Okay but no like–if you’re a sessile organism (i.e. one that does not move), you still have to be able to adapt to changing environmental conditions, including potentially dangerous environmental conditions like fire and drought. Plant behavior, then, is a whole growing behavioral ecology field that studies how plants do change their behavior and physiology in response to their environment–without ever moving on a scale humans can see in the moment. (There are also sessile animals, like anemones and bivalves and barnacles, and plant behavior often pulls from studies of the way that these sessile animals respond to threat and danger by changing their behavior, too.) Trees, because they are long-lived and will therefore experience a wide range of environmental conditions in their one spot before they die, have to be particularly adaptable to change.
So what does it mean for a plant to change behavior in response to a threat? What’s going on with that ethylene gas? What’s the threat?
Well, acacia trees (not Arcadia) are infamously dry, prickly, nasty little trees that aren’t worth eating to most species. They have one big predator: the mighty giraffe.
Pictured: an acacia’s worst nightmare. Oh, it might look louche and unassuming, but giraffes eat fucktons of acacia by preference! And they particularly enjoy snacking on the flowers–the plant’s reproductive organs! What’s worse, acacias traditionally protect themselves in Africa by getting too tall for herbivores to reach and leaves growing enormous thorns all over themselves at lower levels. Giraffes, those bastards, circumvent all those lovely spiky thorns through being tall enough to just casually lean over them and snacking away. The WORST. Even if the acacia puts thorns all over itself, not just at the top, giraffes will cheerfully snake through the thorns using their long, prehensile tongues.
So what can an acacia do to combat the giraffe menace?
Different species try different tactics. Several species, especially bullthorn acacias, carefully feed and host aggressive colonies of ants to attack giraffes and other predators to drive them away. The ant colonies also often helpfully attack all of the competing plants within a radius of the tree, freeing up resources for the host tree.
But more commonly, acacias respond to getting bits eaten off itself by pumping their tissues (especially the tender leaves and shoots) full of tannins. That’s the nasty thing that makes red wine, black tea, and coffee taste so bitter to many humans… and taste interesting and astringent to others. At high enough concentrations, tannins are lethal. Acacias are good enough at producing tannins that they’re sometimes farmed for the purpose. But tannins aren’t cheap, and if there aren’t any herbivores around, maybe it’s better for the tree to put more energy into making new growth. It’s best not to waste them, after all. So plants keep them in reserve and only start really shoving them into the foliage when those bastard giraffes (or other herbivores) start damaging the plant’s growth. That damage triggers ethylene emission, which is an important stress hormone in plants. The ethylene coursing through the plant triggers release of the tannins with which the plant hopes to drive off or kill those bastard giraffes.
Well, wouldn’t it be great to know when that bastard giraffe might be coming before it eats all your nice delicate shoots and precious flowers? That way, you could get a head start on producing your tannins and minimize the damage that the fucking giraffe can do before you either drive it off or kill it. So acacias–and other plants–have ethylene receptors not just for their own hormonal signaling, but also so that they can smell the ethylene produced by other trees getting currently eaten by hungry giraffes. Remember, those leggy fuckers move, so you’d best prepare for them before they can get to you. Plants being able to communicate with one another about changes in environments before any given plant actually has to fight the giraffe allows them to adjust to change on the fly even without being able to change location or position in response to circumstances. Because the ethylene is carried on the wind, then, giraffes will actually move farther distances to graze on acacia trees, especially trees that are upwind–and therefore haven’t had a chance to prepare for the coming of the giraffe before it starts tonguing away.
Anyway that’s how I just spent an hour rolling around in bed composing this reply instead of getting up to take my meds. *fingerguns* those bastard giraffes strike again!
it’s absolutely valid in context but the phrase ‘giraffes and other predators’ sent me a place
to an acacia tree, the lion is a gentle, perhaps even benevolent source of tasty tasty nitrogen droppings, while the giraffe is a fearsome hunter
what I’m saying is that everyone needs to take a plant’s-eye view of the world some time just to get some practice stepping out of our anthropocentric perspective and reflecting on what utter bastards giraffes can be
#for the record the other predator I was thinking of as I wrote that was cattle #based on a bit I was reading about cattle farmers observing that cattle displayed similar eat only the upwind acacia behaviors
#there was also a bit in a kudu farm that fenced in all the kudu in a very small paddock #and went all wtf when the kudu all died from tannin poisoning because they didn’t have anything else to eat besides angry acacia tree
^^^
And acacia trees aren’t the only ones to do something like this!!
Nearly every plant species has some type of defense against herbivory. A lot of these are induced defenses, which the plant will activate when being attacked, such as in the case of acacia trees increasing tannin production in leaves.
There are all sorts of defenses like this, and many plants release chemicals to communicate with other plants around them!
My personal favorite defense is when a plant releases a chemical known as a volatile, which attracts the predators of whatever is eating it! Volatiles signal to predators that there is a tasty source of food around, and provides a beacon to the tree. This defense is often seen in instances of insect herbivory!!
Image description by aranealux
[Image Description:
1: A screenshot of a twitter post by izzitmichaela with text “I spent some time doing research on ADHD coping techniques and did you know that Arcadia trees can warn other trees of danger by emitting a cloud of ethylene gas”
2: a screenshot of a tumblr tag with text “this post fucks me up tf is a tree supposed to do about oncoming danger? Move?
3: Picture of a girraffe viewed from the side. It stands in dead grass, and there are trees in the background, stretching out into the distance.
/End ID]
Plants which release volatile (read: smell-up-the-air) compounds in this defensive way include … grass.
Yeah, they think maybe we like the scent of cut grass because it meant there was uncooked barbecue nearby in the form of grazing ruminants. (After all, we can’t eat grass.)
I’ve also detected and dispatched young hornworm caterpillars on my tomato plants which I wouldn’t have seen because my tomato plant smelled too nice that afternoon. Had the caterpillars gotten bigger they’d have stripped the plant to stems, so this was an urgent SOS from its perspective.
Anyway, ADHD really just be like that.
