You are currently browsing the tag archive for the ‘mutualism’ tag.

ant farm.jpg

The first known farmers were apparently…ants. Leafcutter ants have been growing fungus on chopped up leaves for at least 50 million years. It is an amazingly long time.  Yet, when one thinks of the astonishing range of different “breeds” of animals and crops which humankind has created through artificial selection during the 10 millenia or so years since we started farming, the ants seem a bit lackluster.  For all of their workaholic zeal, ants are not as relentless as us in selecting for traits in their crops.

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Yet, as we learn more about the ants and their empire, the amazing extent of their symbiosis with the plants they use is beginning to become more apparent to us.   Because of the vastly greater timeline of their endeavors, they have coevolved in astonishing ways. An example of this can be found in the homes of Philidris nagasau, a species of leaf cutterant native to Fiji.  These ants literally grow their homes out of Squamellaria, an epiphytic plant which grows on tropical trees.

The Economist described the mechanism through which the ants grow a home (or, alternately, the way the epiphytic plant obtains an army of insect servants):

P. nagasau worker ants harvest seeds from their epiphytic homes, carry them away, and then insert them into cracks in the bark of suitable trees. That done, they patrol the sites of the plantings to keep away herbivores, and also fertilise the seedlings as they grow by defecating into hollow structures called domatia that develop in the bases of the plants’ stems. As a Squamellaria grows, its domatium swells (see picture) and develops galleries that can accommodate ants—which then move in. This, and the plant’s habit of growing flowers that generate nectar long after they have been pollinated, provide the evolutionary quid pro quo that makes the relationship between insect and epiphyte work.

It is incredible that the ants grow their own houses.  Yet, as one looks more closely at familiar domestic arrangements with this story in mind, they start to seem less familiar.  Is farming really as unique as we make it out to be, or does it resemble mutualistic arrangements found throughout the natural world.

Squallemaria.jpg

We would never say we co-evolved with goats, cows, and horses: their domestication seems like a one way exchange to us. Yet an outside observer might look at our leather sofas, cheeseburgers, cavalry charges, or angora sweaters and come to a different conclusion.

Magical Tree by JourneyArtist (deviantart)

Magical Tree
by JourneyArtist (deviantart)

Today is Arbor Day, the annual international celebration of trees. Like my distant heathen ancestors, I partake in a bit of tree worship.  Because of their immense size, strength, beauty and longevity, trees are an obvious metaphor for the numinous.  However there are also more subtle and compelling reasons that trees are the ideal symbol of divinity.

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Trees are at the center of a vast web of commensal relationships between living things.  They rely on large mutualistic collections of organisms to survive. Trees cannot live without an unseen world of symbiotic organisms in the soil. The towering plants rely on nitrogen fixing bacteria, fungi, and actinomycetes to take nutrients from the earth.  Likewise trees communicate through fungal networks which link them together in improbable ways we are only now learning about.

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Trees utilize bees, flies, monkeys, and birds for pollination…and to disseminate their seeds.  They call on different parasitoid wasps for defense through elaborate biochemicals. We should really envision a tree not as a big spiky discreet thing sitting in the lawn, but as a vast flow chart/rolodex of connections with other living organisms.

ecosystem_movement

Of course trees are not unique in being an interconnected node within a vast web of life—that is really the way all life is.  It is a grotesque human conceit that humans stand outside and above nature.  I have always thought of humanity as a problematic youngest child.  We are the favorite (for the moment). We have such gifts…but we are so arrogant, unhappy, and unstable.  And we are so so monstrously greedy.  I sometimes like to imagine trees as a gentle stable elder brother.

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Actually though, mammals are much older than flowering trees.  For hundreds of millions of years our pathetic little ancestors cowered beneath the roots of conifers, cycads, ginkgoes, tree-ferns and such.  Then, at the end of the Mesozoic, the ascent of mammals happened at the same time that the angiosperms took over the land.  Our shrewlike ancestors evolved into arboreal primates as the angiosperms themselves were becoming the forests.

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We grew up together! While the great angiosperm forests of the Eocene may not have required much from our squirrel-like grandparents, today’s forests desperately require our good graces so that they are not all converted into parking lots. Plywood, and ugly discount furniture.

space_tree_by_mattwork-d3hvdc2

Anyway, my thoughts are getting away from me.  I only wanted today’s post to be a reminder of Arbor Day and how wonderful and beautiful trees are. Here is a small gallery of lurid yet evocative images of sacred trees!  I especially like the pictures of trees together with outer space or the cosmos (like the big portal tree at the top).  Happy Arbor Day!

