DOI: 10.5061/dryad.jq2bvq88v
title: Numbers of individuals and endemicity for birds in three study sites of the Himalayas-Hengduan mountains of China

description: 1. Describing the patterns and revealing the underlying mechanisms
 responsible for variations in community structure remains a central focus
 in ecology. However, important gaps remain, including our understanding of
 species abundance. Most studies on abundance-based relationships are from
 either temperate ecosystems or tropical ecosystems, and few have
 explicitly tested abundance-based relationships across a temperate to
 tropical ecotone. 2. Here, we use a comprehensive dataset of breeding
 birds across elevation spanning a temperate to subtropical gradient in the
 Himalayas-Hengduan mountains of China to examine the relationship between
 species abundance and (1) elevational range size, (2) body size, (3)
 elevational range center and, (4) endemicity. 3. We tested a priori
 predictions for abundance-elevational range size relationship,
 abundance-body size relationship, and abundance-elevational range center
 relationship and explored how these relationships change along this
 temperate to subtropical mountain ecosystem. 4. We found that species
 abundance was significantly positively correlated with elevational range
 size across the study sites, demonstrating the key importance of
 elevational range size towards species abundance. Body size and
 elevational range center are weakly correlated with abundance. A novel
 finding of our study is that the abundance-elevational range size
 relationship gradually weakened from temperate to subtropical ecosystems,
 adding to a growing body of evidence suggesting that abundance-elevational
 range size tracks a temperate to tropical ecotone. 5. Our study
 demonstrates that abundance range-size relationship can transition across
 ecotones where faunas of different evolutionary origins converge.
 Furthermore, measuring abundance relationships across different
 environmental variables at the same spatial scale with comparable
 biogeography is a key strategy that can reveal the underlying mechanisms
 behind abundance patterns.
description: We made four replicated bird censuses during the breeding season
 (April to October) in each site along the elevational gradient. These bird
 censuses were from May-June 2012, August 2012, September-October 2012, and
 July-August 2013 in Gyirong; April and May in 2017 and 2018 in Gaoligong;
 August and September in 2018, and June and July in 2019 in Lebu. For each
 site, we divided the entire elevational range into 300-m elevational
 bands: twelve in Gyirong from 1,800 to 5,400 m, nine in Lebu from 2,300 to
 5,000 m, and nine in Gaoligong from 700 to 3,400 m. We did not survey at
 lower or higher elevations because of inaccessible topography (e.g.
 turbulent rivers and cliffs) or harsh climate. We set two to four
 transects in each 300-m elevational band, all transects were in areas with
 the most undisturbed habitat available at each elevational band, and the
 habitat was contiguous between all elevational bands. We standardized
 sampling effort in each gradient by limiting the total length of transects
 in each 300-m elevational band around 7.5 km in Gyirong, and 7 km in Lebu
 and Gaoligong (length of individual transects were between 1.5 and 4 km).
 Details of the transects are provided in Tables S1-S3 of Appendix 1. We
 carried out field surveys between 30 minutes after dawn and 10:00 hours,
 and between 16:00 hours and 30 minutes before sunset (local time) but not
 conducted in extreme weather conditions (heavy rain or fog). For each
 transect, we made two replicated surveys in morning and two at dusk,
 respectively. We sampled the elevational bands in a random order to reduce
 the temporal autocorrelation. Field surveys in each elevational gradient
 were conducted by the same proficient observers. The observers recorded
 all bird species within 50 m of each transect. We used taxonomy of birds
 following Zheng (2011), and only used breeding birds (residents and birds
 breeding in the Himalayas-Hengduan mountains) for statistical analyses,
 because migratory birds could cause potential bias of the elevational
 range since we sampled the birds during breeding season.