Potential disruptions to seed dispersal mutualisms in Tonga, Western Polynesia

Aim As a result of the extinctions of several large frugivorous birds (and one flying fox) and the diminished ranges of others, Tonga now has a relatively small number of effective seed dispersers, especially for large-seeded plants. We estimate the bill width and gape of the two largest known extinct pigeon species (Ducula) present in Tonga prior to human arrival. We then estimate which native rain forest species had fruits that would have probably been included in the diets of these extinct birds but which may not be dispersed regularly by the smaller frugivores present today. Methods Bill width for the two largest extinct pigeons was derived by extrapolating from the size of fossil bones and measurements of skeletons of extant Ducula species. Ducula and Ptilinopus species have distensible jaws, and therefore their gape allows them to swallow objects broader than their bill width. We estimated the gape width and maximum ingestible fruit size that both the extinct and extant species could consume, by extrapolating from feeding observations of extant Ducula and Ptilinopus species (using known bill widths and fruit sizes). We determined whether those plant species whose fruit diameters exceed the mean gape width of the largest remaining avian frugivore in Tonga (Ducula pacifica Gmelin) appeared adapted for dispersal by birds or flying foxes. Results Approximately 79% of the native rain forest trees and lianas in Tonga produce fruits whose morphology suggests that they are adapted to vertebrate dispersal. Bill width estimates derived from the bones of the extinct Ducula sp. nova suggest that this pigeon could swallow fruit up to 48 mm in diameter, whilst the smaller Ducula cf. david Balouet & Olson is estimated to have been able to swallow fruit up to 36 mm in diameter. Plant species whose fruits are too large to be swallowed by Tonga's largest extant frugivorous bird (D. pacifica) yet display fruit characteristics suggesting birds were likely to have been their predominant vertebrate disperser in pre-human times include: Calophyllum inophyllum L., Cerbera odollam Gaertn, Planchonella garberi Christophersen, P. membranacea Lam., Pometia pinnata J. R. & G. Forst., Syzygium quandrangulatum (A. Gray) Merr. & Perry, Syzygium richii (A. Gray) Merr. & Perry and Terminalia catappa L. Main conclusions There is now no avian disperser in Tonga for plant species whose fruits measure >28 mm in diameter (the maximum gape of D. pacifica ). Although frugivorous flying foxes (Pteropus tonganus Quoy & Gaimard) may eat the fruit and disperse the seeds of the eight plant species identified with large fruits, the frequency of dispersal by birds was probably much higher in the past. Species that once relied on the large extinct pigeons for dispersal may have become reduced in abundance in their absence.