Plant Defenses – myriad strategies

Plants, as you might imagine, devote a great deal of energy to defending themselves from predators.  We humans have a natural bias towards animals, creatures that are like us in that they are mobile and respond to stimulus on a timescale similar to ours.  Plants operate, with a few notable exceptions, on a slower timescale, but this in no way should be taken to imply that they are any less interactive vis-a-vis their surroundings.

Plants react to light and dark, sense gravity, moisture, nutrients, and toxins; some can “feel” other organisms (Venus fly traps for example) or “hear” sounds (sensitive plants).  One thing all organisms must cope with is predators and competitors, and all organisms need defenses against these threats.  Plants are no different in their needs, but they are largely immobile, so some of their defenses tend to take a different form than they do in animals.  Surprisingly, their defenses are not as different from animal defenses as one might expect.  I would break plant defenses into three broad categories: chemical, physical, and co-optive.

Chemical defenses often involve toxins of one sort or another or pungent aromas.  Some of these we assiduously avoid, such as certain members of the Sumac (Anacardiaceae) family like poison oak (Toxicodendron diversilobum) and poison ivy (Toxicodendron radicans) due to the allergen urushiol found in the sap.  Other plants using chemical defenses we consume with great relish, many of our foods and spices, for example, derive their strong flavors from the defenses the plant manufactures to deter herbivorous predators.  Mints (Lamiaceae), rosemary, cinnamon, peppers, and onions are good example of common foods we consume that utilize strong chemical defenses.  Other chemical defenses we find recreational and/or medical uses for; ephedrine from plants in the Ephedra family, THC from Cannibus, and cocaine refined from alkaloids found in the coca family (Erythroxylaceae) all have enormous economies reliant on them.

Coca cultivation in Bolivia near Coroico

Coca cultivation in Bolivia near Coroico

Chemical defenses are enormously interesting and extremely sophisticated, but they are largely hidden from us until we are affected by them.  This is part of the reason why eating unfamiliar plants is so dangerous, there are few good ways to determine if a plant is edible upon first encounter.

Physical defenses are the most obvious to us, especially when they come in the form of thorns and barbs, but those defenses barely scratch the surface of the types of physical defenses plants can employ.

An impressive but unsubtle defense - Ceiba speciosa in the Bolivian Amazon

An impressive but unsubtle defense – Ceiba speciosa in the Bolivian Amazon

Sharp pokey bits may defend plants against larger herbivores and chemicals help to protect them from insects or pathogens, but other plants themselves can be, if not predatory, at least detrimental to large trees.  Lianas and other climbing plants, epiphytes, parasitic plants, and even other large trees may need to be defended against.

Strangler fig (Ficus spp.) overwhelming a palm tree's defenses - Bolivia

Strangler fig (Ficus spp.) overwhelming a palm tree’s defenses – Bolivia

The photo above I find particular interesting as the palm tree being overwhelmed by the strangler fig usually has an effective counter to this sort of attack.  Palm trees and tree ferns both allow their old fronds to droop as they age, sheathing the trunk and providing a structure for climbing plants to adhere to.  Eventually these canny plants shed their dead fronds, and with them the uninvited plant guests that have taken up residence.  Many trees employ a similar strategy, eucalyptus and madrone have smooth bark that regularly sloughs off in strips.  The combination of smoothness and shedding makes it difficult for other plants to gain purchase.

Strangler fig is a generic term for a wide variety of tropical fig trees sharing a similar life strategy.  These are the “matapalo” or killer trees.  Rather than growing from the ground and climbing up these trees co-opt animals to carry their tiny seeds through the canopy.  A small portion of these seeds wind up in a place like the crotch of a branch or a broken limb where organic material has built up.  The young fig sends dangling roots down from the canopy in search of nutrients, eventually reaching the ground and transitioning from a vine-like life style to a more tree-like life style.  More and more ground-seeking tendrils make their way downwards, eventually ringing the host tree and strangling it.  As this happens the strangler fig uses the original host as a scaffold and sends its own canopy high enough to overshadow the unfortunate host.  The palm tree in the photo above was underneath a tree the strangler fig took root in and had the misfortune to be attacked from above rather than from below.

Color is an oft overlooked plant defense, the role of which is still being debated.  I don’t mean fruit color, that is blatant advertising and animal bribery.  The color and pattern of the leaves and trunk of plants may serve as defense against predators.

The most familiar example of this is variegation in leaves, that is the white or colored mottling seen most often in ornamental plants, but also occasionally found in the wild.

