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Patterns and drivers of leaf-litter ant diversity along a tropical elevational gradient in Mexico

Citation

Perez-Toledo, Gibrán et al. (2021), Patterns and drivers of leaf-litter ant diversity along a tropical elevational gradient in Mexico, Dryad, Dataset, https://doi.org/10.5061/dryad.573n5tb7s

Abstract

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.

Methods

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 m2 per transect x 2 transects = 320 m2). In each 1-m2 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.

Funding

Consejo Nacional de Ciencia y Tecnología, Award: 487739

Instituto de Ecologia, A.C., Award: 902-11-10204

National Science Foundation, Award: DEB-1354739