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

Abstract

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.

The nutritional geography of ants: Gradients of sodium and sugar limitation across North American grasslands

“…There is growing evidence for the pervasive role of Na as the ‘fourth macronutrient’ (Clay, Donoso, & Kaspari, ; Kaspari, Yanoviak, Dudley, Yuan, & Clay, ; NRC, ; Prather et al, ; Welti, Sanders, Beurs, & Kaspari, ) Our results support these findings, showing that sodium cravings by grassland ant communities declined as sodium availability in plants increased. If true, we would expect sodium additions to enhance consumer abundance in grasslands, and for this effect to strengthen as levels of plant sodium decrease.…”

“…There is growing evidence for the pervasive role of Na as the 'fourth macronutrient' (Clay, Donoso, & Kaspari, 2014;Kaspari, Yanoviak, Dudley, Yuan, & Clay, 2009;NRC, 2005;Prather et al, 2018b;Welti, Sanders, Beurs, & Kaspari, 2019) Our results support these findings,…”

On the geography of activity: productivity but not temperature constrains discovery rates by ectotherm consumers

“…Indeed, activity of an ant community at baits showed the predicted acceleration with environmental temperature at daily and seasonal time scales (Prather et al. ). We predicted a similar relationship between ectotherm activity—summed over days—and ecosystem temperature.…”

“…Global change models typically assume organisms start activity above a thermal minimum and cease activity above a thermal maximum (Buckley and Kingsolver 2012) although it should increase or even accelerate with temperature (as it does, for example, with frog vocalizations; Gerhardt 1978, Gunderson andLeal 2015). Indeed, activity of an ant community at baits showed the predicted acceleration with environmental temperature at daily and seasonal time scales (Prather et al 2018). We predicted a similar relationship between ectotherm activity-summed over days-and ecosystem temperature.…”

“…They noted that the strongest trend was among arthropods, notably ants, whose activity greatly increased toward the tropics. Here, like Prather et al (2018), we harness the explanatory power of continuous variation in NPP and the maximum summer monthly temperature (when most of the studies were conducted) to test the hypothesis that NPP and temperature co-constrain the geography of activity, and in doing so, provides a possible mechanism for their latitudinal gradient in activity.…”