Evolution they say is by natural selection. Just ask Charles Darwin, and you’ll need not a crash course about how nature diversifies the world. Talking of evolution, we big brain apes are at the top of everything; at the top of the food chain, collective learning and you’re more likely to survive just to pass on your genes. But nature is pretty cold, preferring certain species over others. There’s this peculiar tree that does some horrifying deed to another bird species; causing a menace to their entire eco-livelihood. Dear friends, ever heard of the “birdcatcher” tree? This is one of nature’s murderers. But you don’t need to worry.
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The “Birdcatcher” Tree
The Pisonia tree, also known by the caption above, has a dark secret. This tree belongs to the genus Pisonia, which are typically found in the Pacific — from Hawaii and New Zealand all the way to India. Hidden under these trees are bones of dead birds. The tree produces some sort of sticky seedpods that’s known to entrap insects, and birds (particularly seabirds) are found of eating these insects. The seabirds become stuck in the clutches of the tree, as the sticky seeds entangled them, it weighs them down to point of not being able to fly. They die in the sight of escape.
Back in the 1990s, Canadian ecologist Alan Burger from the University of Victoria, heard about the menace and decided to find out more. This led him to the Cousin Island in the Seychelles for an up close investigation at a population of Pisonia grandis and a colony of seabirds. The tree produces long seeds that are coated with a sort of thick mucus and some thorns. These will almost stick to anything it brushes against, however, the seeds usually fall to the ground which can easily pile up.
The seeds often weigh down smaller birds, making it impossible for them to fly away — if not attacked by a passing scavenger or predator — they decompose underneath the foot of the tree. Some are even left hanging in the branches of the trees, “like macabre Christmas tree ornaments.” And this what made Burger wondered if there might be some evolutionary answers for such crimes against an entire species, or perhaps the tree was not the root cause (pun intended).
Burger conducted a series of experiments for over 10 months, from 1999 through 2000. He wanted to determine if there were any benefits the trees got from the dead birds getting stuck with it’s seeds. First, he started with the corpses of the birds decomposing into the soil under the tree. Oddly, he found that the seeds that grew close to the dead birds weren’t any better than the ones that grew further away from the trees. This indicates that the corpses were not beneficial to the Pisonia tree.
Additionally, the trees were far more fertilized by bird droppings than while they were dead — indicating that the trees benefited more from the birds (alive) than their corpses. Burger later immersed Pisonia seeds into seawater with the notion that the dead birds might serve as rafts in dispersing the seeds to nearby islands. But unfortunately, the seeds didn’t survive after a few days upon germination. This ruled out the likelihood of the birds’ corpses as a raft.
“The results from my experiments showed quite convincingly that the Pisonia derived no obvious benefit from fatally entangling birds,” Burger told The Washington Post. “Having the birds alive seems to be the key to dispersal, but an unfortunate consequence of having extremely sticky seeds, and producing many seeds in a cluster, is that some birds get fatally entangled.”
For his final test, Burger occasionally dunked some seeds into the seawater over a period of four weeks. The results? They still germinated. Henceforth, he came to the conclusion that the seeds rather evolved to hitch a ride on the birds, but how cold can you be to murder someone who’s just lending you a ride? Evolution is not fair, at all.
Seabirds absolutely love Pisonia trees, and “it’s rare to see a Pisonia tree that doesn’t have seabirds in it, at least in places where there are seabirds left,” says Beth Flint, a wildlife biologist for the US Fish and Wildlife Service, who also manages habitats within the Pacific Remote Islands Marine National Monument.
Flint and her colleagues work to encourage Pisonia groves — a prime nesting habitat for red-footed boobies, frigate birds, and black noddies. According to Flint, the island of Lisianski — roughly 1,600 kilometers (1,000 miles) northwest of Hawaii — is dotted with Pisonia invasion. This has been going on for over 30 years, she says, but in the mid-1990s, researchers investigated a bird burrow and found that a single Pisonia tree was sprouting up from the brush. Today, it is a dense thicket of clones nearly 30 meters (100 feet) wide.
“And now that there’s that Pisonia patch, the birds love it,” Flint says. “There are lots of boobies and terns nesting in it.”
(Fun fact: Did you know that you can even see this Pisonia tree using Google Earth?)
Burger’s work originally appeared in the Journal of Tropical Ecology all the way back in 2005. But in 2017, the new British Broadcasting Corporation (BBC) documentary series Planet Earth II picked up on the so-called “birdcatcher” tree.
The catch? Evolution sure does have some shady tactics up it sleeves for determining who survives (and who perishes). And fortunately, the Pisonia tree might not be left far behind. Hard miss, seabirds. However, there is one species of turtle doves that feed on Pisonia trees, but manage to escape the ill-fated entanglements of the sticky seeds. Nature is weird, and doesn’t give AF, after all.
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Written by: Nana Kwadwo, Wed, Jan 13, 2021.