Tree Diversity Reduces Herbivory by Forest Insects. Jactel, H. & Brockerhoff, E. G. 10(9):835–848.
Tree Diversity Reduces Herbivory by Forest Insects [link]Paper  doi  abstract   bibtex   
Biodiversity loss from plant communities is often acknowledged to affect primary production but little is known about effects on herbivores. We conducted a meta-analysis of a worldwide data set of 119 studies to compare herbivory in single-species and mixed forests. This showed a significant reduction of herbivory in more diverse forests but this varied with the host specificity of insects. In diverse forests, herbivory by oligophagous species was virtually always reduced, whereas the response of polyphagous species was variable. Further analyses revealed that the composition of tree mixtures may be more important than species richness per se because diversity effects on herbivory were greater when mixed forests comprised taxonomically more distant tree species, and when the proportion of non-host trees was greater than that of host trees. These findings provide new support for the role of biodiversity in ecosystem functioning across trophic levels. [Excerpt: Discussion] Based on the evidence from 119 comparative studies of 47 different insect-tree interactions, our quantitative review showed that overall tree species growing in mixtures suffer significantly less herbivory than those in pure stands. This diversity effect on herbivory appears to be generally applicable at this forest stand scale as the studies used in our meta-analysis include a wide range of insect taxa and feeding guilds that affect trees from numerous plant families and orders in boreal, temperate and tropical biomes (see also Jactel et al. 2005). Overall, our study provides new and convincing support for the theory that species diversity of producer assemblages may reduce the magnitude of consumer effects on producers. Three similar meta-analyses that examined diversity effects in crop plant and algal communities (Tonhasca & Byrne 1994; Hillebrand & Cardinale 2004; Balvanera et al. 2006) also showed that more diverse communities were less affected by herbivore consumption. Our analysis gives support to the extension of this global pattern to long-lived producers such as tree species. However, the magnitude of this effect varied greatly in relation to an additional important factor that has been overlooked in previous studies, the host specificity of the primary consumer. The contrast between the responses of oligophagous and polyphagous herbivores to increased tree diversity was striking and merits further exploration. [\n] What distinguished the subpopulation of studies involving polyphagous insects in our meta-analysis was the significantly different effect size in those studies where other, more palatable species were present in mixed stands. In at least six of these studies (Brown et al. 1988; Gottschalk & Twery 1989; White & Whitham 2000) this result can most likely be attributed to 'associational susceptibility', an outcome of the occurrence of several host tree species in situations where polyphagous herbivores may be able to use a more palatable host to build up their populations, exploit those resources and then 'spill over' to the associated hosts (Brown & Ewel 1987; White & Whitham 2000). For example, gypsy moth (Lymantria dispar) can feed on conifers once its preferred hosts, oaks and other broadleaved species have been defoliated. As a result of this polyphagy, white pine growing in mixed stands with oaks is more likely to be attacked by L. dispar than white pine in pure stands (Brown et al. 1988). We were able to confirm this process as a general pattern by separating studies with polyphagous insects according to the presence or absence of other host tree species in the mixture. Our results clearly showed that such a detrimental effect of tree diversity is not uncommon but only when a more palatable host is present in the tree species assemblage. Similar effects may apply to generalist mammalian grazers which appear to prefer mixed stands in boreal forests (Koricheva et al. 2006). [\n] It is noteworthy that our analysis considers the overall effects of many different studies of which each examined a particular herbivore species on a particular focus tree species growing either in a pure or a mixed stand. It is conceivable that the observed overall reduction in herbivory by a particular oligophagous herbivore on a particular tree species could be outweighed by a corresponding overall increase in total herbivory by polyphagous species on all tree species present in mixed stands. To our knowledge, there has been no published study that specifically addressed this question in forest ecosystems; however, two observations can be offered that oppose this view: (i) collectively, overall herbivory by polyphagous species was not greater in mixed stands than in pure stands and (ii) oligophagous, i.e. specialist herbivores, are likely to exert a higher herbivory pressure than generalist, i.e. polyphagous species. This has been demonstrated particularly in tropical forests (Barone 1998, 2000). [\n] [...] [\n] Thus, our results are consistent with two of the main points in relation to the effects of biodiversity on ecosystem functioning on which ecologists recently agreed (Hooper et al. 2005): [::(i)] species composition can be more important than species richness per se and [::(ii)] the strength or the sign of the relationship can vary with the functional traits of the species. [\n] Our results confirm this because [::(i)] the beneficial effects of tree diversity on herbivory increase with the relative proportion of associated trees, [::(ii)] the association of phylogenetically less similar tree species, e.g. angiosperms associated with gymnosperms, is more effective in preventing herbivory in mixed stands and [::(iii)] the association of more palatable host trees is likely to increase damage from polyphagous herbivores in the mixture. [\n] These findings also highlight the fact that insufficient consideration of such factors may lead to erroneous conclusions about the existence of such effects of biodiversity, and whether or not they are widely applicable. This also has important implications for the sustainable management of forests, particularly planted forests which are predominantly managed as monocultures and are expected to soon supply the majority of the world's demand for wood and fibre products (FAO 2001). Enhancing biodiversity by enriching forests with additional tree species that are less palatable for pest insects may increase food web stability while at the same time potentially offering conservation benefits. However, more comprehensive studies are needed to determine whether there is a concomitant effect of forest diversity on herbivory, productivity and conservation.
@article{jactelTreeDiversityReduces2007,
  title = {Tree Diversity Reduces Herbivory by Forest Insects},
  author = {Jactel, Hervé and Brockerhoff, Eckehard G.},
  date = {2007-09},
  journaltitle = {Ecology Letters},
  volume = {10},
  pages = {835--848},
  issn = {1461-0248},
  doi = {10.1111/j.1461-0248.2007.01073.x},
  url = {https://doi.org/10.1111/j.1461-0248.2007.01073.x},
  abstract = {Biodiversity loss from plant communities is often acknowledged to affect primary production but little is known about effects on herbivores. We conducted a meta-analysis of a worldwide data set of 119 studies to compare herbivory in single-species and mixed forests. This showed a significant reduction of herbivory in more diverse forests but this varied with the host specificity of insects. In diverse forests, herbivory by oligophagous species was virtually always reduced, whereas the response of polyphagous species was variable. Further analyses revealed that the composition of tree mixtures may be more important than species richness per se because diversity effects on herbivory were greater when mixed forests comprised taxonomically more distant tree species, and when the proportion of non-host trees was greater than that of host trees. These findings provide new support for the role of biodiversity in ecosystem functioning across trophic levels.