Space Tree by MartijnVn on DeviantArt

Space Tree by MartijnVn on DeviantArt

Nepenthes rafflesiana elongata (upper pitcher)

Nepenthes rafflesiana elongata (upper pitcher)

Yesterday’s post described the carnivorous nepenthes plants which entice organisms into their slippery liquid-filled depths where the tiny creatures are killed and digested.  The plants however are after different nutrients than carnivorous animals are.  Instead of hungering for proteins, carbohydrates, minerals, and complex amino acids (and all that other stuff nutritionists and zookeepers are always going on about) plants simply want phosphorus and nitrogen.

The small wooly bat (Kerivoula intermedia)

The small wooly bat (Kerivoula intermedia)

The small wooly bat (Kerivoula intermedia) is a tiny vesper bat which lives in Malaysia (the portion on Borneo). The small wooly bat weighs between 2.5 to 4 g (0.08 to 0.14 ounces) and, at most, measures 40 mm (1.6 in) from nose to tail.  It is one of the smallest mammals alive—it is even smaller than the miniscule lesser bamboo bat (which lives inside of single segment chambers in bamboo stalks).  The small wooly bat has found an equally fine home: the tiny creatures live inside a Bornean subspecies of nepenthes– Nepenthes rafflesiana elongata.  The little bats fit perfectly inside the long tapered chambers of Nepenthes rafflesiana elongata—the taper even prevents the tiny aerial hunters from falling in.  In exchange for providing a perfect home for the tiny bats, the plants also get something.  Bat guano is a famous source of nitrogen and phosphorus—so much so that humans have been known to mine old bat caves to use the deep layers of excrement for an agricultural fertilizer.

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Nepenthes rafflesiana elongate does not need to be an effective hunter.  The bats which live inside its tube shaped pitchers provide it with the nutrients it needs on a continuing basis: the two organisms provide a beautiful example of a symbiotic relationship.

woolly bat with pitcher plant

The last tulips in my garden this morning...

The last tulips in my garden this morning…

It is finally flower season! How I love it! However the happiness of the season is constrained somewhat by the gray squirrels, which have systematically beheaded my tulips (despite the fact that I have been simultaneously trying to ward the pests away with foul chemical sprays and appease them with nuts).  Alas, most of my tulips now lie sad and beheaded beneath the cherry blossoms.

My (ineffective) struggles to protect my beloved tulips remind me of the struggles of wild flowers which face a similar arms race.  The tulips I plant are propagated by big nurseries, and the squirrels don’t really want to eat the blossoms: they merely tear them apart to see if there is any food inside (and (probably) because the miserable rodents enjoy my suffering).   Flowers are plant reproductive organs which exist to repopulate the species.  In the case of garden tulips this involves a complicated relationship between myself, Lowes, tulip farms, nurserymen, and squirrels.  In the world of wildflowers, the players are fewer and the stakes are much higher.

Buff-tailed Sicklebill (Eutoxeres condamini) by Ernst Haeckel

Buff-tailed Sicklebill (Eutoxeres condamini) by Ernst Haeckel

Flowers and their pollinators have a mutualistic relationship:  the hummingbird –or bee, or moth, or bat, or whatever–gets a meal while the flower directly shares its gametes (in the form of pollen stuck to the beak or fur) with distant members of the same plant species.  Some blossoms coevolve to provide nectar to specialized pollinators as with the famous sicklebill hummingbird (which feeds on the nectar of specialized Centropogon and Heliconia flowers which fit the bird’s beak and produce colors appealing to the hummingbirds).

stolenbees

This whole relationship falls apart sometimes though, thanks to a behavior first reported by Charles Darwin. Some animals are nectar robbers.  Lacking the long proboscis or curved beak or special senses necessary to obtain the sweet nectar which the plant offers as a reward for its reproductive interlocutors, some animals simply cut through the blossoms or rip them apart to take the pollen.  Although this can be beneficial (if a robber ends up pollinating a flower anyway, or forces a legitimate pollinating species to travel over a larger area—and thus provide greater genetic diversity), more often it is destructive.

Um, sure I guess...thanks, art department.

Um, sure I guess…thanks, art department.

Interestingly, a recent study determined that bumble bees learn how to cut holes in flowers and steal the nectar directly from other bumble bees (you can read about the particulars of the study here).  Bumble bees are not the only pollen robbers–various lepidopterans, bats, and birds are guilty in various ways–but the bumble bee example is the first case to prove Darwin’s thesis that such robbing behavior was learned by insects.

