Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores

Abstract

Although the effects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra‐ and interspecific), most studies have analyzed these effects separately. We compared the effects of genotypic diversity of big‐leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large‐scale (7.2 ha) forest diversity experiment in southern Mexico. The experiment consisted of fifty‐nine 21 × 21‐m plots, with 64 tree saplings each (3‐m spacing between plants). Plots were either mahogany monocultures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Throughout a five‐month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no effect of mahogany genotypic 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, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such effects were not mediated by effects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not influence spider abundance. This study is one of few to directly compare sources of plant diversity, and uniquely compares such effects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity effects outweigh those of genotypes, and our results suggest that such effects are stronger on specialist than generalist herbivores.

Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions

“…For example, in a meta-analysis, Jactel and Brockerhoff [33] found that increased tree diversity reduced herbivory by specialist species, whereas the response of generalist herbivores was variable. These predictions are supported by recent studies in both tropical [9 ] and temperate [31 ] forest systems.…”

“…For example, in a meta-analysis, Jactel and Brockerhoff [33] found that increased tree diversity reduced herbivory by specialist species, whereas the response of generalist herbivores was variable. These predictions are supported by recent studies in both tropical [9 ] and temperate [31 ] forest systems.Other traits such as herbivore mobility [13,34,35] and feeding mode [36] are also thought to be important predictors of plant diversity effects on herbivores but have received comparatively less attention. In an insightful review, Bommarco and Banks [35] reported that highly mobile herbivores responded more strongly to plant diversity than sedentary herbivores because they can disperse more readily and choose among plant patches of varying diversity.…”

“…Accordingly, the range of trait variation among genotypes within a species may not be large enough in some cases for plant intra-specific diversity to influence herbivores ( [6,9 ,24], but see [23]). For instance, Abdala-Roberts et al [9 ] recently reported that tree species diversity notably reduced leaf miner attack on mahogany (Swietenia macrophylla) trees, but found no evidence of mahogany genotypic diversity effects on these herbivores. Similarly, Cook-Patton et al [6] found that plant genotypic and species diversity had effects of similar magnitude on insect herbivores, but that species diversity had stronger effects on predators.…”

Effects of Tree Genotypic Diversity and Species Diversity on the Arthropod Community Associated with Big‐leaf Mahogany

“…, Abdala‐Roberts et al . ), and overall effects of SD and GD on plant growth across all tree species were possibly also weak due to limited interactions between widely spaced tree saplings early in the establishment of this experiment. Biomass‐mediated effects of diversity on herbivore abundance in this system may emerge as plant–plant interactions become stronger.…”

Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment

“…Levels of leaf damage were already in a range that can substantially reduce tree growth if occurring chronically over several years [48], but lower than in most related studies (e.g. [18,20–22,49]), likely because the data collection was carried out relatively early in the season. Nevertheless, by studying herbivory on congeneric native and exotic tree species simultaneously, our results contribute to the lively debate on the direction of the tree diversity-herbivory relationship (e.g.…”