Pisonia grandis belongs to the Bougainvillea family and is a native tree to the Seychelles as well as other tropical areas of the world. It looks like a normal tree but there are several rather special adaptations which make it one of the most interesting tree species I have encountered, not to mention the most deadly!
Seabirds are intrinsically linked to the success of this species via zoochory but quite often lethal zoochory! The tree’s fruit produce hooked, sticky seeds (aka athocarps). The extreme stickiness of the seeds evidently evolved to stick to birds and resist removal, facilitating long-distance dispersal (Burger, 2005). Seabirds are the sole, long- range, dispersal vector of Pisonia (Walker 1991). The seeds are borne on multi-branched infructescences, each bearing from 12 to over 200 seeds. These usually fall to the ground when ripe. Birds become entangled in one or more infructescences and depending on where the seeds attach to the bird as few as 2-5 seeds can impair flight (). Once entangled and unable to fly the bird will become starved, exhausted and eventually die.
Either way the zoochory via this method is effective albeit the death of the vector is somewhat unnecessary.
The effect that Pisonia has on seabird populations has been studied on Cousin Island, initially it was thought that mortality via Pisonia was an unfortunate consequence but had little impact on the large populations of tree and ground nesting seabirds (Burger, 2005), however it is directly proportionate to the extent of the flowering event. Throughout the year there appears to be 3 main flowering events of varying extent, studies of the impacts of these events have been documented (Davies, 2010; Andrews, 2009). Results showed in both cases there was a significant impact on seabird populations, notably Davies (2010) recorded that both White Tern and Tropical Shearwater appeared particularly impacted with an estimated proportion of the populations entangled and killed of 24.52% (±7.92) and 9.38% (±3.95) for Tropical Shearwater respectively. Subsequent studies show that the species most impacted depends on the time of year the event takes place (Derand, 2010). Davies (2010) concluded that the current vegetation on Cousin may not represent the natural climax community with a current dominance of Pisonia which may be beyond natural levels, thus having the potential to cause substantial population declines, with the mortality rates appearing unsustainable in the long term, notably for White Tern, Tropical Shearwater, and White-Tailed Tropicbird.
Pisonia is found on many of the Seychelles islands that have had habitat restoration and subsequently is a key part of the habitat associated with high biodiversity and a complex food web. It is therefore not as easy as replacing Pisonia with other native tree species; It was discovered by Komdeur & Kats (2000) that Pisonia is the most common nest tree for the Seychelles Warbler, an endemic land bird brought back from near extinction by careful habitat management and translocation, thus showing that careful consideration of the entire island ecosystem is essential.
The decision was made by Nature Seychelles after analysis of the above mentioned studies to tentatively clear several forest plots of Pisonia and plant other native tree species. There will be ongoing monitoring of these plots so that a comparison can be made and impacts monitored closely. The desired outcome is to maintain as natural a habitat as possible, whilst reducing the Pisonia induced mortality with no impact to associated fauna.
Burger, A. (2005). Dispersal and germination of seeds of Pisonia grandis, an Indo-Pacific tropical tree associated with insular seabird colonies. Journal of Tropical Ecology, 21, 263-271.
Davies (2010). Lethal zoochory in pelagic birds – The impact of Pisonia grandis seed dispersal on seabird mortality on Cousin Island (West Indian Ocean). Masters of Science in Applied Ecology and Conservation. University of East Anglia, Norwich, 65 p.
Derand G. D. (2010). Monitoring the impact of Pisonia grandis on seabirds on Cousin island, February 2010. Unpublished report, Nature Seychelles, Republic of Seychelles.
Komdeur, J., & Kats, R. K. (1999). Predation risk affects trade-off between nest guarding and foraging in Seychelles warblers. Behavioral Ecology, 10(6), 648-658.
Walker, T. A., Chaloupka, M. Y. & King, B. R. (1991). The vascular floras of Bushy and Redbill Islands. Atoll Research Bulletin 350