The recruitment of seedlings after disturbance sets the trajectory for future forest composition. Seedling recruitment depends on seeds reaching a given area and then successfully navigating the gauntlet of abiotic and biotic challenges in that spot. The importance of the dispersal step in this process is often under-appreciated, perhaps because the vast majority of seeds that reach an area are unable to establish. However, we hypothesize that the composition of frugivores in a forest community influences seedling recruitment patterns and ultimately sets the trajectory for future forest composition. To assess the influence of dispersers on forest regeneration, we compared seedling regeneration across three islands that vary in their frugivore communities. Guam has lost nearly all frugivores due to a non-native snake, leaving only feral pigs for dispersal. Rota has a depauperate bird community but still has fruit bats, and Saipan has a relatively intact bird community, but no bats or pigs. We censused seedlings and adult trees along transects in highly degraded, regenerating, and intact forest on all three islands. We recorded the abundance of seedlings, saplings, and adults. We distinguished seedlings that were likely dispersed from a nearby adult via gravity or from afar through animal-mediated seed dispersal.
Results/Conclusions
The disperser community present on each island is reflected in the seedling community composition. First, there are few seedlings in the intact forest away from their conspecific adults on Guam, compared to other islands. This is problematic for the trajectory of intact forest on Guam, as the future is likely to be less diverse. The opposite pattern is seen in the highly degraded forest, where there are more dispersed seedlings on Guam than on nearby islands, likely due to the pervasive impacts of pigs. However, many of these dispersed seedlings are from non-native species, foretelling a future forest dominated by non-natives. Rota, which is the only island with a significant bat population, shows a clear signal of bats with increased richness and seedling abundance of bat-dispersed species in intact forest. This accidental experiment highlights the importance of dispersal as the first step of community assembly and demonstrates the critical importance of considering the frugivore community composition in passive forest restoration projects.
