Effects of tree diversity on insects
Submitted by editor on 7 April 2015.Get the paper!Plant diversity, typically measured as the number of plant species at a local site, plays a primary role in regulating and maintaining many ecosystem processes such as decomposition rates, plant growth, and resistance against disturbance. In addition, plant diversity has been shown to influence species at higher trophic levels (herbivores, predators) either through effects on plant biomass (increased resource base for consumers) or through increased habitat heterogeneity which influences consumer movement. Similarly, recent studies have also shown that plant genotypic diversity (number of genotypes of a plant species at a local site) can also result in equivalent dynamics to those previously described for species diversity. While the effects of plant genotypic and species diversity have been well documented, they have usually been studied separately and this has limited our ability to determine their relative importance and joint influence on ecosystem function and consumer interactions.
In December 2011, we established a long-term tropical forest diversity experiment in southeast México (Yucatan) to evaluate the effects of plant diversity on insect herbivores feeding on these trees. This system is unique in that we simultaneously evaluate the effects of tree genotypic and species diversity. To test for species diversity effects we created monoculture plots (of one species) of each of six species and polyculture plots represented by mixtures of four out of the six species, whereas to evaluate genotypic diversity we selected one of the tree species, big-leaf mahogany (Swietenia macrophylla), and created monocultures of one and four (out of six possible) genotypes of this species. During the first year of establishment of the experiment, we measured the abundances of a stem-boring caterpillar, a leaf-mining caterpillar, and of three species sap-feeding leafhoppers. Both caterpillar species are dietary specialists that they only feed on mahogany in this system, whereas the leafhoppers are generalists that feed on all tree species.
Data collected during the first growing season (2012) revealed contrasting effects of each source of tree diversity. Moreover such effects were contingent upon the specific herbivore species looked at. Specifically, we found no effect of mahogany genotypic diversity on any of the insect herbivore groups looked at, whereas tree species diversity had strong effects on the abundance of both specialist caterpillars but neither of the generalist leafhoppers. Species diversity effects on leaf-mining caterpillars were consistently negative (lower abundance in polyculture than monoculture), whereas effects on stem-boring caterpillars ranged from negative to positive throughout the sampled period.
Why did tree species diversity effects depend upon herbivore identity? Total plant number per plot was held constant, so that each species was less abundant in polyculture than in monoculture. Therefore, tree species diversity effects on specialist caterpillars could have taken place due to reductions in the density of mahogany (their host plant) at high relative to low diversity, thus lowering herbivore recruitment to diverse plots. In contrast, generalist leafhoppers fed equally on all tree species and are thus not expected to be influenced by reductions in density of any given tree species with increasing diversity. These findings are in keeping with previous studies showing that dietary specialist herbivores are more responsive to changes in plant diversity than generalists. This is an important consideration in predicting the effects of plant diversity on associated fauna: plant diversity may promote or reduce insect diversity and abundance depending on the representation of generalist vs. specialist herbivore species at a given site, which has important implications for conservation purposes in natural systems as well as for pest management in cultivated systems.
Why were mahogany genotypic diversity effects on herbivores absent? Trait variation among mahogany genotypes may have not been great enough to influence these caterpillar species, resulting in weak diversity effects. Thus, following the same logic as above, reductions in density of specific genotypes in genetically diverse plots would have not had an influence on herbivore recruitment as there would be no preference for a particular genotype or set of genotypes. However, we found substantial variation among mahogany genotypes in anti-herbivore defenses as well as in damage caused by these caterpillars, suggesting some other underlying mechanism for these findings. It is possible that genotypic diversity effects were weak because of the spatial scale of our experiment. The magnitude of plant trait variation needed to manifest a diversity effects on insect herbivores must increase with increasing spatial scale. Based on this, our experiment may straddle this threshold such that the effects of tree species diversity were strong enough to matter at the studied scale whereas genotypic diversity effects were not. Such findings have important implications for understanding the relative role of plant genotypic and species diversity on associated faunas, and suggest that unaccounted effects of spatial scale, magnitude of plant trait variation, and insect movement are underlying such dynamics.
Photograph legends:
Photo 1. Tree species polyculture plot at the forest diversity experiment in Yucatan (Mexico); photo taken in November 2012. Person standing in middle of plot is Nicolas Salinas, field assistant in the project.
Photo 2. Stem-boring caterpillar (Hpysipyla grandella) feeding inside a stem of a big-leaf mahogany (Swietenia macrophylla) sapling.