Abstract
Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant anti-herbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses.
Highlights
Ecological research conducted over the last decade has shown that plant intra- and inter-specific diversity have large effects on ecosystem processes, such as decomposition rates and productivity [1,2,3,4,5,6], as well as on the structure of associated communities of consumers [1,2,3,7,8,9,10,11]
Consequences of plant genotypic and species diversity on mahogany defenses Genotypic diversity had a significant effect on the concentration of chemical defenses in stems (Model 0, Table 2a)
Our results showed that plant genotypic and species diversity had strong positive effects on stem anti-herbivore defenses in big-leaf mahogany, namely the concentration of stem polyphenolics and condensed tannins
Summary
Ecological research conducted over the last decade has shown that plant intra- and inter-specific diversity have large effects on ecosystem processes, such as decomposition rates and productivity [1,2,3,4,5,6], as well as on the structure of associated communities of consumers [1,2,3,7,8,9,10,11]. Numerous studies have found that plant diversity increases plant biomass production due to niche partitioning and more efficient resource use among species or genotypes within a given species (i.e., complementarity effect) [12,13] Such increases in plant biomass, as well as greater habitat complexity may in turn cascade up to influence consumers, in the case of arthropods associated with plant canopies [1,2,8,14,15]. High diversity might lead to reduced herbivore abundance (and damage) due to mechanisms of associational resistance such as reduction in host plant density (i.e. resource concentration effects) [24,25], and in turn reduced investment in plant defenses. This latter mechanism is predicted for specialist insect herbivores which are more sensitive to changes in the density of specific host plants [26,27]
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