Adventures Among Ants - Mark W. Moffett [83]
That morning at sunrise I had been on the ground with Terry Erwin, a Smithsonian beetle expert who inventories the species diversity of canopy insects. To get close to our targets I had sent a fishing line into the tree with a slingshot and used it to pull a climbing rope over a branch; I then got into my climbing harness, clipped two ascenders to the harness and the rope, and wriggled skyward.
But as I rose, my support rope shifted; I abruptly fell several inches and began to spin in space. Plant bits shaken loose from the branch above whirled into my eyes and blinded me. My hands were full of cameras and entomology gear. To stabilize myself, I threw my legs around a branch high to one side.
Big mistake! Swinging through the air, my foot smashed a mass of canopy-rooted plants, or epiphytes, that concealed a well-defended ants’ nest. In an instant, workers covered my legs and then dropped like dive bombers onto the rest of my body. As they gashed my skin with their mandibles and sprayed formic acid into my wounds, I recognized not only the species, Camponotus femoratus, but also the fact that I’d found my first “ant garden”—albeit the hard way.
Regaining my balance while slapping at this vicious species of carpenter ant, I noticed the presence of a second ant on my skin—the smaller Crematogaster levior, a shy species of acrobat ant that does not bite. The ant garden is a result of their collaboration and represents an infrequent instance of harmony between ant species. Nestled in this mass of epiphytes, a confederation of these two ants had constructed a quarter-meter-wide treetop house of carton, papery sheets they produced by masticating plant matter and soil. The workers then collected seeds and embedded them in the carton. There the seeds grew into cacti, bromeliads, figs, orchids, philodendrons, and anthuriums, creating a bounteous garden.
The plants and ants depend on one another. The plant roots strengthen the carton, keeping it from disintegrating in rain and giving the ants a stable home.2 The ants, in turn, seem to be necessary for the plants’ survival, since these particular species of flora never occur on their own.3 (Though we can’t say yet if the seeds die if the ants don’t find them, or if the ants are so thorough at snapping them up that these plants have no opportunity to germinate elsewhere.) In any case, the ants were clearly protecting both nest and garden with zeal.
In this striking example of mutualism, the Camponotus and Crematogaster jointly created the nest and protected the epiphytes. They shared trails, helped each other find prey (though Camponotus can be a bit of a thief), and tended the same sap-sucking insect “cattle” as an additional source of fuel. The acrobat ants then drank the honeydew excreted by the smaller Homoptera, or aphid relatives, and reared them to a size suitable for milking by the carpenter ants, which played the more important role in finding and planting the seeds that developed into fresh garden plants.4
Nauseated by an overdose of ant toxins in my bloodstream, I pushed myself away from the ant garden to another tree trunk. The garden was truly elegant, I could see, though for the moment the ants on it seethed. As I recovered my balance, I pondered what it was about life in the canopy that fostered both mutualism and belligerence.
BIOLOGICAL SUCCESS
Success in nature is often described in terms of the number of species in a group. By this measure, the ant-garden ants I had stumbled upon belong to two of the three most successful genera of ants (the third is Pheidole, or big-headed ant), with hundreds of species each. But success isn’t always associated with a proliferation of species; the number