The Coupled Influence of Thermal Physiology and Biotic Interactions on the Distribution and Density of Ant Species along an Elevational Gradient

Abstract

A fundamental tenet of biogeography is that abiotic and biotic factors interact to shape the distributions of species and the organization of communities, with interactions being more important in benign environments, and environmental filtering more important in stressful environments. This pattern is often inferred using large databases or phylogenetic signal, but physiological mechanisms underlying such patterns are rarely examined. We focused on 18 ant species at 29 sites along an extensive elevational gradient, coupling experimental data on critical thermal limits, null model analyses, and observational data of density and abundance to elucidate factors governing species’ elevational range limits. Thermal tolerance data showed that environmental conditions were likely to be more important in colder, more stressful environments, where physiology was the most important constraint on the distribution and density of ant species. Conversely, the evidence for species interactions was strongest in warmer, more benign conditions, as indicated by our observational data and null model analyses. Our results provide a strong test that biotic interactions drive the distributions and density of species in warm climates, but that environmental filtering predominates at colder, high-elevation sites. Such a pattern suggests that the responses of species to climate change are likely to be context-dependent and more specifically, geographically-dependent.

Critical thermal limits in ants and their implications under climate change

“…In general, species in regions with higher maximum temperatures were more heat tolerant, and species in regions with higher minimum temperatures were less cold tolerant; species in regions with higher mean temperatures showed greater tolerance to both heat and cold. Chick et al (2020) found that CT max and CT min of 18 ant species found along an elevational gradient in the Great Smoky Mountains National Park (USA) were correlated with mean annual temperature: species occurring at the warmest sites all had the highest CT max values, and species occurring at the coldest sites all had the lowest CT min values, a pattern that was consistent both among populations of the same species and among communities. Bujan et al (2020a) measured CT max and CT min for 132 ant species in 31 communities across North America (latitudinal range of 15.7 ) to assess whether these two metrics were correlated with environmental temperatures (monthly mean, maximum, and minimum).…”

“…Diamond & Chick (2018b) characterised thermal tolerance breadth for 41 species from 22 genera, showing that species with narrow thermal tolerance breadth had more restricted, more equatorial ranges, suggesting they may be at greater extinction risk under climate change. Similarly, Chick et al (2020) estimated thermal tolerance breadth for 18 ant species along an elevational gradient in Great Smoky Mountains National Park (USA). They found that thermal breadth was moderately positively correlated (50%) with elevational distribution range.…”

“…The second most common method was the water bath (29 studies, 27.1%) (Table S2). In this technique, individual ants are placed in a container such as a glass test tube that is plugged with cotton and placed in a water bath (Chick et al ., 2020) that can be heated or cooled to the desired temperature. In the hot‐plate method (11 studies; 10.3%), ants are placed in an open container with Fluon‐covered sides that prevent escape; the container is then heated to a given temperature (Cerdá, Retana & Cros, 1998).…”

Diversity, Biogeography and Community Ecology of Ants: Introduction to the Special Issue

“…The proximate mechanisms underlying species distributions are poorly understood, and this is the focus of a study of ant species occurring along an elevational gradient in Great Smoky Mountains National Park in south-eastern USA [5]. Thermal tolerance appeared to be the most important constraint on the distribution and density of ant species in colder (higher elevation) environments, whereas competition was identified as a key factor in warmer (lowland) environments.…”

“…The second paper on community ecology [8] examines the extent to which a numerically dominant ant (Formica subsericea) affects the diversity and performance of co-occurring species. As in [5], the study was conducted in the Great Smoky Mountains National Park, where F. subsericea is particularly common. There was no relationship between the abundance of F. subsericea and the total abundance of non-dominant species across the ten sites studied, and the relationship with total species richness was weak.…”