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

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.

研究目的：山地在相对较短的空间尺度内具有极高的环境异质性，是探究塑造多样性梯度的潜在影响因子的理想研究系统。尽管学界对生态梯度的研究已有悠久历史，但山地蚂蚁的多样性格局及其相关形成机制仍未得到充分阐释。本研究（i）描述墨西哥韦拉克鲁斯州科弗雷德佩罗特山东坡枯落物蚂蚁（leaf-litter ants）的物种多样性格局（α多样性（α-diversity）与β多样性（β-diversity））；（ii）评估气候与空间因子对该多样性格局的调控作用。
研究地点：墨西哥韦拉克鲁斯州
研究类群：枯落物蚂蚁
研究方法：我们在从海平面至海拔3500米的范围内设置了8个等间距样地，共采集了320平方米的枯落物样本。采用回归模型，以气候因子（温度与降水量）和空间因子（几何约束变量）预测α多样性格局。同时，通过基于距离矩阵的多元回归（Multiple Regression on Distance Matrices, MRM）分析，评估生境过滤和扩散限制对构建总β多样性（βsor，总物种差异）、物种周转组分（βsim）以及嵌套性组分（βnes）的相对重要性。
研究结果：α多样性呈现驼峰型格局，在海拔600~1000米处达到峰值，该格局可由温度梯度最佳解释，其方差解释率约为80%。β多样性沿海拔梯度呈现非线性格局，总物种差异与物种周转组分主要由生境过滤（即温度距离）解释。
主要结论：温度对α和β多样性格局的重要性，进一步印证了其在调控沿海拔梯度的枯落物蚂蚁多样性格局中的广泛作用。物种丰富度的驼峰型格局，可能是山地底部与顶部的严酷非生物条件，加之低地生境的生物群落损耗共同作用的结果。蚂蚁物种在其最适温区的生态位特化，可解释研究梯度上的总物种差异与蚂蚁物种替换现象。综合来看，本研究结果表明，温度驱动的调控机制在变温类群生物多样性的起源与维持中具有极高的相关性。
方法补充说明：本研究沿墨西哥韦拉克鲁斯州的科弗雷德佩罗特山东坡开展。该区域位于跨墨西哥火山带（Trans-Mexican volcanic belt）与东马德雷山脉（Sierra Madre Oriental）的交汇处。我们设置了8个研究样地，覆盖3500米的海拔梯度。无论地理距离如何，所有样地的平均海拔间距均为500米。我们将样地设置在以下海拔区间（以海平面为基准）：30~50米、610~670米、900~1010米、1470~1650米、2020~2230米、2470~2600米、3070~3160米以及3480~3540米；为简化表述，后续将以离散海拔值指代各对应样地，即0、600、1000、1500、2100、2500、3100、3500米。样地的详细特征参见表1与支持信息表S1。
采样方案：所有采样样地均为原始成熟林，无明显人为森林利用活动，以成熟林占绝对优势，但最低海拔样地（拉曼查）除外——该样地的原始植被已大部分被改造。为消除人为扰动对研究格局的影响，我们在该样地选取了拥有30年以上恢复年限的次生林进行采样。所有采样样地均为密闭冠层森林，可保证枯落物层的存在。2018年雨季（7~9月），我们在8个研究样地各设置1条长300米的横向样带，每条样带内设置10个等间距采样点（相邻采样点间距30米）。在每个采样点的垂直方向上采集两份独立的1平方米枯落物样本：分别位于采样点左侧和右侧各10米处。为提升样本覆盖度并降低季节效应对多样性格局的影响，我们在2019年旱季（3~5月）重复设置了第二条样带。同一海拔样地内的两条样带间距至少为1千米。因此，整个山地共采集了320平方米的枯落物样本（8个研究样地 × 每条样带20平方米 × 2条样带 = 320平方米）。在每个1平方米样方内，我们收集所有枯落物，并用孔径为1厘米的粗网筛进行过筛，以去除较大的碎屑并浓缩细枯落物。将每份样本浓缩后的细枯落物放入独立的迷你温克勒袋（mini-Winkler sacks），在实验室中悬挂3天，使节肢动物掉落至装有95%乙醇的收集容器中。从每个容器中挑出工蚁进行鉴定。若可鉴定至物种水平，则鉴定至种；若无法鉴定，则赋予形态种编号。