Physiological limits to thermal extremes are often thought to determine the abundance and geographical distribution of species, but more recent evidence suggests that species interactions might be equally important. Moreover, the relative importance of these constraints might shift with changing abiotic conditions, such as climate change. Here, we explore the relative importance of physiological tolerances to heat and species interactions in determining the distribution of insects along two elevational gradients. The gradients contrast in precipitation but not temperature, allowing us to separate these two climatic factors.
Montane rainforest in Costa Rica.
Major taxa studied
Bromeliad‐dwelling aquatic insect larvae.
We estimated the elevational preferences of five insect taxa by surveying 170 bromeliads along the moist Atlantic and the dry Pacific slopes of Monteverde and determined their critical thermal maxima (CTmax) experimentally. We determined whether taxon‐specific heat tolerances predicted their elevational preferences, using Deming regressions, and tested whether potential predators mediated effects on the elevation of insect distributions, using structural equation models.
On the moist Atlantic slope, heat tolerances of insects explained their elevational distributions: taxa with high heat tolerances preferred low elevations where conditions were warmest, whereas taxa with low heat tolerances preferred high elevations where conditions were coldest. In contrast, on the drier Pacific slope, the elevational abundance pattern of many insects reflected negative interactions from crane fly larvae. These larvae are known to become predatory in drought conditions and were disproportionally abundant at low elevations on the Pacific slope.
We show that under drought, indirect effects mediated by species interactions can override any direct physiological effects of environmental conditions on insect distributions. The relative importance of limits to physiological tolerance and species interactions thus depends on the environmental context, an important insight given that environmental conditions are expected to shift with climate change.