[Excerpt: Discussion]

Based on the evidence from 119 comparative studies of 47 different insect-tree interactions, our quantitative review showed that overall tree species growing in mixtures suffer significantly less herbivory than those in pure stands. This diversity effect on herbivory appears to be generally applicable at this forest stand scale as the studies used in our meta-analysis include a wide range of insect taxa and feeding guilds that affect trees from numerous plant families and orders in boreal, temperate and tropical biomes (see also Jactel et al. 2005). Overall, our study provides new and convincing support for the theory that species diversity of producer assemblages may reduce the magnitude of consumer effects on producers. Three similar meta-analyses that examined diversity effects in crop plant and algal communities (Tonhasca \& Byrne 1994; Hillebrand \& Cardinale 2004; Balvanera et al. 2006) also showed that more diverse communities were less affected by herbivore consumption. Our analysis gives support to the extension of this global pattern to long-lived producers such as tree species. However, the magnitude of this effect varied greatly in relation to an additional important factor that has been overlooked in previous studies, the host specificity of the primary consumer. The contrast between the responses of oligophagous and polyphagous herbivores to increased tree diversity was striking and merits further exploration.

[\textbackslash n] What distinguished the subpopulation of studies involving polyphagous insects in our meta-analysis was the significantly different effect size in those studies where other, more palatable species were present in mixed stands. In at least six of these studies (Brown et al. 1988; Gottschalk \& Twery 1989; White \& Whitham 2000) this result can most likely be attributed to 'associational susceptibility', an outcome of the occurrence of several host tree species in situations where polyphagous herbivores may be able to use a more palatable host to build up their populations, exploit those resources and then 'spill over' to the associated hosts (Brown \& Ewel 1987; White \& Whitham 2000). For example, gypsy moth (Lymantria dispar) can feed on conifers once its preferred hosts, oaks and other broadleaved species have been defoliated. As a result of this polyphagy, white pine growing in mixed stands with oaks is more likely to be attacked by L. dispar than white pine in pure stands (Brown et al. 1988). We were able to confirm this process as a general pattern by separating studies with polyphagous insects according to the presence or absence of other host tree species in the mixture. Our results clearly showed that such a detrimental effect of tree diversity is not uncommon but only when a more palatable host is present in the tree species assemblage. Similar effects may apply to generalist mammalian grazers which appear to prefer mixed stands in boreal forests (Koricheva et al. 2006).

[\textbackslash n] It is noteworthy that our analysis considers the overall effects of many different studies of which each examined a particular herbivore species on a particular focus tree species growing either in a pure or a mixed stand. It is conceivable that the observed overall reduction in herbivory by a particular oligophagous herbivore on a particular tree species could be outweighed by a corresponding overall increase in total herbivory by polyphagous species on all tree species present in mixed stands. To our knowledge, there has been no published study that specifically addressed this question in forest ecosystems; however, two observations can be offered that oppose this view: (i) collectively, overall herbivory by polyphagous species was not greater in mixed stands than in pure stands and (ii) oligophagous, i.e. specialist herbivores, are likely to exert a higher herbivory pressure than generalist, i.e. polyphagous species. This has been demonstrated particularly in tropical forests (Barone 1998, 2000).

[\textbackslash n] [...]

[\textbackslash n] Thus, our results are consistent with two of the main points in relation to the effects of biodiversity on ecosystem functioning on which ecologists recently agreed (Hooper et al. 2005): [::(i)] species composition can be more important than species richness per se and [::(ii)] the strength or the sign of the relationship can vary with the functional traits of the species. 

[\textbackslash n] Our results confirm this because [::(i)] the beneficial effects of tree diversity on herbivory increase with the relative proportion of associated trees, [::(ii)] the association of phylogenetically less similar tree species, e.g. angiosperms associated with gymnosperms, is more effective in preventing herbivory in mixed stands and [::(iii)] the association of more palatable host trees is likely to increase damage from polyphagous herbivores in the mixture. 

[\textbackslash n] These findings also highlight the fact that insufficient consideration of such factors may lead to erroneous conclusions about the existence of such effects of biodiversity, and whether or not they are widely applicable. This also has important implications for the sustainable management of forests, particularly planted forests which are predominantly managed as monocultures and are expected to soon supply the majority of the world's demand for wood and fibre products (FAO 2001). Enhancing biodiversity by enriching forests with additional tree species that are less palatable for pest insects may increase food web stability while at the same time potentially offering conservation benefits. However, more comprehensive studies are needed to determine whether there is a concomitant effect of forest diversity on herbivory, productivity and conservation.},
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}
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