Print PageA crucial step for understanding the impact of ecosystem engineers on community structure is to quantify the abundance and distribution of the resources they produce. In Brazilian cerrado (savannah) trees, stem-boring beetles act as ecosystem engineers by creating cavities in branches that are subsequently occupied by ants. As a starting point for understanding the consequences of stem-boring for arthropod community structure on savannah trees, we quantified the impact of host plant characteristics on the frequency and size of stem-bored cavities. Specifically, we tested the hypothesis that tree species with extrafloral nectaries would have fewer cavities (due to ant protection) but have a greater percentage of cavities occupied by ants (because of resources provided by the nectaries) than tree species without extrafloral nectaries. Three of the study tree species [(Caryocar brasiliense (Caryocaraceae), Stryphnodendron polyphyllum (Fabaceae), and Qualea grandiflora (Vochysiaceae)] have extrafloral nectaries while three do not [Kielmeyera coriacea (Clusiaceae), Machaerium opacum (Fabaceae), and Sclerolobium aureum (Fabaceae)]. We destructively sampled 5-15 stems per tree, 8 trees per species, for each of the 6 tree species. Stems were sectioned, stem diameter measured, length and diameter measured of any cavities, cavity occupants recorded, and diameter of exit holes for beetles measured.
Results/Conclusions
We found that the six tree species varied in their potential as a resource for exploitation by canopy ants. The number of cavities per stem length and percent of stem length bored by beetles varied 5-fold across tree species. Both variables were significantly greater with increasing tree size in Q. grandiflora but not in the other five species. Cavity length and mean cavity diameter also varied by tree species, both potentially contributing to larger nest size. Mean exit hole diameter (made by beetles after completing the larval portion of their life cycle) did not vary by tree species, but the range of hole sizes available did. We believe that exit hole size determines the size of the ant species that can use the cavity. Thus a greater range of hole sizes should translate into a greater richness of ant species. Finally, there were significant differences among plant species in occupancy by ants of those cavities that had exit holes, ranging from 28% to 41%. Thus ant species do not consider all tree species equal. These interspecific differences do not appear to be related to the presence of extrafloral nectaries as originally hypothesized.
