The disappearance of large vertebrate seed dispersers from a tropical forest: what happens a hundred years later?

Background/Question/Methods Many tropical trees rely on large animals for the dispersal of their seeds, and the interactions between plants and their dispersal agents in tropical forests have long been a source of fascination for tropical ecologists. Singapore, an island of 700 km2 in the heart of the Asian tropics, once shared the same vertebrate fauna with the Malay Peninsula to the north. However, rapid development of the island led to equally hasty and large-scale habitat clearance and fragmentation which resulted in the catastrophic extinction of most of its large vertebrates. Large birds, such as hornbills and imperial pigeons, and most medium to large-sized ground dwelling mammals have been absent since the late 19th century. Results/Conclusions We have monitored the dynamics of a 2-ha primary forest plot in Singapore's Bukit Timah Nature Reserve for the past fifteen years. Tree species diversity appears not to have declined appreciably at Bukit Timah based on the results of our surveys, although there have been short-term fluctuations in stem abundance and basal area. However, to what extent has the disappearance of large seed-dispersing birds and mammals left a signature on tree community structure and distribution patterns at Bukit Timah? We looked for levels of clumping that might have resulted from severe dispersal limitation following the loss of seed predators, testing distribution patterns using methods of neighborhood density analysis. We have in fact found the distribution of certain species of nutmegs (family Myristicaceae), for example, which are hornbill and imperial pigeon-dispersed to be highly clumped, suggesting the imprint of disperser disappearance on their distribution. However, larger seeded nutmegs and other species with similarly large seeds (Durio, Canarium) do not appear to have been as badly impacted. The limited size of the plot and the small sample sizes for some tree species may have influenced our findings, but results suggest that the loss of large seed-dispersing vertebrates may not impact plants with large seeds and propagules equally. It is possible that smaller mammals may help maintain tree community structure long after larger seed dispersal agents are gone.