Herbivory in tropical trees: Relative influence of tree genotype diversity, tree species diversity, and herbivore identity

Background/Question/Methods

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity, most studies have analyzed these effects separately. In this study, we compared the effects of genotype diversity of big-leaf mahogany (Swietenia macrophylla) with that of tree species diversity on specialist and generalist insect herbivores feeding on mahogany in a recently established (six-month old), large-scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of 74, 20 m × 20 m plots of 64 saplings each, with 3 m spacing between plants (n = 4780 plants). We sampled 59 of these plots which were either mahogany monocultures or species polycultures of four species (including mahogany). Within each of these two plot types, mahogany monocultures were composed of either one or four genotypes, and species polycultures had mahogany saplings represented by either one genotype or four genotypes (i.e. mahogany genotype diversity replicated across levels of tree species diversity). Mahogany genotypes represented seeds from maternal trees originating from different populations. Throughout a six-month period, we measured mahogany growth and monitored the abundance of two specialist caterpillars (Hypsipyla grandella stem borers and Phylocnistis meliacella leaf miners) and three generalist phloem-feeding (Cicadellidae) herbivores. 

Results/Conclusions

We found no effect of mahogany genotype diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had significant effects on both specialists but neither of the generalist herbivores. Specifically, we found that species diversity lowered H. grandella stem borer attack at the peak of the rainy season, but increased attack at the end of the season, whereas P. meliacella leaf miner attack was consistently reduced. Such effects were not mediated by effects of species diversity on plant biomass, but instead presumably by resource heterogeneity. In contrast, generalist leafhoppers were not influenced by tree species diversity. Collectively, these findings highlight that the specialist herbivores in this system are more responsive to plant species diversity than generalist herbivores, and that plant species diversity effects on herbivores outweigh plant genotype diversity effects.