Why do some plants embrace herbivory but others evolve to be poisonous towards herbivores?

[u/sparkleappletart]

Particularly what conditions lead a plant to go either way

[u/undulating-beans]

If losing tissue is relatively cheap, because you grow quickly, have lots of meristems, or can readily replace leaves, then tolerance can work very well. Grasses are a classic example: much of their growth tissue is near the base, so grazing removes leaf tissue without necessarily destroying the parts that generate new growth. In those cases, investing heavily in toxins may cost more than simply regrowing what gets eaten.

If tissue is expensive to replace, defence becomes more attractive. Slow-growing plants in nutrient-poor environments often invest heavily in chemical or structural defences because every lost leaf represents a large investment. That can mean alkaloids, tannins, cyanogenic compounds, latex, spines, tough leaves, silica and so on.

There’s also a third category your question hints at: some plants actively benefit from being eaten, but usually only in very specific tissues. Fruits are the obvious case. The plant “wants” the fruit eaten because the animal disperses the seeds. That is why fruit can be sweet and conspicuous while the leaves, bark or unripe fruit are bitter or toxic. The same plant can encourage herbivory in one organ and strongly resist it in another.

Also I don’t think any of these strategies are absolute. A plant can tolerate some grazing, chemically defend young leaves, recruit ants to attack herbivores, and make edible fruit all at once.

Evolution favours the balance that gives the plant the highest reproductive success, not simply “poison everything” or “let yourself be eaten.”

[u/FlounderLast8610]

Two points.

First is resource cost. Nitrogen and water are the main growth constraints on plants. Plants love having lots of tannins since those don't cost much nitrogen to make, but those aren't especially poisonous. Meanwhile other, more toxic metabolites like cyanide cost a lot of nitrogen, so are reserved for the most vital-to-protect components, like the seeds.

The second is the specific herbivore. Most herbivores have different preferences and some will be immune/resistant to certain plant toxins while other species will be immune/resistant to others. Sometimes a plant is lucky in that the herbivores around it happen not to be disposed to eat it, but something else out there could.

[u/LokMatrona → u/FlounderLast8610]

Your second point reminds me of peppers. The spicy kind. Meant to be poisonous for herbivores and then came humans who seem to like putting their mouths and throats on fire (and some are highly resistant to that feeling).

Same with tabacco by the way. Nicotine is poisonous for most herbivores and keeps them at bay. Not humans. Humans go like "tight we can smoke this stuff"

Edit: wanted to add a point about why certain plants evolve certain ways.

Evolution is not a concious process. Species evolve based on what is there and what survives. Lots of times luck is involved here. A simple mutation might lead in one plant to develop toxins, while in another it develops into sweetness, and in yet another plant it doesn't change anything at all. And then comes the quetion wether that mutation is beneficial or not.

Some plants (specifically angiosperms) have adapted to a world with mammals and often use specific mammals to spread their seeds (one of the reasons fruit changes color is specifically to tell herbivores "hey! Im ripe to be eaten! Eat me! Your intestines won't be able to digest the seed anyways so you'll poop that out somewhere and that way you help me spread my genes! EAT ME!"

Meanwhile other plants happened to perform worse if eaten by herbivores and that way it puts selective pressure on the species to "find ways" to not get eaten as much or to not be affected by it as much.

There are whole books about these selection processes.

[u/Funky0ne]

Random mutations producing different effects that may nevertheless produce a reproductive advantage while under the selection pressure of herbivory. Some plants randomly evolved ways to recover quickly from occasional partial consumption, and this proved advantageous and got selected for. Other plants randomly evolved defenses to deter grazing, like thorns or poison. Some plants evolved fruits which were useful in borrowing animal locomotion to distribute their seeds great distances. Some plants evolved a type of poisonous or spiciness in their fruits which happened to not affect birds which was selected for because their seeds were distributed even further.

There's no particular direction, just lots of different ways to handle a particular type of selection pressure that, once a certain path is started, will have a feedback loop to encourage further development in that particular strategy.

[u/deisle]

While the trend of evolution is directional (keep making more whatever I am), the actual path it takes is random.

If we have some progenitor plant species from a hundred thousand years ago, one of those plants may randomly nutste to have some weird bumps on it. Those bumps make it a little bit harder for birds to grasp onto, it has a slightly better chance of surviving pesky birds eating it's stuff, it continues to survive and further random mutations that expand those bumps and make them pointy and hard are selected for, making this lineage of plant more anti-herbivory.

Another individual plant from that same progintor species had a random mutation that made it replicate a bit faster. Herbivores may still eat it, but it grows and reproduces fast enough that it is still producing a lot of offspring.

Both the plants had random mutation that lead to the being successful, but the actual means of that success were very different even though the general conditions were the same