The Devil's Gardens

PimlicoDan
Published on February 1st 2020
10
a devils garden in a rainforest
Deep in the dark heart of the Amazon basin, within the jungle thickets of Ecuador, lie irregular patches of rainforest floor with a shortage of understory vegetation.
These barren patches were long believed by native tribes to be the work of a forest demon, called the Chullachaki. So when scientists eventually came across them, they christened them 'devil's gardens'.
Although there is a biological explanation, the truth is almost as odd as the myth.
A tree in a forest
An unusually bare patch of the forest in the Amazon is a sure sign that there's devilish work afoot, though not of the occult kind.

Satanic plots

If you were to take a turn through a devil's garden, you would notice that not only is the forest floor bare, but the trees belong to a single species, most likely Duroia hirsuta.
To call it a garden diminishes just how large these plots can grow, with the most extensive containing up to 600 trees. And the key to their forest monopoly? Ants.
Duroia, and a small number of other trees in South America, have evolved a mutualistic relationship with the lemon ant (Myrmelachista schumanni).
Mutualism is when two separate species work together to benefit both organisms.
Another example of mutualism is between bees and flowers, where the bee gets food and in return, fertilises the flowers.
Plants that have developed a relationship with ants are known as ant plants or myrmecophytes, from the Greek mýrmēx (ant) and phutón (plant).
A small bird perched on a tree branch
This structure, seen here on an Acacia sp. is called a domatium, and houses the ants, which are dispatched when disturbed by a potential threat
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Ants in their plants

Duroia have such a close relationship with these ants as they provide them with plenty of food and lodgings for the colony to thrive.
The trees have hollow stems and produce swellings, actually small chambers, along the stems of each leaf in which the ants can form their homes. These are called domatia.
Many ant plants also produce a sugary, nectar-like liquid from leaf glands, however, these extra-floral nectaries are not present on D. hirsuta.
Instead, the ants farm small colonies of scale insects and mealybugs within the domatia. This may slightly damage the plant, but producing abundant leaf nectar would also drain resources, so it's a small sacrifice for Duroia's survival.
The plant-ant relationship is so successful that a large Devil's garden can hold up to 15,000 queens and three million worker ants, with some gardens suspected to be around 800 years old.
But how does this seemingly-innocent bit of mutualism lead to the creation of the dreaded devil's gardens?
A close up of a green plant
Lemon ants are named after their sharply acidic flavour- yes, flavour! Seen here on another ant plant's domatia (Cordia sp.)

Little devils

Ants or no ants, Duroia hirsuta has a bit of an issue when it comes to other plants.
It has a growth inhibitor present in its roots called plumericin which can prevent other plant species growing nearby. This process is called allelopathy and is seen in several plant species, as well as bacteria, fungi and coral.
However, Duroia has added another layer to its attack strategy - by recruiting an army of ants.
When a sapling starts to grow within the perimeters of the devil's garden, it finds itself under attack from the lemon ants.
They mercilessly inject the stems with formic acid, in what is the only known case of an insect using formic acid as a herbicide.
Soon the sapling's leaves and shoots become crisp and the plant withers, leaving the Duroia to monopolise that patch of forest.
The cost of mutualism for Duroia is that it finds itself subject to attacks from herbivores frequently as there's nothing else to eat, despite its ant bodyguards.
A green plant
Some plants, including this maidenhair tree fern, have shown resistance to the effects of Devil's gardens.

Other ant plants

Duroia is only one of a large number of plant species that work with ants. They fall into two groups, and are either obligate or facultative species.
With obligate mutualism, the two species cannot survive without each other.
For example, the acrobat ant exists in an obligate mutualism with the macaranga tree and cannot survive outside its domatia.
In facultative mutualism, the species are not dependent on each other for survival. Most ant-plant relationships fall into this category, including the Duroia-lemon ant relationship.
Other examples include some species of acacia, which are infamous for their savage ant defenders, that can fend of the large herbivores of the African savannah.
An epiphytic ant-plant species, Myrmecodia, is beginning to appear more often on the UK as a houseplant. Unfortunately – or perhaps fortunately – ants are not included!
A bird sitting on a branch
The main body of the Myrmecodia plant forms the home for its ant tenants. It grows in similar environments to bromeliads and some epiphytic orchids.
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