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

Of the 25 elements required to build most organisms, sodium has a unique
set of characteristics that ramify through terrestrial ecology. In plants,
sodium is found in low concentrations and has little metabolic function;
in plant consumers, particularly animals, sodium is essential to running
costly Na-K ATPases. Here I synthesize a diverse literature from
physiology, agronomy, and ecology, toward identifying sodium’s place as
the “7th macronutrient”, one whose shortfall targets two trophic
levels—herbivores and detritivores. I propose that sodium also plays a
central, though unheralded role in herbivore digestion, via its importance
to maintaining microbiomes and denaturing tannins. I highlight how sodium
availability is a key determinant of consumer abundance and the geography
of herbivory and detritivory. And I propose a re-appraisal of the
assumption that, because sodium is metabolically unimportant to most
plants, it is of little use. Instead, I suggest that sodium’s critical
role in limiting herbivore performance makes it a commodity used by plants
to manipulate their herbivores and mutualists, and by consumers like bison
and elephants to generate grazing lawns: dependable sources of sodium.