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.
https://seychellesseabirdgroup.wordpress.com/2013/06/02/pisonia-grandisa-grand-problem/