It all begins to make more sense now...

It all begins to make more sense now…

Flowers, though passive, are not helpless.  Over generations, they coevolve with both the robbers and the pollinators—which is how they obtain so many convoluted and fanciful forms (and why there are so many toxicologically and pharmacologically active compounds therein).  It is worth thinking about when you encounter a spring landscape of beautiful flowers—beneath the surface lies a world of sex, appetite, and larceny.

The horror!

The horror!

Agriculture is almost unknown in the natural world. Human beings are the only vertebrates known to grow crops or keep livestock (with the possible exception of damselfish which carefully tend little algae gardens).  And yet we were not the first animals to invent the concept.  Ants have farmed fungi within their tunnels for tens of millions of years.  Ants also keep aphids in captivity in order to “milk” them of sugary secretions–or to eat them outright.  It is possible that beetles, termite, or snails came up with the concept first, but most evidence points to ants as the first farmers.

An Ant Milking Aphids

Ants do not have a shabby operation either. Leaf cutter ants form the largest and most complicated animal societies known on Earth (other than our own) and a single colony can have over 8 million individuals.  Leaf cutters are an ideal example of how adept ants are at farming fungi.  Four different castes of worker ants work together to bring back leaf fragments and integrate them into huge fungal gardens. Different species of leafcutters cultivate different fungi from the Lepiotaceae family.  Certain bacteria with antifungicidal and antibacterial properties grow within the metapleural glands of the ants.  The worker ants use these bacteria to “prune and weed” dangerous or unproductive organisms out of their gardens.  Older (more expendable) worker ants carry waste products from the hive to a waste pile where they stir the hive wastes together to aid in decomposition.  The waste-management job brings the danger of fungal or bacterial contamination and contaminated ants are exiled to certain death in order to keep the gardens safe.  Additionally dead ants from within the hive are carefully placed around the waste pile so as to protect the hive from their decomposition.

Leaf Cutter Ants at the Cameron Currie Lab arrange cut-up leaves into their fungal garden.

According to geneticists who study the rates of mutation within the various fungal cultivars, ants began their farm relationship with fungi around 50 million years ago in the warm Eocene epoch (an era which saw many of the critical relationships in modern ecosystems begin).

 

Digital Cut-away of an underground leaf-cutter nest

Scientists are also beginning to understand the means by which ants herd their little flocks of aphids.  The aphids are smaller insects which feed on the saps and juices of plants (which they suck out by means of specialized mouthparts called stylets).  The ants prevent the aphids by flying away by tearing off their wings.  The feet of the ants produce chemicals which tranquilize and subdue the aphids and keep them from escaping the “pastures” near the ant colonies.  It is believed that aphids also derive certain benefits from this arrangement since the aggressive ants protect them from many of their natural predators.

An Ant with a “herd” of Aphids

For years naysayers belittled the farming achievements of ants suggesting they were little more than symbiotic arrangements.  However as entomologists study the ants more carefully they increasingly discover just how complicated and sophisticated those relationships are (involving as they do numerous symbiotic relationships with bacteria in order to produce the chemicals necessary for agricultural control).  Additionally, what are humankind’s relationships with our crops and animals if not huge harrowing examples of symbiosis?

Chullachaqui (painting by David Hewson)

If you are wondering through the great untouched rainforests of the Amazon basin, you will sometimes come across a clearing devoid of all vegetation save for a few trees.  These bare patches are known as devil’s gardens and are said to be the haunt of the fearsome Chuyachaqui (or Chullachaqui), a shape shifting demon which delights in causing misfortune to travelers.  Although the Chuyachaqui’s default form is that of a small misshapen man with one hoof and one human foot, the demon can change shape into a person known to the traveler in order to mislead the latter to doom.

Lemon ants (Myrmelachista schumanni), so named because they are said to have an acidic lemony taste

Scientists were curious about these small bare patches of forest. After carefully studying the ecosystem, they discovered that a force nearly as diabolical as the Chuyachaqui is responsible.  The lemon ant, Myrmelachista schumanni, produces formic acid, a natural herbicide which it methodically injects into the plants in a “devil’s clearing”.  The only plants which the ants leaves alone are Duroia hirsuta, “lemon ant trees” which have evolved a mutualistic relationship with the ants.  The lemon ants keep the forest free of competing trees and plants, while the lemon ant tree is hollow inside—a perfect natural ant hive and its leaves provide a source of nutrition for the lemon ants (which are a sort of leaf-cutter).

