Bacteria let ants become herbivores

Cephalotes grandinosus, an herbivorous ant

Why are there so many ants?

This is a more perplexing question than it may seem.  At first glance ants are predators and scavengers.  Yet predators should be few in number, balanced on a narrow trophic peak and depending on high prey biomass to exist.  Why are terrestrial ecosystems dominated by these little hunters?

A landmark study several years ago by Dinah Davidson provided an answer:  many ants are not predators at all.  They're herbivores.  Sure, they snack now and again on flesh.  But ants get most of their energy from plants, either through nectaries or channeled through hemipteran honeydew.  They are lower on the trophic pyramid than we thought, fueling their immense populations on a broader energy base than that available to strict carnivores.

The ability of ants to gain nitrogen and other nutrients from an herbivorous diet has long been linked to gut microbes.  This week PNAS provides the broadest study yet on the bacterial-ant association.  A paper by Jacob Russell et al, aptly titled "Bacterial gut symbionts are tightly linked with the evolution of herbivory in ants", examines the phylogenetic distribution of gut bacteria across the ants.

Herbivorous ants are more likely to have Rhizobiales bacteria (from Figure 2 of Russel et al 2009).

Russell and colleagues screened hundreds of ant species with general bacterial primers.  They turned up an astounding diversity of microbes, including some that showed a consistent relationship to the trophic habits of the ants.  In particular, bacteria from the order Rhizobiales- these are the same little microbes associated with nitrogen fixing in plants- were found in many of the herbivorous lineages.

Distribution of herbivory and Rhizobiales across a simplified ant phylogeny (from Figure 3).

This pattern is strengthened by an even more astute observation.  Not only do herbivorous ants tend to have Rhizobiales, it's that completely unrelated herbivorous ants have similar Rhizobiales.  The bacteria are not mere evolutionary hitchhikers passively tracking the geneaology.  They show up in a highly non-random fashion in ants that have need of nitrogen.

Of course, the finer details of the ant-microbe interaction remain unknown.  What the bacteria are actually doing in ant guts, how they are passed about among the ants, and the chemistry of the nitrogen cycling all need to be worked out.  But we have every indication that whatever the specifics, these microbes are important.

The Russell et al study is timely, arriving on the heels of a Science paper by Pinto-Tomás et al showing that leafcutter ants get their nitrogen via symbiotic microbes in the fungus garden.  Both papers point to a vast unstudied realm of microorganisms lurking behind the scenes of social insect ecology.  It's long been known that the secret to the success of the ants was their cooperation with each other.  We are only now realizing that their cooperation with other organisms may be just as vital.

Jacob A. Russell, Corrie S. Moreau, Benjamin Goldman-Huertas, Mikiko Fujiwara, David J. Lohman, and Naomi E. Pierce. 2009. Bacterial gut symbionts are tightly linked with the evolution of herbivory in ants. PNAS 2009 : 0907926106v1-pnas.0907926106.