Dr Alan E Burger of University of Victoria, Canada, has been working for Nature Seychelles, and investigated why the Pisonia tree kills birds.
Pisonia tree fruiting © Alan Burger
The Mapou tree of Seychelles, known to science as Pisonia grandis, is widespread across the tropical Indo-Pacific. It is found most often on small islands that have seabird colonies, where it is often the dominant forest tree and provides favoured nesting sites for terns and noddies. Its seeds, produced in clusters of 50-200, exude a resin that makes them stick readily to feathers.
Birds are often ‘used’ by plants as agents of seed dispersal, and Pisonia seeds apparently get carried from one island to another while stuck on the feathers of a bird. What makes the Mapou tree unusual is the frequency with which its seeds actually entangle the birds on which they are stuck, making the birds unable to fly and thus killing them. Pisonia seeds are produced periodically several times a year and during these periods hundreds of birds might die after being entangled in the seed clusters.
These interactions between Pisonia and birds have long fascinated visitors to tropical seabird colonies, but had not previously been quantitatively studied. The frequency of the fatal encounters raises the possibility that the tree gains some benefit, perhaps by ensuring that its seeds germinate in the presence of a carcass which might provide essential nutrients. So, are the bird deaths ‘deliberate’ and advantageous to the tree in some way, or are they an accident of its seed dispersal method?
Working with Nature Seychelles, I carried out a study of the phenomenon on Cousin Island. I investigated the production, germination, survival, and tolerance to seawater of Pisonia seeds, and the occurrence of entanglement with birds.
I examined three possible benefits to Pisonia that might result from producing such extremely sticky seeds:
1. Enhanced long-distance dispersal
2. Enhanced germination and seedling survival
3. Increased nutrients benefiting the established trees.
|A fairy tern got caught in pisonia seeds © Alan Burger|
Cousin Island is an important nesting colony for many seabirds, and all seven nesting species were recorded to carry Pisonia seeds stuck to their feathers. Land birds seldom had seeds attached. Through a series of experiments I discovered that Pisonia seeds could not withstand prolonged immersion in seawater. Seeds floating ashore on a bird carcass would therefore be unlikely to germinate. Furthermore, a seed-covered carcass which washed ashore would not be likely to end up in soil where Pisonia might grow. On Cousin, bird carcasses that washed ashore were rapidly eaten and buried by ghost crabs within the salty inter-tidal zone where Pisonia seeds would not germinate or seedlings survive.
On the other hand, my experiments showed that the seeds could tolerate periodic dipping into seawater for up to four weeks. Seeds stuck on a living seabird might therefore persist and be deposited ashore when the bird preened its feathers. Dr. Chris Feare, a long-time researcher of Seychelles seabirds, reported the establishment of Pisonia trees on Bird Island under palm trees where Noddies roosted and preened during their annual moult. Long-distance island-to-island dispersal via living seabirds is thus highly likely.
Additional experiments using thousands of Pisonia seeds showed that seeds attached to a carcass did not germinate or survive any better than those planted nearby without carcasses. Seedling production was actually slightly lower when attached to carcasses, probably as a result of disturbance by crabs and other scavengers at the carcass. With or without carcasses, very few seedlings were produced and few survived (0.1% of 6,020 seeds survived as seedlings after 2–8 months). Seed production therefore is not essential to maintain the local Mapou forests and bird carcasses do not help to establish new Pisonia plants. Nearly all Pisonia trees in these forests grow as vegetative sprouts from fallen branches. The seeds have evidently not evolved their extreme stickiness as a means of deliberately killing birds in order to enhance the germination and success of their seeds.
It seems doubtful that the extreme stickiness of the seeds would have evolved primarily for dispersal within an island either. This could be achieved just as readily through less-sticky seeds that are preened off the feathers of birds remaining on the island where they encountered the seeds. Long-distance dispersal seems to be the main benefit.
Although birds killed by entanglement add nutrients to the soil, the amounts are trivial compared with the massive inputs from guano, failed eggs and dead chicks coming from the tens of thousands of breeding seabirds typically found on Pisonia-dominated islands.
My study suggests that killing birds confers no benefit to Pisonia grandis. Why then are the seeds so sticky that they can cause the deaths of hundreds of birds a year? I found that immersion in water gradually reduced the stickiness of the resin. In order to remain on a seabird long enough to reach another island, the seeds have to start out extremely sticky indeed. Seeds that were not sticky enough were lost along the way whereas those that were extremely sticky remained on the birds and had a chance of establishing a Pisonia tree to colonise a new island. The entanglement and deaths of some of the birds can be viewed as an unfortunate negative side-effect of natural selection for such extremely sticky seeds. Pisonia has clearly evolved to grow on seabird-dominated islands and it is one of the few trees that thrives in the acidic guano deposits produced by large numbers of nesting seabirds.
This study was funded by BirdLife Seychelles (now Nature Seychelles), and was published in full in the Journal of Tropical Ecology Vol. 21 pp. 263-271 (2005).