Imagine, if you will, walking through the dense understory somewhere in the Amazon basin. Diversity reigns supreme here and it would seem that every few steps reveals myriad new plant species. As you walk along, something in the vegetation changes. You stumble into a clearing in the middle of the forest dominated entirely by a single species of tree. Why the sudden change? How did this monoculture develop? You, my friend, have just found yourself on the edge of a Devil's garden.
Devil's gardens are said to be the resting place of an evil spirit known to local tribes as Chullachaki. Anyone unlucky enough to stumble into his garden is said at risk of attack or curse. In reality, these gardens have a biological origin. The real gardeners are a handful of ant species which seem to have rather specific gardening preferences. Careful inspection would reveal that the gardens largely consist of trees in one of three genera - Duroia, Tococa, or Clidemia.
The reason that ants are so fond of these genera has to do with housing. These plant groups contain species which produce swellings along their stems and petioles known as domatia. These domatia are hollow and are the favorite nesting spots of various ant species. Ant colonies set up shop within. As anyone who has ever blundered into an ant colony can attest, ants are quite voracious at defending their home.
By providing ant colonies with a home base, these plants have essentially hired body guards. It is a wonderful form of symbiosis in which the ants aggressively defend against anything that might want to take a bite out of their host tree. Any herbivore trying to take up residence or lay eggs within the Devil's garden is viciously attacked. In doing so, the ants are protecting their host trees at the cost of all other plants unlucky enough to germinate within the garden. Still, this anti-herbivore behavior doesn't totally explain the monoculture status these host trees achieve within the garden itself. Why are these gardens so ominously devoid of other plant species?
To answer this, one would have to watch how the ants behave as they forage. While scouting, if ants encounter a seedling of their host tree, nothing really happens. They go about their business and let the seedling grow into a future home. When they encounter a non-host tree, however, their behavior completely changes.
The ants begin biting the stem of the plant, exposing its vascular tissue. As they bite, the ants also sting the foreign seedling, injecting minute amounts of formic acid into the wound. One or two ants isn't enough to bring down a seedling but one thing ants have on their side are numbers. Soon an entire platoon of ants descend upon the hapless seedling, stinging it repeatedly. In no time at all, the seedling succumbs to the formic acid injections and dies. By repeating this process any time a foreign plant is found growing within the vicinity of the garden, the resident ants ensure that only trees that will produce domatia are allowed to grow in their garden. Thus, a Devil's garden has been formed.
Although this relationship seems incredibly beneficial for each party, it does come at some cost to the plants themselves. Certainly forming the domatia is a costly endeavor on the part of the plant, but research has also shown that growing in such high, monoculture-like densities in the jungle has its downsides. It has been found that individual host trees can actually experience more herbivore pressures when growing within a Devil's garden than if it was growing alone, elsewhere in the forest.
Despite their aggression towards herbivores, the ants simply cannot be everywhere at once. As such, the high densities of host tree species within a Devil's garden act like a dinner bell for any insect that enjoys feeding on that particular type of plant. Essentially, the ants are concentrating a potential food source. Experts believe that this might explain why Devil's gardens never completely take over entire swaths of forest. Essentially, there are diminishing returns to living in such high densities. Still, benefits must outweigh costs if such mutualisms are to be maintained and it is quite obvious that both plant and ant benefit from this interaction to a great degree.