Cecropia Ants

One of the most interesting symbioses in the rainforest exists between the Cecropia tree and the Cecropia ant (Azteca sp.) The Cecropia trees recruit ants to live in them and protect the tree. The ants drive off herbivorous insects, attack herbivorous vertebrates, and remove epiphytes and competing plants. Their wastes also provide a lot of nitrogen to the plant. The plant provides them with housing (inside hollow stems) and gylcogen-rich M�llerian bodies. At least, that's the simple explanation of the relationship. As with many symbioses, a deeper look reveals additional complexities. Let's start by looking at the trees.

The Cecropia Tree

Cecropia trees are common in the rainforest - and in the tropical dry forest as well. The trees are often seen along the roadsides as well. They are related to the mulberry tree. They often invade disturbed areas (hence their presence on roadsides) and are considered a pioneer species. They don't do well in the shade; in the rainforest they depend on the appearance of openings due to treefalls, landslides, fires, etc. They are very quick-growing and shed the lower limbs (this shedding epiphytes as well). In addition to the ants, the Cecropia trees have a number of chemical defenses including latex ducts (which gum up the mouthparts of feeding insects) and tannins. Despite the defenses, the Cecropia trees fall prey to a number of herbivores, most noticeably sloths (below) as well as a number of insect herbivores. The Cecropia leaves below have been attacked by insect herbivores, which demonstrates that the defenses of the plants - chemical and ants combined - are not able to drive off all the potential herbivores. The damage below was probably done by Orthoptera (katydids, grasshoppers, etc.) or Lepidoptera (caterpillars).

Some studies suggest that the principle contribution of the Azteca ants to the defense of the tree is to reduce herbivory by Leafcutter Ants (Atta sp.). Since the leafcutters are responsible for considerable herbivory in their range, if the Azteca ants only ran off the leafcutters and allowed other herbivores free reign then they would still benefit the Cecropia trees in areas with leafcutter ant nests.

The Ants

Left: The ants live in the hollow stems of the tree. As the plant lays down new stems these hollow areas are likely to be colonized by the ants. The founding queen needs only to chew through a thin membrane in the area where the opening is made; this region of the plant also lacks the latex ducts which would otherwise inhibit the queen from chewing through.

In addition to a nest site, the plants induce the ants to stay by the production of M�llerian bodies. These bodies are produced below the attachment point of the leaves, in an area covered with a dense growth of trichomes (hairs). This is the brownish-orange patch in the photo to the right. The M�llerian bodies (white, football-shaped structures) grow from the trichomes and are harvested by the ants. Apparently, they do not contain any glycogen until very late in their development; this probably prevents the ants from taking them before they are mature (ants that did would die of starvation and not pass on the genes that encourage early harvest of M�llerian bodies).

Left: An ant approachs the entrance to the nest, carrying a M�llerian body.

What does the tree get from the relationship? Well, in addition to defense (no matter how effective) it appears that another significant advantage to the tree is the supply of nitrogen that the tree is able to obtain from the ants. Sources differ as to the origin of the nitrogen; some say it comes from the bodies of decaying ants, others from their wastes. A few sources debate whether or not this should properly be called a mutualism at all. These arguments fall into two camps: 1. stating that the tree benefits much more than the ant (especially given the fact that some studies show the plant receiving more benefit from the nitrogen than the ants get from the glycogen of the M�llerian bodies, which may constitute only about 1/5th of their caloric intake. 2. Wondering if the tree gets any benefit from the presence of the ants. These arguments miss the point - both species do benefit from the relationship. In the case of the second argument it might simply be that the studies about the nitrogen transfer from the ants to the trees had not been completed when that argument was developed. As for the first, it seems patently false. Regardless of how much nutrition is supplied by the tree to the ants, it's clear that the ants could not survive without the nesting sites; some are apparently obligate residents of the trees.

Undoubtedly, Cecropia trees could survive without the ants (one study which set out to demonstrate the positive effect the ants could have on the trees was inconclusive because the trees the researchers chose in their experimental site (a region where the ants were known to occur) in fact lacked colonies of ants! Still, in order to benefit all the ants have to do is increase the chances for survival and reproduction of the trees, and it would seem that their contributions in terms of nitrogen and defense (against leafcutter ants, at least) would do just that.

Video of Spider Monkey eating Cecropia Ants at Tortuguero National Park.

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