Subsidient: Christiaan Spencer / Wageningen University
Subsidy: S192.61
The Beni-savannah is a forest-savannah mosaic composed of seasonally flooded savannah, isolated forest islands, and larger gallery forests which are dominated by the Bolivian endemic and key-stone species, the Motacú palm (Attalea princeps). Currently, the Motacú population on the forest islands is facing a collapse because of decreasing adult densities and reduced regeneration rates. This raises great concerns regarding the stability of the forest-savannah mosaic. This study aims to test whether the reduced Motacú regeneration on forest islands could be explained by distance- and density-dependent mortality caused by bruchid beetle seed predation. I used 45 sample plots in gallery forest and 21 plots on forest islands, and measured the adult-, seed- and seedling density and calculated the palm fruit predation rate by bruchid beetles at the plot level. I tested if palm seedling density decreases with distance-dependent mortality (i.e., the distance to adult palms), density-dependent-mortality (i.e., the seed density) and seed predation by bruchid beetles (i.e., number of exit holes per fruit). Bruchid predation levels were significantly lower on the forest islands compared to the gallery forests (49.2 vs. 76.4 % mortality) where Motacú regenerates abundantly (40.8 vs. 5.0 seedlings per plot). Seed predation by bruchids and seedling densities were not related to adult- and seed density and were therefore not found to be distance and density-dependent. The observed lack of regeneration on the forest islands is therefore not a result of bruchid beetle seed predation.