Variegated hibiscus leaf. Source

Color mottling in leaves is often a symptom of nutrient deficiency, insect predation, viral infection, or genetic chimerism (expression of more than one genetic sequence in a single organism).  In most of the above cases this indicates poor health in the plant, and a plant in poor health makes for an unappetizing meal.  Some plants seem to capitalize on this and mimic the effects of various types of poor health (eg. false leaf damage and variegation) to trick predators into avoiding what appears to a be an un-nutritious meal(1)(2).  Bark color, whether natural or as a result of mutualistic lichen growth may be a predator deterrent as well, as lighter colors may make predators more visible to other predators higher up on the food chain.

Before moving on to animal co-option I should mention one other strategy employed by some plants.  Outgrow your predators.  In this case a plant allocates few resources to defense and focuses on growth and/or reproduction.  Balsa trees follow this strategy, they grow astoundingly rapidly and produce copious numbers of seeds.  They are not long lived and have few toxins, as a result they are subject to immense amounts of predation from a wide range of species.  Some of these, such as tapirs, they avoid by growing out of their reach.  Others are more problematic.  I saw a 30 foot tall young balsa tree completely stripped of leaves by leaf cutter ants in less than two days.

Basla saplings - Bolivia

Basla saplings – Bolivia

The most interesting of the plant defenses, to my mind, is animal co-option for defense.  Ants are probably the animal most often co-opted by plants.  We don’t often think of plants as being the ones to manipulate animals, but that is more a reflection of our animal bias than of the true nature of things.  Plants are highly manipulative, in their slow manner.  Like many effective manipulators, they accomplish their ends via bribes (and in a few cases by outright lies – orchids tricking bees into trying to mate with the flowers is a classic example of vegetative duplicity).  Ants are employed as guards by a great number of plant species.

When I first arrived in the Amazon I recall thinking to myself, “Cool, I hope I get to see some of the ant/plant mutualism.”  The first plant I looked at closely was a common understory shrub in the widespread and diverse Melastomataceae family.

Melastomataceae with ant sheltering nodes at the base of the leaves - Peru

Melastomataceae with ant sheltering nodes at the base of the leaves – Peru

At the base of each leaf there was a hollow, swollen node with two small openings on the underside.  Tiny ants occupied these nodes and would rush out to defend the plant when it was bumped.  This is a surprisingly effective defense against herbivores of all sorts, insect and mammalian.  All through the Amazon (and elsewhere in the world) ants and plants have banded together.  I found arboreal ferns with hollow rootballs harboring and colonies, tall cecropia and smoke trees (“palo diablo” – devil trees) with hollow trunks harboring vast numbers of fierce and painful fire ants, and evocative single species plant stands called Chullachaqui gardens.

According to legend the Chullachaqui is a forest guardian spirit that keeps small monoculture garden plots scattered here and there in the forest.  One should be wary entering these areas, ask permission first, and be careful not to damage any of the plants the Chullachaqui grows.  This is good advice as the Chullachaqui gardens are home to a species of ant that lives underground and kills all the plants growing on the surface except for one species.  Animals that interlope are attacked also attacked and the bite and sting of the ant is painful as it contains formic acid.

One of the neatest ant-plant interactions comes in the form of hanging “ant gardens”.

Ant garden in Peru with Monstera spp.

Ant garden in Peru with Monstera spp.

The dense cluster of plants in the above photo is growing from an arboreal ant hive.  Certain plants produce seeds with fatty nodules on them that ants eat.  Ants collect the seeds and store them in their hives, clipping the edible portions off as needed.  Some of the sees sprout and send their roots into the rich material of the ant hive, reinforcing it and protecting it from rain and predators as they grow.  These hanging ant gardens are found throughout the tropics.

Ant-plant interactions are not limited to the tropics.  Some trillium species bribe ants with food to carry their seeds to good growing locations and elderberries in certain portions of North America keep a protective coterie of ants nearby by bribing them with sugar produces from nectaries grown specifically for the ants.

Co-opting an animal may well be the most sophisticated of plant strategies for its subtly, specificity, and efficiency.  Energy cost is at the root of all these strategies.  A plant only has as much energy as it can collect from the sun and soil nutrients.  It must balance its energy use amongst growth, reproduction, and defense.  Every defense a plant employs lessens the amount of energy it can devote to growth and reproduction.  Combining forces with other species can provide a relatively low-cost way for a plant to gain an aggressive, mobile, multi-pronged defense force.  Ants, for example defend their plant hosts with both physically damaging attacks and with chemicals.  That’s a two-for-one defense with an added bonus of rapid deployment for the relatively minimal investment of a home or some food.

Humans may well be mammal plants have trained best.

***

A note about the images and film vs digital in the field:

These photos (with the exception of the hibiscus leaf which is not mine) were taken in 2005 with a well used and abused Canon AE1 with a 50mm 1.4 lens and scanned from the negatives by the developing kiosks in Peru and Bolivia.  The quality of images reflects both the environmental stresses put on the camera and film and the irregular scanning quality.

