Growing hand in hand in the tropical rainforest

Andres Camacho is a biologist from the University of Costa Rica studying bird song and genetic divergence in the Costa Rican highlands. Some of his favorite things are mountains, birds, ants, music and studying evolution.
By: Andres Camacho

Who said being a tree in a tropical rainforest would be easy? Growing up in this highly diverse ecosystem can bring severe hardships for a tree. Competition for light in the first years of life is fierce. Some trees never get enough sunlight to grow, unless a tree falls. But it doesn’t end there for the trees: besides the hard competition for sunlight and other threats like plants growing on them or logging, rainforests are full of thousands of different kinds of animals that feed on plants–most of them insects. Most plants produce their own chemical or physical defenses against these insects. But some species survive in this harsh environment with a different strategy: finding a friend.

This is the case of Ocotea atirrensis and Ocotea dendrodaphne, two mid-sized trees with relatively thin stems that grow to around three meters high. These trees have found a way to avoid being eaten by bugs, by providing home for the small ant Myrmelachista flavocotea. Local people and ant experts have known these ants live inside the Ocotea trees’ stems for many years, but it wasn´t until Air Force Academy biologist Kellie Kuhn started studying them in late 2007 at La Selva Biological Station in Costa Rica that she confirmed that the ants’ presence actually helped the plant. “They grow up together,” Kellie tells me as we walk a kilometer and a half into the primary forest, towards one of her study plots. The trees, it turns out, cannot live without the ants. For better or worse, they completely depend on the ants to survive. It’s a complex and endearing relationship. Still, if one thing is certain about friendships, it’s that not all friendships last forever.

Kellie tells me some of the interesting facts she has learned from this interaction and her eyes grow wide with excitement. A Myrmelachista flavocotea queen colonizes an Ocotea tree once the plant reaches a certain size and produces a small knot-like protuberance on its stem, called a nodule. Once inside, the queen ant starts producing workers and its colony starts to grow. The ants live inside the stem and defend their home against bugs that visit to eat the leaves.

Kellie shows me the little holes the ants use to go in or out of the plant, and, when she gently taps the stem, a few tiny yellowish ants come out to see what’s going on. The response would normally have been more intense, Kellie says, with more ants emerging to spray formic acid–the chemical substance ants produce to defend themselves–on the intruder. But it’s been pouring for most of the day, and under these conditions it’s the ants that take advantage of the relationship by seeking shelter inside the tree. Ants not only bite and spray formic acid at caterpillars and other herbivores but also groom the tree´s leaves, taking off litter like lichens, moss and caterpillar eggs.

Kellie’s earlier work proved that both participants get benefits from this relationship, which scientists call mutualism: a win-win situation for both participants. But Kellie explains that although a tree without ants doesn’t live long, the ants are most beneficial at particular stages of the tree’s development. “If you were to come and look only at old trees,” she says, “you would think it’s parasitic and not helpful.” The old tree next to us looks battered, with many damaged and eaten leaves. Then she recalls the medium-sized tree we just saw, healthy looking with many green uneaten leaves. “Beautiful!” she sighs.

Turns out that the time when the trees are producing the most fruit coincides with the time the ant colony is the biggest and hardest working. So it’s during this period that it’s most beneficial to host ants. At early stages, Kellie believes, ants are also beneficial for the plant by bringing in nutrients in the form of small bugs they prey on in the forest floor.

Kellie hopes to understand how a single species impacts a mini-community of organisms on a tree, and scale this understanding up to a larger community like a whole forest. “Hopefully this could help understand, with enough data, how we get hyper diverse systems,” she says. We already know these ants have an effect on the presence of different bug species in these trees. But it doesn’t end there. Kellie points out a small Ocotea and explains how these trees are usually found in groups of the same species. That is, until one gets colonized and the ants kill the other small plants around their tree, spraying their formic acid directly into the plant. The ants probably do this to open more space for their tree to get sunlight. So, just the behavior of the ant is already impacting the plants that grow around an Ocotea.

Still, friendship doesn’t solve everything. In harsh and diverse environments like a tropical rainforest, there are threats that can’t be avoided, even through an alliance with ants. One of these is the random event of a big tree falling over the Ocotea. Some of the time this will only harm the plant without killing it, unless the ant colony gets killed by the tree fall. Then, after three years, the tree without ants will die too. But there’s another source of mortality that almost always kills the tree. Kellie tells me about a beetle that cuts clean through the tree below the part of the stem where the ant queen lives, and then lays its eggs on the fallen tree. Since the queen chamber has been cut, the tree loses its ants and dies. “The first time I saw a fallen tree I got really mad, because I thought someone was cutting off my trees,” she says. “Until after a few days I saw the beetle doing it. Then it turned into something very interesting.”

Kellie suspects than when a tree gets to about 30 years old, the ants will be old too and the queen will probably start producing more males to go out and reproduce instead of worker ants to defend the tree. This is a sign that the colony is dying of old age–and if it does, the tree will die too. That is, unless the tree produces a sprout on its base. Then, even though the adult tree dies, its sprout could be colonized by a young queen, probably a daughter of the dying one. Now both tree and ants have a second chance and can start all over again. Their life cycles are intimately tied together. I recall Kellie’s words as we walk through the forest: “They grow up together.” But it seems they die together too. And sometimes, begin again.

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