Microhabitat and body size effects on heat tolerance: implications for responses to climate change (army ants: Formicidae, Ecitoninae)

Abstract

Summary1. Models that predict organismal and population responses to climate change may be improved by considering ecological factors that affect species thermal tolerance. Species differences in microhabitat use can expose animals to diverse thermal selective environments at a given site and may cause sympatric species to evolve different thermal tolerances. 2. We tested the hypothesis that species differences in body size and microhabitat use (abovevs. below-ground activity) would correspond to differences in thermal tolerance (maximum critical temperatures: CT max ). Thermal buffering effects of soil can reduce exposure to extreme high temperatures for below-ground active species. We predicted larger-bodied individuals and species would have higher CT max and that species mean CT max would covary positively with degree of above-ground activity. We used Neotropical army ants (Formicidae: Ecitoninae) as models. Army ants vary in microhabitat use from largely subterranean to largely above-ground active species and are highly size polymorphic. 3. We collected data on above-and below-ground temperatures in habitats used by army ants to test for microhabitat temperature differences, and we conducted CT max assays for army ant species with varying degrees of surface activity and with different body sizes within and between species. We then tested whether microhabitat use was associated with species differences in CT max and whether microhabitat was a better predictor of CT max than body size for species that overlapped in size. 4. Microhabitat use was a highly significant predictor of species' upper thermal tolerance limits, both for raw data and after accounting for the effects of phylogeny. Below-ground species were more thermally sensitive, with lower maximum critical temperatures (CT max ). The smallest workers within each species were the least heat tolerant, but the magnitude of CT max change with body size was greater in below-ground species. Species-typical microhabitat was a stronger predictor of CT max than body size for species that overlapped in size. Compared to the soil surface, 10-cm subsoil was a significantly moderated thermal environment for below-ground army ants, while maximum surface raid temperatures sometimes exceeded CT max for the most thermally sensitive army ant castes. 5. We conclude sympatric species differences in thermal physiology correspond to microhabitat use. These patterns should be accounted for in models of species and community responses to thermal variation and climate change.

Interactions between rates of temperature change and acclimation affect latitudinal patterns of warming tolerance

“…Given the growing exploration of CTmax and warming tolerance in the context of both fundamental physiological ecology and the conservation implications of climate change, and a broadening in the range of experimental ramping rates used (for discussions and/or examples, see Terblanche et al . 2011; Sunday et al ., 2012, 2014; Baudier et al ., 2015; Kaspari et al ., 2015; García-Robledo et al ., 2016), these results have important implications for future work. We explore these implications in more detail here.…”

“…Such consideration is important given that a wide range of studies are now routinely investigating critical thermal limits (e.g. Baudier et al ., 2015; Kaspari et al ., 2015; Verble-Pearson et al ., 2015; García-Robledo et al ., 2016). A contrary view is that both temperature and time should be investigated fully owing to the significance of a third parameter, the sensitivity to temperature change (Rezende et al ., 2014).…”

Effects of increasing aridity and chronic anthropogenic disturbance on seed dispersal by ants in Brazilian Caatinga

“…Large ant species are especially sensitive to disturbance (Gibb et al., ; Leal, Andersen, & Leal, ), and this can result in severe reductions in the quality of seed dispersal services in disturbed habitats (Almeida et al., ; Gove, Majer, & Dunn, ; Leal, Andersen, & Leal, ; Ness, Bronstein, Andersen, & Holland, ). Although smaller insects are considered more sensitive to high temperatures through desiccation (Baudier, Mudd, Erickson, & O'Donnell, ; Kühsel, Brückner, Schmelzle, Heethoff, & Blüthgen, ), recent studies have found that the most sensitive ants to climate change are in fact the largest (Andrew, Miller, Hall, Hemmings, & Oliver, ; Gibb et al., ). This might be because large ants require more resources and take longer to mature, thus reducing their adaptive capacity (Gibb et al., ; McCain & King, ; Savage, Gillooly, Brown, West, & Charnov, ).…”

Critical thermal maxima and body size positively correlate in red imported fire ants,Solenopsis invicta

“…In particular, small changes in temperature may represent a big challenge for ectotherms with low heat tolerance or those that already survive close to their upper thermal tolerance limit (Baudier et al, 2015). Among terrestrial invertebrates, ants represent an ideal group to investigate the link between thermal ecology and climate warming (Del Toro et al, 2015), as they are relatively easy to collect, house, and identify.…”

“…In particular, an investigation of the association between thermal tolerance, body size, and microhabitat use in fire ant colonies is needed. Recent studies (Baudier et al, 2015) highlight the relationship between CTmax and microhabitat use; above-ground ant species showed a higher heat tolerance than the below-ground species. The utilization of similar microhabitats (and thus, the experience of similar microclimates) by medium and large fire ant workers may explain the lack of difference in CTmax between the two groups.…”