DOI: 10.5061/dryad.573n5tb7s
title: Patterns and drivers of leaf-litter ant diversity along a tropical elevational gradient in Mexico

description: Aim: Given their high environmental variation over relatively short
 distances, mountains represent ideal systems for evaluating potential
 factors shaping diversity gradients. Despite a long-standing interest in
 ecological gradients, ant diversity patterns and their related mechanisms
 occurring on mountains are still not well understood. Here, we (i)
 describe species diversity patterns (α and β) of leaf-litter ants along
 the eastern slope of Cofre de Perote in Veracruz, Mexico, and (ii)
 evaluate climatic and spatial factors in determining these patterns.
 Location: Veracruz, Mexico Taxon: Leaf-litter ants Methods: We sampled 320
 m2 of leaf litter spread across 8 equally-spaced sites from sea level to
 3500 m of elevation. We used regression models to predict α-diversity
 patterns with climatic (temperature and precipitation) and spatial
 (geometric constraints) variables. We also assessed, through multiple
 regression based on distance matrices (MRM), the relative importance of
 habitat filtering and dispersal limitations for shaping total
 dissimilarity (βsor), turnover (βsim) or nestedness (βnes). Results: A
 hump-shaped pattern was observed in the α- diversity with a peak at
 600-1000 meters above sea level. This pattern is best explained by the
 temperature gradient with around 80% of variance explanation. β-diversity
 showed a non-linear pattern along the elevational gradient with total
 dissimilarity and turnover components better explained by habitat
 filtering (i.e., temperature distances). Main conclusions: The importance
 of temperature on both α- and β-diversity patterns reinforces its
 widespread importance in shaping litter ant diversity patterns across
 elevational gradients. The hump-shaped pattern in species richness is
 probably the result of harsh abiotic conditions at the base and the top of
 the mountain combined with biotic attrition in lowland sites. Niche
 specialization of ant species in their optimal thermal zones may explain
 total dissimilarity and ant species replacement along the studied
 gradient. Taken all together, these results suggest a high relevance of
 temperature-driven mechanisms in the origin and maintenance of the
 biodiversity of ectothermic taxa.
description: This study was conducted along the eastern slope of the Cofre de
 Perote mountain, in Veracruz, Mexico. This region is located at the
 junction of the Trans-Mexican volcanic belt and the Sierra Madre Oriental.
 We selected eight study sites spanning an elevational gradient of 3500
 meters of altitude. Regardless the geographical
 distance, all sites were systematically separated with an elevational
 difference of 500 meters on average between each other. We placed our
 study sites at the following elevations above sea level: 30-50 m, 610-670
 m, 900-1010 m, 1470-1650 m, 2020-2230 m, 2470-2600 m, 3070-3160 m and
 3480-3540 m, however, for simplicity, we will refer to each site as the
 discrete unit (i.e. 0, 600, 1000, 1500, 2100, 2500, 3100, 3500 m). For
 summarized characteristics of sites see Table 1 and Supporting Information
 Table S1. Sampling sites were old-growth
 forests characterized by no obvious forest use and highly dominance of
 mature forests, except in the case of the lowest site (i.e La Mancha),
 where most of its original vegetation has been transformed. To overcome
 the effect of perturbation in the studied patterns, we sampled La Mancha
 in a secondary forest with up to 30 years of regeneration. All sampling
 sites were closed-canopy forests in which a leaf-litter layer could be
 guaranteed. During the rainy season (July-September) of 2018 one 300-m
 transversal transect was located at each one of the eight study locations
 where we established 10 equidistantly sampling points (i.e. 30 meters
 between each other). Two independent 1-m2 samples were taken
 perpendicularly to each sampling point: one 10 meters on the right side
 and the other 10 meters from the left side. This procedure was repeated in
 a second transect placed during the dry season (March-May) of 2019 in
 order to increase our sample coverage (see below) as well as reduce any
 seasonality effect on our diversity patterns. Transects within an
 elevational site were separated at least 1 km away from each other. Thus,
 320 leaf-litter samples characterized the whole mountain (8 study sites x
 20 m<sup>2 </sup>per transect x 2 transects = 320
 m<sup>2</sup>). In each 1-m<sup>2</sup> quadrat,
 we collected the leaf litter inside and sifted it through a coarse mesh
 screen of 1-cm grid size to remove the largest fragments and concentrate
 the fine litter. The concentrated fine litter from each sample was
 suspended in independent mini-Winkler sacks for 3 days in the laboratory.
 Falling arthropods were collected into a container with 95% ethanol. Ant
 workers were removed from each container for identification. When
 possible, specimens were identified at the species level. If not, we
 assigned a morphospecies number.