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

Oikos 000: 001–009, 2015 doi: 10.1111/oik.02033 © 2015 Th e Authors. Oikos © 2015 Nordic Society Oikos Subject Editor: Martijn Bezemer. Editor-in-Chief: Dries Bonte. Accepted 13 January 2015 Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores Luis Abdala-Roberts , Kailen A. Mooney , Teresa Quijano-Medina , Mar i a Jos e Campos-Navarrete , Alejandra Gonz a lez-Moreno and V i ctor Parra-Tabla L. Abdala-Roberts (labdala@uci.edu) and K. A. Mooney, Dept of Ecology and Evolutionary Biology, Univ. of California, Irvine, 321 Steinhaus Hall, Irvine, CA 92697-2525, USA. – LAR, M. J. Campos-Navarrete and V. Parra-Tabla, Depto de Ecolog i a Tropical, Campus de Ciencias Biol o gicas y Agropecuarias, Univ. Aut o noma de Yucat a n, Apartado Postal 4-116, Itzimn a , MX-97000 M e rida, Yucat a n, M e xico. – T. Quijano-Medina, Wageningen Univ., Droevendaalsesteeg 2, NL-6708 PB Wageningen, the Netherlands. – A. Gonz a lez-Moreno, Inst. Tecnol o gico de Conkal, Km. 16.3 Antigua Carretera M e rida-Motul, MX-97345 Conkal, Yucat a n, M e xico. Although the eﬀ ects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra- and interspecifi c), most studies have analyzed these eﬀ ects separately. We compared the eﬀ ects 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. Th e experiment consisted of fi fty-nine 21 ! 21-m plots, with 64 tree saplings each (3-m spacing between plants). Plots were either mahogany monocul- tures 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. Th roughout a fi ve-month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no eﬀ ect 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 signifi cant eﬀ ects on both specialists but neither of the generalist herbivores. Specifi cally, 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 eﬀ ects were not mediated by eﬀ ects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not infl uence spider abundance. Th is study is one of few to directly compare sources of plant diversity, and uniquely compares such eﬀ ects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity eﬀ ects outweigh those of genotypes, and our results suggest that such eﬀ ects are stronger on specialist than generalist herbivores. Evidence has mounted for the eﬀ ects of intra- (reviewed by Bailey et al. 2009) and inter-specifi c (Siemann 1998, Haddad et al. 2009) plant diversity on higher trophic levels. Numerous studies have found eﬀ ects of plant diversity on arthropod species richness and abundance (Koricheva et al. 2000, Crutsinger et al. 2006, Haddad et al. 2009) and on consumptive interactions at higher trophic levels (Moreira and Mooney 2013, Abdala-Roberts and Mooney 2014), with the basis of such eﬀ ects being variation in ecologically important traits among plant species or genotypes within species (Hare 2002, Mooney and Singer 2012). Within this context, a widely documented pattern is that greater plant diversity frequently leads to reductions in herbivory (reviewed by Andow 1991, Barbosa et al. 2009). Two hypotheses have been oﬀ ered to explain this phenomenon and invoke the infl u- ence of resource heterogeneity on consumers. According to the ‘ enemies hypothesis ’ (hereafter EH; Root 1973), greater habitat complexity at high plant diversity favors increased predator recruitment (e.g. because of greater availability of shelters or prey) resulting in stronger top – down suppression of herbivore populations and thus lower herbivory (reviewed by Russell 1989). Alternatively, the ‘ resource concentra- tion hypothesis ’ (RCH) (Root 1973) holds that herbivore foraging is density-dependent and increasing plant species diversity at a constant plant density reduces the probabil- ity of fi nding a preferred host plant, which lowers herbivore recruitment and damage on individual plants. While both hypotheses have received considerable attention, evi- dence from natural systems has generated mixed support (Bommarco and Banks 2003, Underwood et al. 2014). Th e inconsistent support for the EH and RCH could be due to unaccounted variation in herbivores traits (Vehvil a inen et al. 2007, Plath et al. 2012). For example, the dynamics predicted by the EH should be more likely to occur for generalist herbivores because they are more sus- ceptible to natural enemies as they frequently lack defense mechanisms found in specialists (e.g. crypsis, sequestration of plant toxic compounds; Mooney et al. 2012, Singer et al. EV-1