Altitudinal variation in the critical thermal limits of tropical pollinators

Pollination is an ecosystem service essential for plant reproduction and food security. Although the effects of climate change on pollinators and pollination services are expected to be greater in the tropics than at other latitudes, the thermal biology of tropical pollinators remains largely unknown. We assessed the effect of altitude on the critical thermal limits of bees and flies across an elevation gradient (200, 1500, 2700, and 3500 m) on the eastern Andes of Central Colombia (Department of Cundinamarca). To measure critical thermal minimum (CT_Min) and critical thermal maximum (CT_Max), we used a portable heating and cooling system with an anodized aluminum sample plate (Elara 3.0^®) and employed a dynamic (ramping temperatures) protocol. We used an initial temperature of 22°C and increased it or decreased it at a rate of 0.5°C_min−1. In general, bees were more heat tolerant and less tolerant to low temperatures than flies. While CT_Min decreased with elevation, CT_Max was similar across the altitudinal gradients. The increase we observed in CT_Min with elevation in these tropical pollinators supports Brett’s heat-invariant hypothesis, which has been documented across a wide range of vertebrates and invertebrates. The differences in the thermal limits between bees and flies could also explain the dominance of the latter group at high elevations.