I think everyone has a bit of a thing for carnivorous plants. We’re used to plants as placid green things, harmlessly soaking up the sun’s rays and in turn feeding the whole pyramid of animal life. They’re peaceful, pretty and interesting, and even the dangerous ones will only cause trouble if you go out of your way to invite it. Plants are meant to be inoffensive in the most literal way, and although the rogues who upend this botanical tranquility might be a little bit mean and a little bit horrible, they’re always interesting.
It helps, I suppose, that carnivorous plants tend to go for smaller prey. A venus fly trap would be less curiosity and more existential horror if it was a venus human trap. And while I’ve heard of the biggest pitcher plants trapping small rodents, like the fly traps they’re mostly a threat to invertebrates. People have trouble empathizing with insects.
Predation in plants evolved in situations where these plants inhabited marginal, low-nutrient soil. In these cases, any extra helps, and structures which might acquire extra nutrients in the form of, say, a fly, or an ant, boosts that plant’s survivability. Through the inexorable forces of natural selection and extended time, we get sundews, pitcher plants, venus fly traps, etc. (I think my favorite is the bladderwort, which uses an intricate vacuum trap to hoover up underwater microorganisms).
This brings us, by way of opposites, to certain species of the Pisonia tree, which catches birds.
Ok, so first the mechanics, which are horrible. Unlike trapping plants, which constrain their prey and then emit a digestive fluid to dissolve and digest it, Pisonias kills bit by bit. It’s only dangerous after its flowers and its seeds are ripe, which happens about twice a year. The seeds are the killers: they’re small and easy to brush against, and they’re also extremely sticky.
Evolution has engineered Pisonia seeds to stick to feathers. They’re covered in tiny hooks which intertwine with the barbs on a feather and become extremely difficult to disentangle, and they are also covered in glue. One seed is not a problem, of course. It’s annoying, but hardly dangerous. But ten? Twenty? When they’re almost impossible to take off? Brush up against enough Pisonia seeds, and birds can’t take off. And when birds can’t fly, they can’t eat.
Starving to death while entangled in a mass of seeds is an absolutely horrible way to go, and it’s not even clear how much the Pisonia gets from inflicting it. Without there being an actual, physical trap (quite often, however, the birds just get tangled up in a ripening bundle of seeds, and hang there like decomposing baubles), the tree doesn’t get first dibs on digesting the birds. Something else — ground scavengers or whatever — tends to benefit.
Even the birds it does manage to keep, left in a sad pile of feathers and bones at its trunk aren’t really necessary for the Pisonia’s survival. We talked earlier about the constraints facing most predatory plants: marginal soils means that additional nutrients are vital for the plants to thrive. That’s not true of the Pisonia trees, which mostly live on rich (sub-)tropical islands and are additionally fertilized by the seabirds they kill, who basically dredge up the riches of the sea and crap them out all over the place. A dead bird might be a nice, macabre, treat, but it’s not life and death (except for the bird).
As one piece put it, the Pisonia appears to be killing these poor birds “for the heck of it”. So what gives here? Do these trees murder their neighbors out of some perverted love of killing? Well, no, obviously not, they’re trees, and not even the most tree-hating person of all time — for my money, this goes to the Roman poet Horace, who blessed posterity with multiple odes about how he hated one specific tree — would ascribe them with active malevolence.
Actually, these birds are the victims of something more interesting. The evolutionary constraint which most impacts plants like Pisonia is not nutrition but seed dispersion. If you live on a tropical island, and are a tree, it’s quite hard to ensure that your seeds end up somewhere that is both hospitable and significantly far away that they’re not competing with you, the parent tree for resources.
Unfortunately, most places which are sufficiently far away are also the ocean. This makes aerial dispersion useless. In theory, one might dump the seeds into the water and hope for friendly currents to cast them up on another island, but seawater is extremely good at killing most seeds, so that’s out too. The best way, as it turns out, is to hijack birds.
Birds travel widely and are very good at finding islands on which to nest. Get them to carry your seeds for you and your dispersion problems are over. This is a strategy used widely by fruiting trees, which lure in animals with a tasty treat and then get, uh, deposited somewhere else.
Seabirds don’t eat fruit, so trees like Pisonia have to disperse via the less-scenic route. If their seeds can attach themselves to feathers, and that feather falls off on another island, there it is. Problem solved. Which explains why pisonia’s seeds are sticky. It does not, however, explain why they’re murderously sticky.
The reason is because evolution basically doesn’t care about minor collateral damage. Pisonia’s seeds are optimized for dispersion, and that requires a great deal of stickiness. Sure, if they were so sticky and so fatal they killed every bird they came into contact with, that might be a problem for the birds. But as it turns out, there is an optimum bird death-to-stickiness ratio, and that ratio is bad news for birds.
It’s important to remember that life on Earth is beautiful. It’s also important to remember that it’s frequently horrible, morbid and totally uncaring. Pisonia trees definitely fall into category two.