Devil's Gardens

Tococa_leaves.jpg

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

Tococa  sp. (Melastomataceae)

Tococa sp. (Melastomataceae)

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. 

Behold - A Devil's Garden

Behold - A Devil's Garden

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. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]

The Truth About Coffee

Mmm mmm coffee. This wonderful elixir has taken over the world. Though individual tastes and preferences vary, there is no denying that most folks who turn to coffee enjoy its effects as a stimulant. Many an In Defense of Plants post has been written in a coffee-fueled frenzy. Even as I write this piece, I am taking breaks to sip on a warm mug of the stuff. Coffee has plenty of proponents as well as its fair share of nay sayers but the health effects don't really concern me much. Today I would rather talk with you about the shrubs that are behind all of this. 

The coffee we drink comes from a handful of shrubs in the genus Coffea. Native to parts of Africa, these shrubs are distant relatives of plants like buttonbush (Cephalanthus occidentalis) and the bedstraws (Galium sp.). The "beans" that we brew coffee from are not beans at all but rather a type of pit or stone found in the center of a bright red berry. Before they are roasted, the "beans" are actually green. Plants in this genus produce an alkaloid compound known as caffeine. Though it may seem strange, the purpose of caffeine is not to stimulate the human nervous system (though it is a wonderful side effect) but rather it is produced as a defense mechanism for the plant. Making this compound is a complex process that involves many metabolic steps within the tissues of the plant. There are certain factions out there who would like to argue that this is proof against evolution but, as always, evidence seems to be the downfall of their argument. 

Creationists will tell you that the adaptations we see throughout the living world are too complex to have happened by accident. In reality, there is a vast amount of evidence that disputes this. Caffeine is one such example. It has evolved independently multiple times in many different plant lineages. Looking at the genome of coffee, researchers at the University at Buffalo (my alma mater) found that the genes involved in the synthesis of caffeine did not arise all at once. Instead, the genes duplicated multiple times throughout the history of this genus with each duplication coding for another step in the process of producing the caffeine molecule. The interesting part is that each step of this evolutionary process produced a chemical that was itself useful to the plant. The precursor compounds are bitter and toxic to the kinds of animals that like to nibble on the plant. 

As it turns out, the benefits that the plants get from caffeine aren't restricted to defense either. Coffee, as well as other flowering plants such as citrus, produce small amounts of caffeine in their nectar. Researchers at Arizona State University found that bees were 3 times more likely to remember a flowers scent when there was caffeine in the nectar than if there wasn't. This serves a great benefit to the plant producing it because it means that its flowers are much more likely to get pollinated. As it turns out, humans aren't the only species that enjoys a good buzz from caffeine.

Before we get too excited over coffee, we must remember that is definitely has its downside. Worldwide, we humans drink roughly 2.25 billion cups of the stuff every day. In order to produce that much coffee, humans have turned somewhere around 11 million hectares of land into coffee plantations. This has come at an extreme cost to the environment. Also, being a tropical species, the types of habitat used to grow coffee were once lush, tropical rain forests. A majority of coffee consumed around the world is produced in monocultures. Where there once stood towering trees and a lush understory is now an open, chemically-laden field of coffee shrubs. There is hope, however, and it is rising in popularity. 

If you enjoy coffee as much as I do, you should certainly consider switching over to shade grown coffee. I have attached a fair amount of literature at the bottom of this post but the long story short of it is that growing coffee is much less harmful to the environment when it is grown in a forest rather than open plantations. The structural complexity of shade grown coffee farms allows a greater diversity of plant and animal species to coexist with one another. Species diversity and richness are significantly higher on shade grown farms than on open field plantations. 

So, there you have it. Coffee is as complex as it is interesting. We humans are simply lucky to have stumbled across a plant that interacts with our brain chemistry in wonderful ways. Certainly coffee has benefitted in the long run. 

Photo Credit: Ria Tan (http://bit.ly/1pFQD1J)

Further Reading:
http://www.sciencemag.org/content/345/6201/1181.full

https://asunews.asu.edu/20130307_beesandcaffeine


http://s.si.edu/1o6wOaj

http://www.sciencedaily.com/releases/2012/08/120807101357.htm

http://bit.ly/1S6dLVV