Data from: Toward a geography of omnivory: omnivores increase carnivory when sodium is limiting

1. Toward 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. 2. 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. 3. We
tested each hypothesis using N stable isotopes (δ 15 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 15 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 NPP. 4. Sodium gradients accounted
for 18% of the variation in δ 15 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 15 N. 5. Our
results reveal a geography of omnivory driven in part by access to Na.