Killing Me Softly, or, The Fatal Embrace of the Strangler Fig

(Via: Wikimedia Commons)

Common Name: Strangler Figs

A.K.A.: Ficus species

Vital Stats:

  • There are around 800 sp. of figs, over half of which are hemi-epiphytes, like stranglers
  • Around 10% of all vascular plants are epiphytes (about 25,000 species)
  • The trees which produce the figs we eat are terrestrial, and do not grow in other trees

Found: Tropical forests of Latin America, Southeast Asia, and Australia

It Does What?!

What does it take to squeeze the life out of a full-grown tree? A lot of time and some very long roots, apparently. Many parasites eventually bring about the untimely death of their hosts, but few do it as slowly and as insidiously as the strangler fig.

Stranglers begin life as a tiny seed that leaves the back end of a bird and happens to land on a tree branch high in the rainforest canopy. The seed germinates, and the young fig begins to grow as an aerial plant, or epiphyte, taking its moisture from the air and its nutrients from the leaf litter on its branch. Thousands of plant species, including most orchids, grow in this manner. But then an odd thing begins to happen. The seedling produces a single long root. Very long. From tens of metres up in the canopy, this root grows all the way down to the ground. Many young stranglers will die before their questing root reaches the earth, but for those that make it, a connection is formed with the soil through which water and nutrients can be extracted. From this point on the great, towering giant which holds this tiny little interloper is in mortal danger.

The strangler fig, playing “harmless epiphyte.”
(Screenshot from The Private Life of Plants, BBC)

A secure connection to the soil allows the fig to speed up its growth and to begin sending more and more roots earthward. Rather than dropping straight down, like the initial root, these later organs will twine around the bark of the host tree. At first, the roots are tiny, like mere vines crawling over the host trunk. Over time, however, they thicken, covering more and more of the trunk’s surface. Where they touch or overlap, the roots actually fuse together, forming a mesh over the surface of the bark. Up above, the stem of the strangler is growing as well. It rises through and above the host branches, soaking up the light and leaving the other tree shaded and starved for energy.

In fact, this is a war fought on two fronts. As the starving host tree struggles to gather light energy to send downward from the leaves, it is also increasingly unable to bring water up from its roots. This is because the tree’s trunk continues to expand even as the strangler’s grip grows tighter around it. These opposing forces effectively girdle the tree, crushing the vascular tissues that carry moisture from the soil. Eventually, the battle is lost and the tree dies. Fortunately for the fig, its major investments in root growth have paid off – the dead host tree does not fall, taking the strangler with it. Instead, it simply rots where it stands. Finally, many years after its arrival on the scene, the strangler fig has achieved independence. It is now a free-standing tree, completely hollow and supported by its interwoven lattice of aerial roots.

The first root finds the ground.
(Screenshot from The Private Life of Plants, BBC)

So what happens when more than one strangler fig seed lands on a particular tree? Something quite unique… the roots of the different individuals fuse and form an organism which is indistinguishable from a single tree, except by molecular testing. These are what biologists refer to as ‘genetic mosaics.’ What’s more, the individuals actually begin to act like a single tree. You see, figs typically have staggered flowering times, such that it is unlikely for numerous trees in a small area to be in bloom at the same time. This helps in keeping their wasp symbionts well nourished. Once trees fuse, however, they seem to become physiologically linked as well, with researchers reporting that they bloom as a single individual.

The most hurricane-proof tree ever.
(Screenshot from The Private Life of Plants, BBC)

[Fun Fact: Some strangler fig species have very high growth rates, and huge individuals have actually been found engulfing abandoned buildings in the tropics.]

Says Who?

  • Harrison (2006) Journal of Tropical Ecology 22(4): 477-480
  • Perry & Merschel (1987) Smithsonian 17: 72-79
  • Schmidt & Tracey (2006) Functional Plant Biology 33: 465-475
  • Thomson et al. (1991) Science 254: 1214-1216
Don’t meditate under strangler figs.
(Via: Flickr, by vincenzooli)
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EVOLUTION TAG TEAM, Part 2: Sex & the Synconium

The second in an ongoing series of biology’s greatest duos. (Check out Parts One and Three)

(Via: Mastering Horticulture)

Common Name (Plants): Fig Trees

  • A.K.A.: Genus Ficus

Common Name (Wasps): Fig Wasps

  • A.K.A.: Family Agaonidae

Vital Stats:

  • Approximately 800 species of figs
  • Most are trees, but some are shrubs and vines
  • Approximately 640 species (20 genera) of fig wasps
  • All are obligate pollinators of figs

Found: Throughout the Tropics

It Does What?!

