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

Abstract

Studies on elevation gradients in Panama and Costa Rica have shown that leaf-litter ants exhibit a mid-elevation peak in diversity. 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 Veracruz, Mexico, to Costa Rica. We sampled 56 sites distributed throughout Middle America. All were 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 and identified to genus or species. Model selection was performed on richness and diversity variables to test if ant diversity best fits a linear model or one allowing for a mid-elevation peak. Linear models were also used to examine the relationships among diversity measures and 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 400 m. A cubic relationship is statistically favored over quadratic or linear. 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. Potential causes of the truncated bell curve include lowland biotic attrition, mid-point attractors, and ecotonal transitions from lowland to montane communities. Analysis of 17 subclades within ants mostly showed the same truncated curve but six clades were anomalous. Distinctive behavioral or historical features potentially explain their patterns.

Ant diversity patterns across tropical elevation gradients: effects of sampling method and subcommunity

“…In wet forests of Middle America, ant occupancy (presence or absence in small sampling units) is nearly 100% from sea level to~2000 m elevation, above which ant-free patches begin to appear (reflected in samples with no ants) Colwell 2011, Longino et al 2014). This pattern occurs not only on individual elevational gradients (Olson 1994, Longino andColwell 2011), but also appears to be robust throughout Middle America, from southern Mexico to Costa Rica (Longino and Branstetter 2019). Ant abundance, measured as average number of individuals per m 2 , is also relatively stable from sea level tõ 1500 m, above which it sharply declines (Longino and Colwell 2011, Longino et al 2014.…”

“…Elevational gradients are of interest because they are replicated across space and have pronounced environmental changes over short distances (Rahbek 1995, Graham et al 2014. Ants on elevational gradients have been the subject of numerous studies (e.g., Olson 1994, Fisher 1996, 1998, 1999a, Sanders 2002, Sanders et al 2007, Malsch et al 2008, Longino and Colwell 2011, Machac et al 2011, Longino et al 2014, Smith et al 2014, Szewczyk and McCain 2016, Liu et al 2018, Longino and Branstetter 2019, revealing a variety of patterns and suggested causes. Among the insects, ants are a prominent group because of their ubiquity and high biomass (H€ olldobler and Wilson 1990, Wilson andH€ olldobler 2005).…”

“…These concepts are related to the midpoint attractor model of Colwell et al (2016), in which species cluster near a favored region of a gradient. Longino and Branstetter (2019) proposed niche conservatism to explain the truncated bell curve of ant diversity in Middle American elevational gradients. Longino and Branstetter (2019) proposed niche conservatism to explain the truncated bell curve of ant diversity in Middle American elevational gradients.…”

“…Previous elevation studies of Middle American wet forest ants (Olson 1994, Longino and Colwell 2011, Longino et al 2014, Longino and Branstetter 2019 have mostly been based on a single sampling method: microarthropod extraction from 1-m 2 patches of forest floor leaf litter and rotten wood (the miniWinkler method, Fisher 1999b). MiniWinkler sampling takes 1 m 2 of the forest floor litter and rotting wood, shakes it up, and lets all the ants fall out.…”

Ecological interactions among insect herbivores, ants and the host plant Baccharis dracunculifolia in a Brazilian mountain ecosystem

“…Changes in abiotic conditions and vegetation with elevation play essential roles in affecting α and β‐diversity of insect communities (e.g. Lawton et al ; Fernandes & Price ; Kumar et al ; Kraft et al ; Fernandes ; Peters et al ; Nunes et al ,b; Longino & Branstetter ). Climatic variation at higher elevations (see Rasmann et al ) can represent an active driver for such changes.…”