Genotypic richness of A. thaliana strongly influenced plant survival and biomass both in the presence and absence of insect herbivores. Plant biomass was 17% higher in genetic mixtures than in monocultures in insect-exclusion treatments, 5% higher in mixtures in insect-inclusion treatments and 11% higher in mixtures when all treatments were analysed together.
Our results are comparable with those of Crutsinger et al. (2006), who found a 36% increase in productivity in 12-genotype mixtures of Solidago altissima in the field. This is higher than the effect size seen here and raises the possibility that productivity may increase with richness in a linear fashion. Similarly, Johnson, Lajeunesse & Agrawal (2006) found a 27% increase in fitness (fruit production) of Oenothera biennis in mixtures containing up to eight genotypes compared with monocultures. Because seed production is highly correlated with above-ground biomass of A. thaliana (Cahill, Kembel & Gustafson 2005), it is possible that growth at high genotypic diversity may ultimately correspond to increased fitness of this species. It must be noted that our results are based on a semi-natural, mesocosm experiment and thus are not entirely comparable to results obtained in the field by Crutsinger et al. (2006) and Johnson, Lajeunesse & Agrawal (2006). However, the demonstration of positive genotypic diversity–productivity relationships in both natural and artificial settings suggests that these findings are biologically real, and may have important consequences for community structure and function in natural ecosystems.
Unlike recent experiments manipulating plant genotypic diversity only (Hughes & Stachowicz 2004; Crutsinger et al. 2006; Johnson, Lajeunesse & Agrawal 2006), the study described here is novel in that it manipulates insect herbivore presence or absence at two levels of diversity. By controlling for the influence of herbivores, we were able to demonstrate that the positive effect of genotypic diversity on plant performance is through plant-level interactions, and not necessarily mediated by insects (i.e. through selective herbivory). This is in contrast to the results of Hughes & Stachowicz (2004), who detected differences between low- and high-diversity eelgrass communities only after a grazing disturbance. A trade-off between fast growth rates, as in A. thaliana, an annual, weedy species, and resistance to disturbance, as in eelgrass (Hughes & Stachowicz 2004), may explain this discrepancy. We emphasize the need for concurrent manipulations of genotypic diversity and arthropods in natural settings to elucidate the role of multiple trophic groups in structuring diversity–productivity relationships.
Although the design of this experiment did not allow us to explicitly test the mechanisms responsible for higher plant biomass and survival at high genotypic diversity, several possibilities must be considered. First, mixtures may contain, by chance, one or more highly productive genotypes which drive total productivity. This ‘sampling’ effect (Huston 1997), however, is not entirely applicable to this experiment: all nine A. thaliana genotypes were present in equal proportions (1/9), thus all genotypes present in monocultures were equally and consistently represented in mixtures.
Rather, our results may be driven by non-additive mechanisms, which occur when productivity in mixture cannot be predicted from each genotype’s performance in monoculture, as in this study. For instance, complementarity (i.e. niche partitioning or facilitation) has been frequently demonstrated at the species level (Tilman et al. 2001; Hector et al. 2002; Cardinale et al. 2007), and may also play a role in diversity–productivity relationships within populations. While it may be argued that the functional differences among genotypes are smaller than those among species, intraspecific variation is nevertheless large enough to alter ecological processes (Hughes et al. 2008). Given the global distribution of the genotypes employed in this experiment, and the large among-genotype variability in important functional traits such as competition (J. F. Cahill, unpubl. data), it is not unreasonable to suppose that niche partitioning or facilitation led to the high performance of genetic mixtures. Variation in resource uptake strategies may lead to reduced intraspecific competition and greater overall resource capture in genetic mixtures. Decreased ammonium concentrations in sediments in diverse eelgrass communities (Hughes & Stachowicz 2004) suggest that niche partitioning may occur at the genotypic scale in a similar manner as at the species scale.
One alternative, although not mutually exclusive, explanation is that of a ‘selection’ effect (Loreau & Hector 2001; Hector et al. 2002), which can occur if species or genotypes with particular functional traits (i.e. high productivity) come to dominate a mixture over time. The duration of this experiment (one generation) did not allow any genotype to become numerous or dominant via enhanced reproductive success, thus the above definition may not apply. If, however, mortality rates within a genotype are a function of diversity, selection effects may occur through greater survival of highly productive genotypes in mixture than in monoculture.
Recent work also sheds light on the importance of genetic identity of neighbours to plant performance in diverse species (Fridley, Grime & Bilton 2007) and genotypic assemblages (Cahill, Kembel & Gustafson 2005). Such ‘neighbour effects’ may occur, for example, if a genotype with low fitness in monoculture expresses higher growth or lower mortality in the presence of particular neighbour genotypes in mixture. Similarly, competitive asymmetry (Weiner 1990) between genotypes may lead to performance in mixture (i.e. high productivity) which cannot be predicted solely from performance in monoculture. The identification of phenotypic plasticity and trait differences associated with diversity effects has been previously omitted from many biodiversity studies and constitutes a promising direction for future research (Hughes et al. 2008).
Non-additive and additive mechanisms, outlined above, are not mutually exclusive and may work in combination to drive biodiversity–productivity relationships. However, a detailed exploration of the relative contribution of each of these mechanisms to our results, although warranted, is beyond the scope of this article.
