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!