For long periods of time in the field I still think that film is the better option.  I was in the jungle for months at a time, sometimes in places with no power (20+ days hiking and on a raft in Bolivia for example) and in hot, humid conditions with frequent thundering downpours and rampant mildew growth.  I love the digital camera I use now, but it would have been completely unusable for the majority of the time I spent in South America.

Film cameras do still have their place.

Brown Pelicans: today’s Pterosaurs

I am a big fan of Pelicans. They may be my favorite birds, though claiming anything to be a favorite is a little silly. I like all pelicans, but it is the Brown Pelican (Pelecanus occidentalis) and its cousin the Peruvian Pelican (Pelecanus thagus) that are at the top of my pelican list.

Brown Pelican (Pelecanus occidentalis) banking away from a landing at Point Dume, in Malibu

Brown Pelican (Pelecanus occidentalis) banking away from a landing at Point Dume, in Malibu

The Brown Pelican is the smallest of the 8 species of pelican in the world. Small is a relative term when it comes to pelicans, the Brown Pelican weighs up to 12 pounds (5.4kg) and has a wingspan up to a little more than 8 feet (2.4 meters). It and the Peruvian Pelican, which is nearly twice the size of the Brown Pelican, have a hunting strategy that differs from all other pelicans and one that is great fun to watch.

Pelicans are extremely successful apex predators. Their primary hunting tool, their beaks, have remained relatively unchanged for 30 million years as evidenced by a remarkably intact fossil from southern France. Pelicans have the largest beaks of any bird, a long affair with a sharp hook at the end and a large pouch underneath. Like baleen whales pelicans gulp huge mouthfuls of water and food (fish for pelicans) and strain the food from the water. Most pelicans do their fishing from the surface of the water, floating along like immense ducks, dipping their heads into schools of fish to grab a meal.

Brown Pelicans have an entirely different strategy.

Brown Pelican diving for fish.  View the fullsize image to see fpanicked fish leaping clear of the water to escape the pelican

Brown Pelican diving for fish. View the full-size image to see panicked fish leaping clear of the water to escape the pelican

Pelicans can see through the water well enough to spot fish near the surface. Brown and Peruvian pelicans hunt from the air in a delightfully cavalier fashion. When they spot a school of fish they dive for them, but this is not the elegant, dagger like dive of the gannet, this is the lumbering crash of a falling boulder. They fold their wings and plummet from the sky, more-or-less beak first, impacting with a great explosion of water. Their version of a dive is more akin to a drunken stumble into the pool than it is the clean Olympian dive. Despite the seeming lack of grace, their hunting strategy is effective.

While the dive of a pelican exhibits a singular lack of grace, they are elegant precision flyers. Pelicans of all species are probably best known for their surface skimming flight.

Brown Pelicans skimming the water - the lead pelican does not seem hindered by the loss of an eye

Brown Pelicans skimming the water – the lead pelican does not seem hindered by the loss of an eye

Being large, heavy birds (the largest species of pelicans weigh upwards of 20 pounds), pelicans use as little energy as possible when flying. We see them most often flying low over the water, wings nearly touching the surface of the ocean. The weight of their bodies compresses the air underneath them, making it more dense. As a result the air provides more lift, in effect they are riding on their own cushion of air. We make vehicles that do this, hover craft, and far more impressively, the Soviet ekranoplan vehicles.

Pelicans are adept surfers, riding the slight updraft of air above the curl of breaking waves.

Surfing Pelican

Surfing Pelican

Large air-sacks under the skin and hollow bones help pelicans float and a tough layer of fiber in their breast muscles helps pelicans keep their wings extended during long flights. Like other large birds pelicans search out thermals and other updrafts to climb into the sky for long flights.

I find Brown Pelicans to be surprisingly colorful.

Pelican eying me with suspicion

Pelican eying me with suspicion

Their heads have yellow, red, and a bluish tint as well. I suspect that the vibrancy of the colors changes in accordance with mating season. Peruvian Pelicans also share this colorful head, perhaps being even more colorful.

Peruvian Pelican (Pelecanus thagus)  in Paracas, Peru

Peruvian Pelican (Pelecanus thagus) in Paracas, Peru

Pelicans have a primeval aspect to them. We no-longer have Pterosaurs, but looking at Pelicans I feel a sense of what it must have been like when the sky was full of those wide-winged, short-tailed flying creatures.

Landing averted

Landing averted

The Mighty Dragonfly

Of all insects there are few that capture our attention and interest the way dragonflies do.  They have, perhaps, the coolest, most evocative name of any group of insects: Dragonfly.  In English there are a great number of other common categorical names: Devil’s Darning Needle, Snake Doctor, and Ear Cutter among others.  Many of these names come from the mystifying apparent fear of nature that crops up over and over in European views of the world.  Many European cultures viewed dragonflies as sinister creatures, servants of the devil, in league with other evils such as snakes and bats.

