Towards a geography of omnivory: Omnivores increase carnivory when sodium is limiting

Abstract

Towards understanding the geography of omnivory, we tested three hypotheses that predict the proportion of animal tissue consumed: the sodium limitation hypothesis predicts that omnivores increase animal consumption in Na-poor environments because Na bioaccumulates from plants to predators; thus, heterotrophs are Na-rich sources. The nitrogen limitation and habitat productivity hypotheses use the same logic to predict more animal consumption in N-poor and productive environments respectively. Omnivory is a common trophic strategy, but what determines the balance of plant and animal tissue omnivores consume is relatively unexplored. Most of what we know comes from single populations at local scales. Here we quantitatively test these three hypotheses at a large geographic scale and across 20 species of omnivorous ants. We tested each hypothesis using N stable isotopes (δ N) to quantify the degree of carnivory in ant populations in 20 forests that span 12° latitude from Georgia to Maine, USA. We used the difference in δ N between 20 ant conspecifics in 10 genera between two paired forests (10 pairs of 20 forests) that consisted of a coastal and inland forests on the same latitude to determine if the proportion of animal tissue consumed could be predicted based on Na, N or net primary productivity. Sodium gradients accounted for 18% of the variation in δ N, 45% if one outlier ant species was omitted. In contrast, the nitrogen limitation and habitat productivity hypotheses, which predict more animal consumption in N-poor and more productive environments respectively, failed to vary with δ N. Our results reveal a geography of omnivory driven in part by access to Na.

Species energy and Thermal Performance Theory predict 20‐yr changes in ant community abundance and richness

“…Temp, mean monthly temperature (°C); NAP, net aboveground productivity (g CÁm À2 Áyr À1 ); ΔAIC c , the change in the Akaike information criterion corrected for sample size. Like the mobile, generalist butterfly populations that are most likely to increase as temperatures rise (Warren et al 2001), ants as a clade are dominated by generalized feeders (Clay et al 2017) with a winged reproductive stage and a simple life cycle (Boulay et al 2010) less prone to phenological mismatch with host plant species (Parmesan 2006). selection of large protected areas; declines are frequently recorded near urban or agro-ecosystems (see review in Leather 2018).…”

“…Most species can retreat to thermally buffered nests. Like the mobile, generalist butterfly populations that are most likely to increase as temperatures rise (Warren et al 2001), ants as a clade are dominated by generalized feeders (Clay et al 2017) with a winged reproductive stage and a simple life cycle (Boulay et al 2010) less prone to phenological mismatch with host plant species (Parmesan 2006). Finally, ant colonies are also important scavengers (Griffiths et al 2018); it is possible that ant communities, at least in the short term, benefit from the stress and mortality inflicted by climate change on more susceptible insect populations.…”

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

“…Omnivoryeating plant and animal tissue-is a common way of achieving an optimal mix of nutrients (Digel, Curtsdotter, Riede, Klarner, & Brose, 2014). Just as basic community properties such as abundance and diversity mechanistically track the abiotic template of temperature, precipitation and biogeochemistry (Kaspari, Alonso, & O'Donnell, 2000;Rosenzweig, 1995;Roy, Jablonski, Valentine, & Rosenberg, 1998), there is increasing evidence that the diets of omnivores can vary predictably with the geography of abiotic drivers (Clay, Lehrter, & Kaspari, 2017;Simpson, Sword, Lorch, & Couzin, 2006).…”

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

“…Arms, Feeny, & Lederhouse, ), (b) plant Na through plant exudates and (c) insect biomass through consumption of animal tissue (e.g. Clay et al, ), and five constraints on plant sugar exudate production: (a) Net primary productivity as the ultimate constraint on sugar production (e.g. Rosenzweig, ), (b) time into growing season as plant production capacity increases as the season progresses, (c) N and P plant content as these are essential for plant productivity (e.g.…”

“…Harpole et al, ), (d) plant K as it promotes plant control of exudates (Carvalhais et al, ; Sardans & Peñuelas, ) and (e) insect biomass because increased access to N, P and Na should increase demand for sugars (e.g. Clay et al, ; Yanoviak & Kaspari, ). For both Na and sugar, their predictions accounted for over half the variation in ant bait usage.…”