Snacked on any Fig Newtons lately? Tasty, right? Like the ad says, “A cookie is just a cookie, but a Newton is fruit and cake.”  …And wasps.

They must have run out of space on the package for that last part.

Before you toss out your favourite teatime treat, I should point out that without those wasps, the figs themselves wouldn’t exist. [Personally, I love Fig Newtons and will eat them regardless of any insects present.] This plant-insect pairing actually represents one of the most stable symbioses out there, with evidence suggesting it has existed for over 65 million years.

Now with 10% more Wings
(Via: Wikipedia)

While it’s not entirely clear how this arrangement evolved in the first place, fig trees produce a unique structure called a synconium, in which the flowers are actually inside the part we think of as the fruit. This synconium, which can contain up to 7000 flowers, depending on the fig species, has a tiny hole at the tip called an ostiole. In order for the flowers to be pollinated and the fruit to grow, a female wasp must squeeze through that hole, often losing her wings and antennae in the process, and distribute pollen that she carries in a sac on her abdomen. As she does so, she also uses her ovipositor to reach down into some of the female flowers and lay her eggs in their ovaries, where a gall is formed and the larvae can develop. Then she dies and ends up in a cookie. The End.

But hold on, let’s remove humans from the equation for a moment. She dies, but her eggs hatch into little moth larvae which use the growing fig for nutrition. Once they’re old enough, the young wasps mate with one another inside the fig (another nice mental image for snacktime), and the females gather pollen from the male flowers and store it inside their abdominal pollen baskets (yes, that’s actually what they’re called). The wingless male wasps have a simple, three step life: 1) mate with females, 2) chew a hole through the fig so they can leave, 3) die. That’s pretty much it for them. They may escape the nursery with the females, but they’ll die shortly thereafter, regardless. In fact, even the females have a pretty rough deal; from the time they’re old enough to mate, they have about forty-eight hours to get their eggs fertilized, gather pollen, find a new synconium, distribute the pollen, and lay their eggs. Two days, and their life is over. No pursuit of happiness for the fig wasp, I’m afraid.

“What does it all mean?”
(Via: BugGuide.net)

As with any long-standing mutualism, there are, of course, parasites ready and waiting to take advantage of it. These parasites are wasps which are able to enter the synconium and lay their eggs, but which do not pollinate the fig. Although their eggs will crowd out those of the fig wasps, decreasing the number of fig wasp larvae born, they are kept in check by the fact that any unpollinated synconium will be aborted by the tree and drop to the ground, taking the parasite eggs with it. The nonpollinating wasps are therefore kept from being a serious threat to the tree’s pollinators.

So there you have it, another of evolution’s great matches. The wasps get an edible nursery, the trees get pollinated, and we get tasty fruits with suspicious crunchy bits that probably aren’t dead wasp bodies, so just try not to think about it too much…

Seeds, or wasp eggs? You be the judge!
(Via: This Site)

[Fun Fact: The symbiosis between fig species and their corresponding wasp partners is so specific (often 1:1), that the shape of the ostiole actually matches the shape of the head of the wasp species which will pollinate it.]

[For those who would like to read about figs and fig wasps in much greater detail (such as how this works when the male and female flowers are in different figs), check out this excellent site for all you could ever want to know.]

Says Who?

  • Compton et al. (2010) Biology Letters 6: 838-842
  • Cook et al. (2004) Journal of Evolutionary Biology 17: 238-246
  • Kjellberg et al. (2001)Proceedings of the Royal Society of London, Biology 268: 1113-1121
  • Proffit et al. (2009) Entomologia Experimentalis et Applicata 131: 46-57
  • Zhang et al. (2009) Naturwissenschaften 96: 543-549