Other cultures, often more agrarian ones, had a far more benign view of dragonflies, based, perhaps, on the recognition of their fundamental role in controlling populations of pest insects of all sorts.  An archaic name for the Japanese Islands is Akitsushima (秋津島), the Dragonfly Islands, where dragonflies symbolized courage, strength, and happiness.  For some native American tribes dragonflies symbolized clean, pure water, swiftness, and agility.  In the modern world dragonflies are good indicators of environmental heath, indicating a robustly functioning ecosystem.

Libellula quadrimaculata – Four Spotted Skimmer
The Alaskan State Insect

Dragonflies and their close relatives, Damselflies, come in a dazzling array of colors and patterns, ranging in size from less than  an inch long up to the South American Megaloprepus caerulatus with a wingspan of over 7 inches.  The largest dragonfly we know of is from the 300 million year old fossil Meganeura that had a wingspan of over 2 feet.

Dragonflies are powerful hunters, both in their nymph and adult stages.  Dragonfly nymphs are aquatic and prey on any animal or insect they can grab with their claws or their extendible jaws.  Insects, small fish, tadpoles, and small amphibians are all food for these voracious predators.  The nymphs are large, and, in turn, are prey for a wide range of other animals, insects, birds, and fish.  Elva Paulson has some wonderful watercolors of a dragonfly emerging from its nymph stage.  Humans are included as predators, many Asian cultures eating both dragonfly nymphs and adult dragonflies as delicacies.  One of the most tasty things I’ve eaten (from a long list of foods most people would consider to be unusual) was a plate of deep fried dragonfly larvae.  Absolutely delicious.  In Beijing I would sometimes find adult dragonflies candied in liquid sugar, their wings crispy with the hardened sugar.

Unknown green dragonfly – note the barbs on the forelegs for catching prey

The adult phase of a dragonfly’s life is short, in temperate climates only the length of the summer.  This is their mating stage and it takes them between 2 months and 6 years living under water to reach this stage.  Dragonflies are extremely active during this mating phase and must eat often.  They have enormous eyes giving nearly 360 vision, incredibly swift reactions, fast, powerful flight, and wicked barbs on their legs to assist capturing insects in flight.  The inset above shows these barbs.

Libellula exusta – White Corporal (I think)
eating its prey

The common names of dragonflies often reflect their speed or their abilities as hunters.  Meadow-hawk is one of my favorite names, and watching one dart away to catch an insect and return to its roost to devour it definitely brings hawks to mind.

Libellula quadrimaculata – Four Spotted Skimmer
note the different wing heights

Dragonflies are powerful fliers.  They have been clocked at over 35 miles an hour, fast enough to get a speeding ticket in a school zone, and, like hummingbirds, can fly forwards, backwards, sideways, up and down, and hover.  Their backs are sloped where their wings anchor, placing each pair at different heights, allowing for tremendous wing mobility.  Some species of dragonfly migrate, but the scale of some of those migrations has only recently been realized.  One dragonfly species in particular, the Globe Skimmer (Pantala flavescens) flies from India to Africa and back, island hopping cross the Indian Ocean, making open water crossings of nearly 1000km (620 miles) between island stops.  The only places they can breed are at the Indian and African ends of the migration, many of the islands they use as stopover points do not have sufficient freshwater for dragonflies to breed.  This is a stunning feat of flying for an insect and may be a behavior that evolved as a result of plate tectonics splitting India and Africa apart, eventually thrusting India into Asia.  If so, this migration could have begun 135 millions years ago.  Unfortunately, we have no reliable way of telling if this is the case.

Last year was a good year for dragonflies in Vermont, and this year looks like it is shaping up to be a good one as well.  The ecologist in me cannot help wondering why and one idea is that it may be linked to the calamitous drop in bat populations as a result of white-nose disease, a fungus that infects hibernating bats, weakening and eventually killing them.  It may be that adult dragonflies have more to eat with fewer bats and a greater percentage of them are surviving through the summer.  There is a historical precedent for this sort of boom in insect populations.  During the Great Leap Forward, Chairman Mao promoted a policy of killing off all things he thought were eating grain, birds amongst these.  With the crash in bird populations in China the insect population exploded.

Unidentified dragonfly – maybe a Darner of some sort

I am happy to see the dragonflies here.  Their presence means that the water is clean, we will have fewer mosquitoes, midges, and black-flies, and they are extraordinarily beautiful creatures.

Three-hundred twenty-five millions years old and going strong.  They have it figured out!