DOI: 10.5061/dryad.ttdz08kw8
title: Mechanisms underlying altitudinal and horizontal range contraction: The western black crested gibbon

description: Aim: Species ranges in mountain areas may shift both horizontally and
 altitudinally, resulting from climate change and anthropogenic impact. Two
 hypotheses (the abundant center hypothesis and the contagion hypothesis)
 have been proposed to account for patterns of horizontal range
 contraction. However, undulating topograph causes a mosaic of unsuitable
 habitats, which may complicate the spatial pattern of range contraction.
 We develop a framework incorporating horizontal and altitudinal range
 contraction patterns of a species living in mountain areas, to better
 understand the underlying mechanisms of species range contraction.
 Location: China, North Laos, and North Vietnam Taxon: Western black
 crested gibbon, Nomascus concolor Methods: We collected occurrence data of
 the gibbons from various sources and modelled their potential distribution
 range in the 1950s, 1980s, and 2010s, using ecological niche modelling. We
 compared distances from the center point of the potential range in 1950s
 to center points of the largest 100 patches in the 1950s and the 2010s to
 understand the patterns of horizontal range contraction. We also
 calculated potential distribution within different altitudinal range for
 six populations in each period to understand the patterns of altitudinal
 range contraction. Results: Potential horizontal distribution of the
 gibbons decreased by 69% from the 1950s to the 2010s. We found the 100
 largest patches in 2010s were further apart from the center point of the
 potential range in 1950s compared to the 100 largest patches in 1950s,
 supporting the contagion hypothesis. No populations extended their range
 to higher altitude, suggesting climate change did not have a profound
 effect on gibbon range upward shift. All populations lost a substantial
 proportion of their ranges in lower altitude (500 – 1,500 m) but to
 different degrees, suggesting that populations experienced different
 anthropogenic pressures. Main conclusions: Anthropogenic threats probably
 including human population increase, agricultural expansion and hunting,
 were more likely than climate change to have caused range contraction in
 western black-crested gibbons. This study highlights the importance of
 studying horizontal and altitudinal range contraction simultaneously.
description: 1.
 Climate variable We downloaded
 climate data from CRU TS v. 4.03 (0.5° * 0.5° grid) (Harris et al., 2014;
 Harris, Osborn, Jones, & Lister, 2020). We calculated monthly
 climate variables at 1km<sup>2</sup> resolution by
 interpolation of complex multivariate data from climate data (CRU TS v.
 4.03) using thin plate smoothing splines for each period (1950s, 1980s,
 and 2010s) following the downscaling method provided in CCAFS (The CGIAR
 Research Program on Climate Change, Agriculture and Food Security)
 (Navarro-Racines, Tarapues, Thornton, Jarvis, & Ramirez-Villegas,
 2020). We then derived the bioclimate variables from the monthly climate
 variables (1km<sup>2</sup>) using “dismo” package in R. We
 also calculated an annual heat-moisture index (AHM) by Bio1 (annual mean
 temperature) and Bio12 (annual precipitation) using “raster” package in R
 (<i>Formula</i>: AHM = [Bio1*0.1 + 10]/[Bio12*0.001]) (Wang,
 Hamann, Spittlehouse, & Murdock, 2012). 2.
 Landcover We downloaded
 landcover maps in 2010 (current) from the MCD12Q1 V6 product (Friedl
 & Sulla-Menashe, 2015). This map provides global land cover at 500
 meter spatial resolution at annual time. Categorical broad-leaf forest and
 anthropogenic influence (including crop and urban area) variables were
 derived from landcover data. We therefore converted all variables to
 continuous variables by calculating Path Distance and resampled as a
 raster layer in 1km<sup>2</sup> resolution using ArcMap 10.2.2
 software. We also downloaded topographic data from the SRTM 90m Digital
 Altitude Database v4.1 (1km<sup>2</sup>) (Reuter, Nelson,
 & Jarvis, 2007). Pleace check
 the details in the paper.