Using metabolic and thermal ecology to predict temperature dependent ecosystem activity: a test with prairie ants

As ecosystems warm, ectotherm consumer activity should also change. Here we use principles from metabolic and thermal ecology to explore how seasonal and diel temperature change shapes a prairie ant community's foraging rate and its demand for two fundamental resources: salt and sugar. From April through October 2016 we ran transects of vials filled with solutions of 0.5% NaCl and 1% sucrose. We first confirm a basic prediction rarely tested: the discovery rate of both food resources accelerated with soil temperature, but this increase was typically capped at midday due to extreme surface temperatures. We then tested the novel prediction that sodium demand accelerates with temperature, premised on a key thermal difference between sugar and sodium: sugar is stored in cells, while salt is pumped out of cells proportional to metabolic rate, and hence temperature. We found strong support for the resulting prediction that recruitment to NaCl baits accelerates with temperature more steeply than recruitment to 1% sucrose baits. A follow up experiment in 2017 verified that temperature-dependent recruitment to sucrose concentrations of 20% (mimicking rich extrafloral nectaries), while noisy, was still only half as temperature dependent as recruitment recorded for 0.5% NaCl. These results demonstrate how ecosystem warming accelerates then curtails the work done by a community of ectotherms, and how the demand and use of fundamental nutrients can be differentially temperature dependent.