The biogeography of omnivory: Do omnivores increase prey consumption relative to plants in sodium-poor environments?

Background/Question/Methods

Detrital food webs have an abundance of omnivores. However, the mechanisms driving omnivore abundance and diet are not well understood. Nitrogen-limitation is one hypothesis for increased heterotroph consumption relative to plants. However, many arthropods obtain N from N-fixing endosymbionts. An alternative hypothesis is sodium-limitation. Sodium bioaccumulates from plants to predators. As such, heterotrophs themselves are an abundant source of Na. In Na-poor landlocked environments, consuming proportionally more prey may alleviate Na-limitation in omnivores. We first tested if inland ants, which are mainly omnivorous, are Na-limited relative to coastal ants. To assess this assumption, we systematically collected ants at ten paired lowland forests from Georgia to Maine by laying transects of randomly placed Na, sugar, and water baits. Second, we tested whether ants increase their prey consumption relative to plants to compensate for low Na availability. We used stable isotope analysis to assess the δ¹⁵N of conspecific and congeneric ants between coastal and inland forests. Third, we tested if Na-hunger and predation is plastic in a laboratory experiment. We collected Solenopsis invicta colonies and fed them Low, High, or Control Na diets. After five weeks, we gave colonies a choice between crickets or sugar and measured recruitment times and strengths.

Results/Conclusions

Inland ants were Na-limited relative to coastal pairs and visited Na baits 2-fold more. Despite increased Na use in inland forests, ant activity (sugar bait usage) was constant between paired forests but decreased with increasing latitude. Preliminary data from stable isotope analysis suggests congeneric inland ants are more predacious than coastal ants and food chain length is longer in inland forests. Further, laboratory Solenopsis invicta fed Low Na diets discovered crickets 3.5-fold faster than Controls. In contrast, sugar discovery was constant among Low, High and Control diet colonies. Our results suggest Na-hunger is inducible and limited access to Na drives increased predation in common omnivores. This in turn has ramifications on the rest of the food web and ultimately ecosystem function. Understanding the forces underlying food web structure including omnivory is increasingly important as anthropogenic activities and climate change alter macro- and microscale biotic and abiotic variables.