, and plants). Methods:
 We tested the effect of elevation on species richness using multilevel
 mixed-effects meta-analytical models. Data from 127 studies spawning
 almost one century of research were integrated to test the effect of
 elevation across distinct 1) SRE models, 2) quality of primary data (e.g.,
 mountain sampling coverage), 3) biogeographic realms, 4) studied taxa, and
 5) organism mobility. Results: The linear negative pattern showed the
 strongest model fit followed by the hump-shaped and the linear positive
 models. We observed that studies with higher sampling sizes showed a
 consistent decrease in the strength of SRE patterns. Further, the larger
 the mountain coverage and sampled range, the stronger the detection of
 some SRE patterns. Overall, the elevational effect on species richness was
 consistent across biogeographical realms, taxonomic groups, and organism
 mobility. Main conclusions: This study indicates a bias in the detection
 of SRE patterns, driven mostly by mountain comprehensiveness, namely the
 number of sampling units, sampled range, and mountain coverage. These
 results call attention to the evidence that undersampled elevation
 gradients may bias our understanding on the complex relationships between
 elevation and biodiversity, thus impairing a broad understanding on the
 ecology, evolution, biogeography, and conservation of mountain biota.
description: To perform a systematic
 quantitative review on SRE patterns we used different approaches,
 following the synthesis guidelines in Haddaway et al. (2020). First, we
 conducted literature searches in the Web of Science and Scopus searching
 engines, using the following keyword combination: (“altitud* gradient*” OR
 “elevation* gradient*” OR “mountain* gradient*” OR altitud* OR elevation*
 OR mountain*) AND (“species richness” OR richness OR diversity OR “species
 diversity" OR biodiversity). Initial searches were conducted between
 July 2020 and March 2021 and covered the timespan of the
 bibliographic
 databases (1945-2021). To
 complement our database, we also included studies provided in the
 reference list of elevational gradients reviews (e.g.,
 Rahbek, 1995; Sanders
 & Rahbek, 2012;
 see Figure S1 and Table
 S1). A total of 644 published studies were fully screened for
 eligibility.  From these, we selected all studies that
 reported data on SRE relationships. This search comprised global data
 across all biogeographical realms, all continents, except Antarctica, and
 insular regions. We focused on the dominant components of terrestrial
 ecosystems by reviewing SRE models reported for microorganisms,
 invertebrates, vertebrates, and plants. Studies were included when meeting
 the following criteria: 1) raw data on species richness (neither rarefied
 nor transformed) provided for each elevational gradient; 2) any
 statistical parameter stated for species richness along an elevation
 gradient (e.g., r, F, Spearman-rho, t or R2); and 3) reported sample
 sizes. In less than 5% of the database, we contacted the authors to obtain
 original raw data and calculated the correlation coefficients whenever
 missing from the original publication. Studies that treated elevation as a
 categoric or qualitative variable were excluded. This selection criteria
 resulted in 143 studies and 479 gradients that were screened for
 quantitative data.  As a conservative approach, we further excluded
 the gradients reporting sample size lower than five elevational quotas
 sampled over a single mountain (n=2), undefined SRE models (i.e., no clear
 pattern reported in published figures or results, n=15 gradients, 3.13% of total cases), infrequent
 SRE models (e.g., polynomial models,
 n=14, 2.92% of total cases), and studies focusing
 exclusively on microorganisms (n=12, 2.5%), due to an
 insufficient number of records. Further, we excluded gradients with
 inconsistent sampling throughout the mountain and/or those with inflated
 sample sizes, likely due to pseudoreplication (n=63, 13% of records).
 Thus, the final database comprised 373 gradients from 127 studies
 comprising the three most
 frequent SRE
 models reported in the
 literature: linear negative, hump-shaped, and linear
 positive.