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

Abstract

Sugar and sodium are essential nutrients to above‐ and below‐ground consumers. Unlike other properties of ecological communities such as abundance and richness, we know relatively little about nutritional geography—the sources and supply rates of nutrients, and how and why they vary across communities and ecosystems. Towards a remedy, we present a suite of hypotheses for how sodium and sugary exudate availability should vary for a common omnivore—the ants—and test them using a survey of 53 North American grasslands. We do so by running transects of salt and sugar baits and inferring the magnitude of environmental supplies as the inverse of their use as exogenous baits. We then use estimates of potential drivers of the availability of salt and sugary exudates—plant and soil nutrients, and bioclimatic variables—to test the best predictors of sodium and salt use by ant communities. Beyond a baseline of ant activity, salt use increased as an inverse of the amount of sodium in an ecosystem's plant tissue, but not its soils. Plant sodium varied by two orders of magnitude in grasslands across 16° latitude. This suggests that plant exudates are an important source of sodium for grassland consumers. The three drivers that best predict exogenous sugar use by ants all point to factors constraining sugar production: net above‐ground productivity, how far the community is into that year's growing season (both reflecting the rates of photosynthesis) and, intriguingly, the potassium content of plant tissue, which is likely linked to exudate production via plant turgor. These data suggest that ants and other consumers across a range of grasslands and climate vary significantly in the demand and supply of sugar and salt. This nutritional geography ultimately arises from gradients of climate and biogeochemistry with implications for the geography of plant–consumer interactions.

The seventh macronutrient: how sodium shortfall ramifies through populations, food webs and ecosystems

“…In this section I develop a second hypothesis: that plants manipulate their sodium stocks to exploit the sodium cravings of their consumers and mutualists (Kaspari et al 2019). Variation in plant sodium thus reflects a variety of adaptive solutions by species within communities, and, at the ecosystem scale, adaptive solutions to shared challenges (Subbarao et al 2003).…”

Salty, mild, and low plant biomass grasslands increase top‐heaviness of invertebrate trophic pyramids

“…N, P, K, Na) are in short supply (Joern, Provin, & Behmer, 2012; Kaspari & Powers, 2016) or when toxic elements bioaccumulate (Cui, Zhang, Zhang, Liu, & Zhang, 2011; Ouédraogo, Chételat, & Amyot, 2015). The role of sodium in shaping trophic pyramids is relatively unique among the elements in that sodium is an essential element for all animals but is generally not limiting for plants, unlike many other nutrients that are often limiting, such as N, P, and K in terrestrial systems (Kaspari, Welti, & de Beurs, 2020). The amount of sodium found in plant tissue is highly variable (Borer et al., 2019; Han, Fang, Reich, Ian Woodward, & Wang, 2011), resulting in spatial variation in sodium limitation for primary consumers (Prather et al., 2018; Seastedt & Crossley, 1981; Snell‐Rood, Espeset, Boser, White, & Smykalski, 2014; Welti, Sanders, Beurs, & Kaspari, 2019), with expected consequences for higher trophic levels that have yet to be explored.…”

The geography of grassland plant chemistry and productivity accounts for ant sodium and sugar usage

“…Sodium (Na) is emerging as a major focal nutrient (Clay, Lehrter, & Kaspari, ; Kaspari, Yanoviak, & Dudley, ; Kaspari, Yanoviak, Dudley, Yuan, & Clay, ) and although many studies have considered the ecological importance of sugar (Fischer & Shingleton, ; Raubenheimer et al, ; Wilder et al, ), spatial gradients of sugar quantity and quality and their impacts on community‐level processes has remained largely unexplored. Kaspari et al () supports the hypothesis that gradients in the sweetness and saltiness of plants underlie variation in omnivore nutrient limitation. Future studies could test whether the geography of plant sugar and salt also predict variation in ant biomass and community structure across grasslands.…”

“…Borer et al, ; Kaspari & Powers, ; Wilder, Holway, Suarez, LeBrun, & Eubanks, ). Kaspari, Welti, and de Beurs () further expands the breadth of this field using a geographical perspective to test a suite of hypotheses for potential sources of sodium (Na) and sugar and how their availability impacts the activity of temperate grassland ant communities across North America. Ants are ecologically important and widespread omnivores in temperate grasslands that impact ecosystem functions like decomposition, nutrient cycling, productivity and biodiversity (Wills & Landis, ).…”