An overwhelming majority of plants depend upon birds and insects for seed dispersal. Plants attract pollinators by releasing aromatic compounds into the air, or by producing sweet nectar that birds and insects feed upon. Species of the Pisonia plant are no different. They entice small birds to build nests on its branches, and when the birds brush against these seed-laden branches, the Pisonia’s sticky seeds get stuck to the birds’ feathers. After some time the seeds fall off, ideally when the birds had travelled to another island. But somewhere along the Pisonia’s evolutionary history, something went horribly wrong. Instead of using the birds as vectors of pollination, the Pisonia ends up killing a large number of birds by ladening them with so much seeds that the poor birds are unable to fly away. They fall to the ground and starve to death, or get eaten by predators.
Black noddies nest on Pisonia trees in Heron Island, Queensland. Photo: Christopheb / Dreamstime.com
Pisonia trees, also nicknamed “bird-catchers”, are found in tropical habitats, mainly islands in the Caribbean and Indo-Pacific. The trees produce long seeds, covered with a thick mucous and tiny hooks that stick to almost anything that brushes against them. The seeds grow in large tangled bunches, and each bunch can bear anything from a dozen to over two hundred seeds.
The Pisonia tree flowers twice a year, and the timing of these events coincide—naturally through years of evolution—with peak seabird traffic to the islands. These seabirds use the Pisonia for nesting, and when the young ones hatch and fledge, the result is often tragic. The young birds get entangled and entrapped in the sticky bunches. Even a handful of seeds can weigh a bird down, making them unable to fly. They fall down and eventually die of starvation, or become food for scavengers. At other times, the birds die in the trees; their mummified corpses hanging from the branches “like macabre Christmas tree ornaments.”
Seeds of Pisonia trees. Photo: Christopheb / Dreamstime.com
Seeds of the Pisonia tree are covered by a sticky resin. Photo: Scott Zona/Flickr
The Pisonia tree’s grisly behavior has a devastating effect on the bird population. A study conducted on seabird population on Cousin Island in Seychelles revealed that the Pisonia tree killed a quarter of White Terns and nearly a tenth of Tropical Shearwaters via fatal seed entanglement.
Scientists have wondered if there is any evolutionary benefit derived by the Pisonia tree by killing birds. One hypothesis stated that the trees get increased nutrients from the decomposition of bird corpses that fall at the foot of the trees. But when Alan Burger, an ecologist at the University of Victoria, analyzed seedlings sprouting near bird carcasses on Cousin Island, he did not find any evidence that seemed to indicate that the seeds were sprouting any better. In addition, the tree received far more fertilizer from the bird's droppings and fallen eggs, indicating they were worth far more to the tree alive than dead.
A wedge-tailed shearwater covered in sticky Pisonia grandis seeds in Tiam'Bouene islet, New-Caledonia. Photo: Tristan Berr/Twitter
A white capped noddy tangled in Pisonia seeds at Heron Island, Queensland, Australia. Photo: Christopheb / Dreamstime.com
Next Burger tried immersing Pisonia seeds in seawater to test another hypothesis that the carcasses of dead birds might wash ashore on another islands, helping seed dispersal. To test the theory, Burger dunked the seeds in salt water and found that they died after just five days, ruling out the possibility that the Pisonia might be using dead birds as rafst to disperse seeds to faraway island.
“The results from my experiments showed quite convincingly that the Pisonia derived no obvious benefit from fatally entangling birds,” Burger concluded in a paper than appeared on the Journal of Tropical Ecology in 2005.
“The entanglement and mortality of some of the potential vectors can be viewed as a negative side-effect of selection for seeds with sufficient adhesion to slick to a bird's plumage for several weeks and resist the efforts of preening birds to remove the seeds,” Burger wrote.
A fledgling brown noddy entangled with seeds across its chest and tail. Photo: BBC
Photo: BBC
In other words, the seeds absolutely need to be sticky so that they do not fall off the birds. And because the resin gradually loses its adhesion, especially when immersed in sea water, the seeds have to start out extremely sticky. Inadvertently, this results in the death of hundreds of thousands of seabirds every year.
Conservationist on Cousin Island are now working to replace Pisonia trees with other native tree species to help the sea bird population to recover. They also actively seek out entangled birds and carefully clean them, before putting them back in a tree.
Surprisingly, many seabirds seems to love Pisonia trees. “It’s rare to see a Pisonia tree that doesn’t have seabirds in it,” says Beth Flint, a wildlife biologist for the U.S. Fish and Wildlife Service.
Flint and her colleagues actually work to encourage Pisonia groves, because they are prime nesting habitats for red-footed boobies, frigatebirds and black noddies.
References:
# Alan E. Burger, Dispersal and Germination of Seeds of Pisonia grandis, an Indo-Pacific Tropical Tree Associated with Insular Seabird Colonies, Journal of Tropical Ecology
# Pisonia grandis…A grand problem?, Seychelles Seabird Group
# Jason Bittel, This tree lures birds with a free lunch and then kills them, Washington Post
# The island paradise with a dark heart, BBC
Actually, the dead birds are an evolutionary advantage for the plants. The dead birds provide additional nutrients for the geminating seeds as well as for the plants rooted in the carcasses vicinity.
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