Abstract
Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) – the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CT max), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CT max were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation‐resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.
Highlights
Small ectotherms are often at risk of desiccation given their high surface area to volume ratio, proportionately low fat storage, and relatively high metabolic rate (Gibbs 2003; Harrison et al 2012)
Ecology and Evolution published by John Wiley & Sons Ltd
We show that desiccation can be a major challenge for small ectotherms even in a moist tropical forest
Summary
Small ectotherms are often at risk of desiccation given their high surface area to volume ratio, proportionately low fat storage, and relatively high metabolic rate (Gibbs 2003; Harrison et al 2012). Desiccation resistance – the ability for an organism to reduce water loss – is a useful trait in small ectotherms, especially in light of predicted increases in the frequency and severity of droughts (IPCC 2014). Tropical forests contain a large fraction of Earth’s species, but, perhaps due to their high relative humidity, little attention has been given to the patterns of desiccation resistance in tropical arthropods (Stanley and Parsons 1981; Karan et al 1998; Hoffmann et al 2003; Lapinski and Tschapka 2014).
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