A clearing filled with lemon ant trees

Large colonies of lemon ant trees have been found which are believed to be more than 800 years old—far older than the life of any ant colony or individual tree.  It is remarkable to think these ant/tree settlements have been part of the rainforest since before the Mongol conquests.

Copidomopsis floridanum injecting its eggs into a caterpillar.

Sometimes horror is a matter of perspective.  For example, parasitoid wasps–some of the most horrifying hymenoptera–are also some of the most beneficial to humankind. The parasitoid wasps are a hugely diverse superfamily among the hymenoptera consisting of more than 6000 different species.  These insects are ancient, successful, and profoundly useful for controlling invasive species or pests (particularly various arthropods), however as soon as one knows what “parasitoid” means it becomes difficult to regard these wasps without revulsion and distaste. A parasitoid is a creature which lives inside another creature (the host) and ultimately kills/destroys that host by consuming it or by bursting out of it.  The detailed dynamics of this relationship are often grisly in the extreme, but they highlight the bizarre (not to say disturbing) mutualism which is such a feature of the natural world.

The emerald cockroach wasp or jewel wasp (Ampulex compressa)

Parasitoid wasps are especially alarming because of the extent to which they can manipulate the behavior of their host.  For example the emerald cockroach wasp (Ampulex compressa) is a solitary hunting wasp which finds a single cockroach and delivers a mildly paralytic sting to the roach’s thorax.  This first sting temporarily incapacitates the roach and allows the wasp to carefully make a second more meaningful sting to a precise spot in the roach’s brain which control’s the roach’s escape response. Not only does the wasp know where to sting, she utilizes a toxin which specifically blocks receptors for the neurotransmitter octopamine. The wasp then chews off a portion of the roach’s antennae and returns to her layer leading the captive roach by holding its damaged antenna like a leash. Inside the wasp’s burrow she plants a single egg on the roach’s belly and then seals the zombified insect inside the chamber with sand and pebbles.  After three days the wasp’s egg hatches and the new larva feeds for 4–5 days on the external portions of the roach.  It then burrow inside the still living roach and devours the creature’s organs in a progression which leaves the roach alive for a maximum length of time.  When the roach is near death the wasp larva builds a cocoon inside it, metamorphoses into an adult, and then bursts out of the roach carcass and flies off.

Aaaagh!

Across the many different parasitoid wasps there are many variations of this behavior involving different arthropod hosts–and specifically targeting the host’s eggs, lava, or adult form.  Additionally there are sundry vectors by which the parasitoid wasps control their hosts.  Not all wasps utilize targeted neuropoisons like the emerald cockroach wasp.  Wikipedia elaborates on how close the biochemical relationship between the parasitoid wasps and their hosts can become:

Endoparasitoid species often display elaborate physiological adaptations to enhance larval survival within the host, such as the co-option of endosymbiotic viruses for compromising host immune defenses. These polydnaviruses are often used by the wasps instead of a venom cocktail. The DNA of the wasp actually contains portions that are the templates for the components of the viral particles and they are assembled in an organ in the female’s abdomen known as the calyx.

In other words some wasps utilize ancient hunks of rogue DNA to directly or indirectly control (and then destroy) their host organisms.

Braconid wasp lavae (Cotesia congregatus) destroying a tomato hornworm

The biochemical sophistication of the parasitoid wasps does not end there.  Certain wasps seem to have a symbiotic relationship with plants.  When these plants are gnawed by harmful insects (especially beetles or caterpillars) the plants release specific chemicals which summon the parasitoid wasps, which, in turn, destroy the insects. An example of this can be found in that most ubiquitous of American staple crops, corn.  When beet armyworm caterpillars  (Spodoptera exigua) start eating a live corn plant, it releases a chemical which attracts  parasitiod wasps of the species Cotesia marginiventris (the larvae of which utilize beet armyworm caterpillars as hosts). If however the corn is invaded by corn earworns (Helicoverpa zea) it will release a different chemical which attracts a different   wasp Microplitis croceipes.  As scientists look further into such relationships, they are discovering that most plants have a vast range of chemical tags which are appealing to specialized parasitoid wasps (and to sawflies).  Perhaps one of the reasons that various blights have been able to make such deep incursions in new ecosystems is the absence of plants’ terrifying little friends.

Cotesia marginiventris on a corn leaf

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