Tuesday, January 5, 2016

The bird-killing tree of the Great Barrier Reef, pisonia grandis

 Black Noddy on Lady Elliot Island, Great Barrier Reef, Australia
 Removing the barbed seeds
some of the sticky seeds - note the stalks that are like grappling hooks

Black Noddy with chick in the pandanus plants on Lady Elliot Island, GBR, Australia

I like to think of non-human nature working for the best in a challenging world: beneficent, evolving with the needs of complex communities. I understand there is killing but, unlike with humans, all to a purpose: nothing wanton, little wasted.
I also thought that there was a hierarchy of power and strength so it did not cross my mind that a tree could kill a large seabird. But one can and it's not a pretty sight, especially on a tourist island in full season.
The Pisonia grandis tree is grand by any terms, especially as a survivor on a small coral cay coping with poor soil and intermittent cyclones. It manages to grow into a mighty tree over twenty metre high with wide branches and a large grey spreading trunk with tentacles like an octopus clinging to the soil. The leaves are large and bright-green and much favoured by seabirds to line their nests. The tree seems to cater for the birds with many spots available for building nests. If you compare it to the other main trees of the coral cay islands, the allocasurinas, you can see why the Black Noddys and the Bridled Terns favour this apparently perfect tree. But there is a nasty side-line to this generosity.
When the birds breeding season is in full swing with thousands of seabirds occupying every nook and cranny of the coral cay, the seeds branches of pisonia trees mature and drop to the ground.
The seeds are born on a branching structure as big as a human hand and each stalk bears many centimetre long seeds, rather like long grains of chocolate-coloured rice, but this is barbed rice. While the seeds are immature and green they are no danger.
However, once the seed branchlets dry and fall to the ground they become a deadly trap for the birds.
At this stage the seeds exude a glue that adheres to whatever touches it - perhaps the tail feathers of the many buff-banded rails that frequent these islands, or most often to the flight feathers one of the thousands of nesting seabirds that alight under the trees for shade, preening, or for meeting their mates.
When one seed adheres the whole branchlet remains with the seed and very soon the other seeds stick to further feathers: the long flight feathers and the fine downy feathers of the head and neck. Before long the bird cannot fly and the more it tries to remove the seeds, the more other seeds catch on. The bird walks around under the trees, trying for days to free itself before it collapses and dies of exhaustion, dehydration and starvation.
Of course, these birds are couples and the partner waiting on the nest for relief and food for their chick must eventually abandon the chick to survive itself.
When we saw birds in this situation on Lady Elliot Island we were distraught but were told by the staff of the marine centre that this was 'nature' at work and that only approximately 1% of the birds die and furthermore that the death of the birds helped the trees to grow as their decaying carcasses would provide nutrient for the seeds to germinate.
However, after some research I find that both these statements are not correct. This is according to current research in the Seychelles Island of Cousins where they have found that depending on the timing of the crops of seeds the death rate can be as high as 25% in some species and that the germination of the pisonia seeds is not facilitated by being within the carcass of a seabird. Nature Seychelles have now adopted a policy of removing stands of pisonia grandis from Cousin Island. Yet on Lady Elliot Island in Australia they are busy planting more.
My husband and I spent some time catching the disabled seabirds that were covered in pisonia seeds and removing them. It is not easy. The barbs and the glue are a horrible combination and it took two of us some time to remove the intertwined branchlets with their lethal seeds from the bird's bodies. However, it gave us great pleasure to set each bird free and see it disappear over the sea.
I have alerted Lady Elliot Island to the new research from the Seychelles and I have written to Birdlife in Australia to ask for their guidance on current Australian research. It appears to me that Lady Elliot Island in particular is operating under old misconceptions.