Gardens Underground

fungus on a tree

Walking in the forest of BCI, it's easy to notice that the forest above is very tall and complicated. It's not so easy to believe that another world exists below ground level. In some areas of BCI, the ground is rather bare, and the red clay soil is apparent. Some areas have a thin covering of fallen leaves, like the leaves in the decomposer picture gallery. The leaves break down into smaller pieces and disappear into the soil. But some areas about 18 feet by 18 feet (6 meters by 6 meters) in size are so clear of debris they seem unnatural, like somebody swept the area with a broom. Even small plants growing in this area have been cleaned of their leaves.

Here I can see ants moving in and out of holes in the ground, and most of them are carrying objects. I made some notes while standing next to a hole where the ants disappear underground: 'A long line of ants stretches off through the forest, and the ants are carrying small circle-shaped pieces of green leaves, each about a half- inch (a little more than a centimeter) in diameter. Some of the ants also have another smaller ant riding on top of their leaf. The leaf pieces are cut by ants who use their sharp jaws as clippers. The ants work on the leaves high up in a tall tree. Even with binoculars I can hardly see where the ants are cutting. Each ant cuts a rough circle from a leaf, and carries the piece down to the ground 100 feet (33 meters) below. Once on the ground, the leaves are transported a distance of 75 feet (25 meters) to the place where the ants go underground. The path they follow is about 8 inches (20 centimeters) wide, and is very smooth, as if a human had cleared a pathway for the ants. Actually, there are two major lanes on this trail, one with empty-handed ants moving away from the hole in the ground, and the other with ants carrying leaf pieces toward the hole. Some large ants are busy clearing a fallen brown leaf that is lying across the trail. They cut the leaf into pieces and carry them to one side of the trail. Another ant trail comes from a greater distance in the forest and I cannot see where it leads. It is traveled by thousands more ants carrying tiny purple flowers. The parade of thousands of tiny flowers looks like people carrying parasols. The flower trail leads into the trail of the ants with the green leaf circles.

This species of ants is one of many fascinating ant species in the BCI forest. Other ant species on BCI include the giant "bullet ants" with very poisonous stings, the tiny ants that love sugar and invade the kitchens, the thorn-dwelling acacia ants, the ants that build big homes out of what looks like concrete attached to tree trunks, the predatory army ants, and many others. The ants I am observing are called 'leafcutters', or sometimes 'parasol ants.' Another name people use tells us what these ants do for a living: 'fungus-gardening ants.' These ants are special because they take pieces of plants underground and use them to grow an edible, nutritious fungus, which the ants and their larvae (babies) can eat. This kind of food production through decomposition is used by many species of ants, including one species of leafcutter that lives in the Pine Barrens of South New Jersey! Maybe some of our readers in South Jersey can make some observations for us when the weather is a little warmer.

This is how the ants on BCI do their gardening. The ants carefully choose leaves from plants that the fungus can feed on. Since many trees on BCI make chemicals to protect them from fungus or from insects, not every leaf in the forest is edible. So the ants have to make a choice. The ants bring their leaf pieces home to underground chambers where a garden is being set up. The ants create a small compost heap for the fungus, just as your class could make a compost heap for your schoolyard. The fungus decomposes the cut up leaves and takes its nutrients from the plant material. The ants help this along by chewing the leaf pieces until they are small, softened bits, easier for the fungus to digest. The ants even clean the leaves to remove any bits of other kinds of fungus that might grow on the leaves. They try to keep their fungus garden clean and pure. The fungus doesn't eat the plant pieces the same way we eat food. It doesn't have a mouth and cannot swallow. It has to produce a chemical that it releases, which breaks down the structure of the leaf. Then the fungus can absorb the nutrients released from the leaves. The fungus grows and forms a fruiting body sort of like a mushroom and this is what the ants feed on.

Leafcutter colonies have small underground caverns, often several feet below ground level, where they plant their fungus gardens. They are ventilated by tunnels that have openings at the ground level, so that the air supply for the fungus is very good. The cavern is cleaned by the ants, and the remaining trash is thrown out on a trash heap outside the underground residence of the ants. This is a heap of small packets of used-up leaf material. Each packet has been carried and dumped in the same spot, building up a trash pile that can be two feet (66 centimeters) high. Once the old leaf material has been cleared out, then the ants bring in a fresh bed of leaf pieces.

The fungus is a species that doesn't grow on its own. It needs the ants to feed it. It needs the ants to take it to new homes underground, into comfortable cool, moist caverns built by ants. And it needs the ants to chew its food, to keep it clean, and to plant more fungus gardens. It even has the ants as 'doctors.' When the fungus is ill with a bacterial infection, the ants have another microbe to produce a chemical cure for the sick fungus. The ants eat only this fungus, and the fungus can grow only if cared for by the ants. The fungus needs the ants and the ants need the fungus. This is what we call "mutualism."

The only ant in the colony who reproduces is the queen. She lays all the eggs, which are raised to become new workers. Some of these will become big-jawed soldiers who protect the colony; some grow into the medium-sized workers who tend the gardens and cut the leaves; and some become the tiny leaf-riders who protect the leaf pieces from parasitic insects that would lay eggs on the fungus-food. Some special ant eggs become new queens, and when they grow up, they leave the colony to make a new colony. When they leave, they grasp a tuft of fungus to take along and start a new garden.

The leafcutter colony I am observing might have as many as 8 million member ants. This is the same colony shown in the entry on decomposition, where a giant mushroom has grown on their trash heap. There are many colonies on BCI, and most are near the laboratory. The leafcutters are so numerous on BCI that biologists have measured their use of leaves. They found that leafcutters use as much plant material as all the vertebrate species combined-- all the howler monkeys and deer and sloths and so on. They are a very important part of the ecosystem here! In fact, ants are important in many other kinds of ecosystems. Certainly the anteaters must think they are essential. And you'll enjoy observing ants this year, if you try it!