Darker ants dominate the canopy: Testing macroecological hypotheses for patterns in colour along a microclimatic gradient

Abstract

Gradients in cuticle lightness of ectotherms have been demonstrated across latitudes and elevations. Three key hypotheses have been used to explain these macroecological patterns: the thermal melanism hypothesis (TMH), the melanism‐desiccation hypothesis (MDH) and the photo‐protection hypothesis (PPH). Yet the broad abiotic measures, such as temperature, humidity and UV‐B radiation, typically used to detect these ecogeographical patterns, are a poor indication of the microenvironment experienced by small, cursorial ectotherms like ants. We tested whether these macroecological hypotheses explaining cuticle lightness held at habitat and microclimatic level by using a vertical gradient within a tropical rainforest. We sampled 222 ant species in lowland, tropical rainforest across four vertical strata: subterranean, ground, understory and canopy. We recorded cuticle lightness, abundance and estimated body size for each species and calculated an assemblage‐weighted mean for cuticle lightness and body size for each vertical stratum. Abiotic variables (air temperature, vapour pressure deficit and UV‐B radiation) were recorded for each vertical stratum. We found that cuticle lightness of ant assemblages was vertically stratified: ant assemblages in the canopy and understory were twice as dark as assemblages in ground and subterranean strata. Cuticle lightness was not correlated with body size, and there was no support for the TMH. Rather, we attribute this cline in cuticle lightness to a combination of the MDH and the PPH. Our findings indicate that broad macroecological patterns can be detected at much smaller spatial scales and that microclimatic gradients can shape trait variation, specifically the cuticle lightness of ants. These results suggest that any changes to microclimate that occur due to land‐use change or climate warming could drive selection of ants based on cuticle colour, altering assemblage structure and potentially ecosystem functioning.

Spatiotemporal dynamics of the ant community in a dry forest differ by vertical strata but not by successional stage

“…Different effects of seasonality on the microhabitats of the canopy and the litter may be linked to the higher temporal stability of species composition of ants, as we found in this study. In tropical forests, there is a vertical gradient of microclimatic conditions from the ground level to the canopy, with an increase of temperatures and solar radiation, and a decrease in air humidity (Kaspari et al., 2015; Law et al., 2019). Moreover, there are not equally regular changes in microclimatic conditions between the litter and canopy strata across seasons.…”

Evidence of phenotypic plasticity along an altitudinal gradient in the dung beetle Onthophagus proteus

“…This is especially pertinent for insects who live in mountainous tropical regions where the difference between atmospheric temperature and objects exposed to direct sunshine can vary greatly ( Mani, 1968 ). The photo-protection hypothesis (PPH) ( Law et al, 2020 ) predicts an increase in cuticle darkness in insects due to the protection increased melanization provides against UV-B radiation. Increased melanization correlates with greater UV-B radiation exposure in butterflies and Drosphilia ( Bastide et al, 2014 ; Katoh, Tatsuta & Tsuji, 2018 ) and higher levels of melanin are able to protect against harmful ultraviolet radiation ( Delmore, Brennan & Orr, 2018 ).…”

Riparian buffers act as microclimatic refugia in oil palm landscapes

“…The decreasing costs of microclimatic dataloggers have also catalysed an increase in research investigating fine‐scale microclimatic conditions (e.g. Hardwick et al., 2015; Law et al., 2019).…”