Physiological traits and their relationships vary along an elevational gradient within and among Fijian bee species

Abstract

Temperature and water availability are independently hypothesised to be important abiotic drivers of the evolution of metabolic rates and gas exchange patterns, respectively. Specifically, the metabolic cold adaptation hypothesis (MCA) predicts that cold environments select for faster metabolic rates to counter the thermodynamics of biochemical reactions while the hygric hypothesis predicts that dry environments select for discontinuous gas exchange to reduce water loss. Although these two hypotheses consider different physiological traits and how they vary along different abiotic gradients, metabolic rate drives gas exchange patterns in insects meaning these two traits are inherently linked. Despite this link, the MCA and hygric hypotheses are rarely considered together and the extent to which metabolic rates and respiratory patterns vary and co-vary with temperature and aridity along climatic gradients remains unclear. We tested the MCA and hygric hypotheses within a species of endemic Fijian bee, Homalictus fijiensis, across an altitudinal gradient of 1100 m, and among four Fijian bee species, including H. fijiensis, that inhabit different altitudinal bands. In Fiji, environmental temperature is ~5 C lower in the central highlands than in the coastal lowlands with the highlands receiving ~100 mm of additional precipitation than the lowlands each month. We found an MCA-like pattern within H. fijiensis and among Fijian bee species, where metabolic rate decreased with increasing temperature, but precipitation also explained variation in metabolic rate. We also found support for the hygric hypothesis within H. fijiensis and across species. Lastly, we found that the relationship between metabolic rate and ventilation rate changed depending on precipitation of the driest month within H. fijiensis and among bee species. Bees that inhabited wet regions had low ventilation rates of greater volume but bees that inhabited drier locations had rapid ventilations of smaller volume, potentially as a mechanism to exchange gas without desiccating. As the correlation between metabolic rate and ventilation rate changes across the precipitation gradient, it is possible that these two traits can evolve independently of each other in response to abiotic variables despite being positively correlated.

Species richness patterns in Fijian bees are explained by constraints in physiological traits

“…Species collected at high elevations tended to have lower heat tolerances and were less desiccation resistant than those found at lower elevations. High elevation regions in Fiji tend to be ∼5°C cooler, and receive about 100 mm more rainfall each month, than the lowland regions (da Silva et al 2022). Thus trends in these traits reflect changes in climate across altitudinal space.…”

“…Finally, limited desiccation resistances are also likely playing a major role in restricting many species’ ranges to high elevations, which are much wetter than the lowlands. High environmental temperatures are associated with increasing vapour pressure deficits (drying power of the air) (da Silva et al 2022), so even though species might have high thermal tolerances, if they are not desiccation tolerant they will not be able to inhabit warm environments if humidity is not high.…”

“…One possible explanation for the pattern in species richness of Fijian Homalictus is that they are broadly adapted to cooler and wetter conditions, and climates have changed faster than has adaptation in their physiological traits, pushing their ranges upslope over time to where the local climates remain cooler and wetter (Ongoma et al 2021, da Silva et al 2022). Species geographic ranges are often linked to their physiological traits such as heat tolerance and desiccation resistance (Kellermann et al 2012a, b, Healy et al 2019).…”