I wish I had time to write about a pair of significant ant-plant papers that came out this week, but as it is I’m preparing for my booth at the Entomological Society of America meetings next week. You’ll want to read both of these, as they indicate the depth and complexity of ant-plant relationships.
1.In water-stressed habitats, plants invest more in their Azteca ant guards. A new study by Beth Pringle:
The strength of ecological interactions, measured as the costs or benefits sustained by each species, depends on the environmental context in which the interaction occurs. Stressful environmental conditions should favor trading between species that can produce a given resource or service at the lowest cost. Mutualisms, in which both interacting species incur a net benefit, may thus strengthen under stressful conditions. Here we examine an ant–plant mutualism, in which plants provide food and housing for ants and ants defend plants against leaf-eating insects, along a four-fold annual precipitation gradient comprising tropical sites in Mexico and Central America. We show that the strength of the mutualism, in terms of carbon investment by plants and leaf defense by ants, increases as water availability decreases. Carbon shortages are more frequent where water is scarce and increase the risk that plants will die if all of their leaves are eaten by herbivores. Trees appear to invest more in ant defenders when water is scarce to insure themselves against extreme herbivory. Water availability thus indirectly determines the outcomes of this ant–plant mutualism, which suggests that the increasing frequency of extreme climate events in the tropics will have important ecological consequences.
(Pringle EG, Akçay E, Raab TK, Dirzo R, Gordon DM (2013) Water Stress Strengthens Mutualism Among Ants, Trees, and Scale Insects. PLoS Biol 11(11): e1001705. doi:10.1371/journal.pbio.1001705)
2. Swollen thorn acacias nutritionally entrap their Pseudomyrmex partners by disabling their ability to digest anything other than Acacia nectar. A new study by Martin Heil:
Mutualisms require protection from non-reciprocating exploiters. Pseudomyrmex workers that engage in an obligate defensive mutualism with Acacia hosts feed exclusively on the sucrose-free extrafloral nectar (EFN) that is secreted by their hosts, a behaviour linking ant energy supply directly to host performance and thus favouring reciprocating behaviour. We tested the hypothesis that Acacia hosts manipulate this digestive specialisation of their ant mutualists. Invertase (sucrose hydrolytic) activity in the ant midguts was inhibited by chitinase, a dominant EFN protein. The inhibition occurred quickly in cell-free gut liquids and in native gels and thus likely results from an enzyme–enzyme interaction. Once a freshly eclosed worker ingests EFN as the first diet available, her invertase becomes inhibited and she, thus, continues feeding on host-derived EFN. Partner manipulation acts at the phenotypic level and means that one partner actively controls the phenotype of the other partner to enhance its dependency on host-derived rewards.