The truncated bell: an enigmatic but pervasive elevational diversity pattern in Middle American ants

Authors: LONGINO, JOHN - University Of Utah; Branstetter, Michael

Submitted to: Ecography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2018
Publication Date: 7/4/2018
Citation: Longino, J.T., Branstetter, M.G. 2018. The truncated bell: an enigmatic but pervasive elevational diversity pattern in Middle American ants. Ecography. 41:1-12. https://doi.org/10.1111/ecog.03871.
DOI: https://doi.org/10.1111/ecog.03871

Interpretive Summary: A major goal in biology is to understand how life is distributed across the planet. From desert to forest, from north to south, and from low to high, biologists want to know how and why species diversity can differ from one place to the next. Documenting and explaining these patterns is of intrinsic value and can help scientists predict how habitat modification and climate change will affect biological diversity in the future. The study of Longino & Branstetter examines patterns of diversity in ants – a diverse group of social insects related to bees – along elevational gradients in the mountains of Central America (Mexico to Panama). Previous research at two sites in Central America identified a diversity pattern for ants in which species richness (i.e., the number of species) increased from low to mid elevations and then decreased from mid to high elevations. This pattern is known as the “mid-elevation peak” diversity pattern and has been observed in other regions and organisms, but it has been difficult to explain. To determine if the pattern extends throughout the mountains of Central America and to try and explain the pattern Longino & Branstetter expanded sampling of ants to 56 wet, rainforest sites spread throughout several mountain ranges. At each site, 100 samples of forest floor leaf litter were collected and most of the ant specimens from each sample were identified to species. Various species richness and diversity estimates were calculated and those estimates were examined against a suite of ecological variables, including temperature, elevation, precipitation, and seasonality. The authors examined diversity patterns for all ants and several subgroups of ants, including several genera that appeared to exhibit contrasting patterns. Remarkably, it was found that ants show a mid-elevation peak in species diversity across all of Central America, indicating that the mid-elevation peak is a pervasive phenomenon throughout the region. But, no set of environmental variables could explain the pattern with high confidence, leaving the cause of the pattern largely uncertain. The authors suggest that the pattern might be due to effects from recent climate warming related to glacial cycles or the overlap of two distinct ant communities (low and high) merging together. Overall, the study demonstrates that the mid-elevation peak is a common pattern for ants and that more research is needed to understand what drives it.

Technical Abstract: Studies on elevation gradients in Panama and Costa Rica have shown that leaf-litter ants exhibit a mid-elevation peak in diversity. Species richness rises from sea-level to 500 m elevation, then declines to very few species above 2000 m. This diversity pattern has been observed in other groups and regions, but uncertainty remains as to just how pervasive it is and what might explain it. Here we examine the robustness of the mid-elevation peak in ant diversity across the entire Middle American corridor, from Costa Rica to Veracruz, Mexico. A study of the Barva Transect in Costa Rica relied on seven sites to demonstrate a mid-elevation peak. We used identical methods to sample an additional 49 sites distributed throughout the Middle American corridor. Sites were all in closed-canopy evergreen wet forest, spanning 11° latitude, from near sea level to 2600 m elevation. Ants were extracted from 100 litter samples from each site. Linear models were used to examine the relationships among diversity measures and elevation (as a proxy for temperature), precipitation, and seasonality. Species richness measures and diversity indices that incorporate relative abundance show a similar relationship to elevation throughout the region: a truncated bell curve with a mode near 500 m. Typical of gradient studies, temperature is a significant correlate with diversity, but does not predict a bell-curve. Precipitation and precipitation seasonality fail to explain much of the variability, and no combination of environmental variables predicts a bell curve. Two major subclades within the ants, Attini (sensu lato) and Pheidole, show the same pattern. Two genera, Adelomyrmex and Stenamma, are exceptional in having distinctive montane radiations, which cause them to have divergent patterns of diversity and elevation. Potential causes of the bell curve include lowland biotic attrition, mid-point attractors, and ecotonal transitions from lowland to montane communities.