Herbivore survival and biomass were both significantly higher in mixtures than in monocultures of A. thaliana (Fig. 2). As with primary productivity, the potential mechanisms responsible for this pattern fall into two broad categories. First, herbivore responses to varying plant diversity may be additive, where observed biomass and survival of insects in genotypic mixtures are wholly predictable from performance in monoculture. Under this scenario, high insect biomass in genotypic mixtures is simply a result of greater plant quantity (see Primary productivity section, above), and we would expect to see a tight correlation between herbivore and plant biomass. Several lines of evidence indicate, however, that insect performance was not entirely dependent on primary productivity. Comparison of plant biomass in herbivore-exclusion and herbivore-addition treatments (roughly approximating plant biomass ‘before’ and ‘after’ herbivory) reveals that insects were unlikely to be limited by available biomass: the reduction of plant biomass due to insect presence was only 18% in monocultures and 26% in mixtures. We also compared herbivore biomass with plant biomass in each treatment. Herbivore biomass reflected general patterns in primary productivity (Fig. 1a,b; Fig. 2), but it did not match these patterns perfectly, nor did it closely follow the productivity of each genotype separately (Fig. 3). Linear regression analysis (not shown) indicated that plant biomass measured in insect-exclusion treatments explained only 22% of the variation in total herbivore biomass (slope = 0.13919, SE = 0.043, F1,37 = 10.663, P = 0.002, r2 = 0.224). Collectively, these results suggest that plant growth was an important, but not absolute, predictor of herbivore biomass.
Figure 2. Mean fresh biomass (± 1 SE) of all insects in an experimental pot in each diversity, density and fertility treatment combination.
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Alternatively, non-additive mechanisms, such as greater food quality, may account for the disparities in insect performance observed between genotypic monocultures and mixtures. Trichoplusia ni is considered a generalist species, known to feed on a multitude of agricultural crops and weeds (Cameron, Isman & Upadhyaya 2007), and as such may benefit from a mixture of food items compared with only one or a few plant species. A mixture of food items differing in nutritional quality and toxin content may act in a complementary fashion to improve herbivore growth rate and fitness, as has been demonstrated in laboratory feeding experiments performed on daphnids (DeMott 1998) and polyphagous insects (Bernays et al. 1994; Mody, Unsicker & Linsenmair 2007). Pfisterer, Diemer & Schmid (2003) found a strong, positive relationship between plant species richness and grasshopper biomass in a natural grassland ecosystem, indicating that the benefits of dietary mixing are not limited to artificial settings.
Although evidence for the benefits of dietary mixing has largely been restricted to studies examining plant species richness, similar principles may apply to genotypic richness. Host plant genotypes have been shown to differentially affect performance and fecundity of aphids on Solidago clones (Moran 1981; Maddox & Cappuccino 1986), aphids on Rudbeckia laciniata (Service 1984) and stem gallers on Solidago altissima (Cronin & Abrahamson 2001). Direct support for intraspecific dietary mixing was recently demonstrated by Mody, Unsicker & Linsenmair (2007), who found that the differences in food quality among host plant conspecifics were large enough to render mixed diets beneficial to lasiocampid caterpillars. In this study, genotype had a significant effect on herbivore biomass, suggesting that A. thaliana genotypes varied considerably in nutritional quality or palatability (Fig. 3). A reasonable explanation for the results of our study is that in mixture, genotypes acted in a complementary fashion to increase herbivore survival and biomass.
Interestingly, the relative palatability of each line changed as a result of fertilization, as evidenced by a marginally significant genotype × fertilization interaction in monocultures (Table 3). Some genotypes which were highly palatable relative to other genotypes at low fertilization were only somewhat palatable at high fertilization, and vice versa. In some cases (i.e. lines 2 and 4, Fig. 3d), fertilization actually had a negative effect on herbivore biomass, despite the strong, positive effect of fertilization in the study overall. The mechanism behind this interaction is unclear, and, unfortunately, impossible to determine from the data available.
Herbivores in natural systems have shown a variety of responses to manipulations of plant species diversity (Knops et al. 1999; Koricheva et al. 2000; Scherber et al. 2006a, 2006b, Unsicker et al. 2006). Studies examining plant intraspecific richness, however, have only recently been undertaken and field experiments have thus far shown either an increase (Crutsinger et al. 2008) or no change (Johnson, Lajeunesse & Agrawal 2006) of insect herbivore abundance. Our study is novel in that it demonstrates direct, positive effects of genotypic diversity on herbivore performance in semi-natural conditions, at multiple levels of resource availability and plant density. Further work is needed to determine whether this relationship is found for other insect species, and whether non-additive mechanisms, such as complementarity (i.e. dietary mixing), can account for these results.
Levels of herbivory
Although insect biomass was consistently higher in mixtures than in monocultures, levels of herbivory (as indicated by plant performance in the presence of T. ni) did not significantly differ between the two treatments. Similarly, despite greater insect biomass and survival at high than at low plant densities, levels of herbivory did not differ between these two treatments. Instead, herbivory was dependent upon an interaction between density and fertilization (Table 1): at high density, herbivory was higher in fertilized treatments, whereas at low density, herbivory was greater at low fertility (Fig. 1). Higher survival of insects in fertilized and high density plots (Table 2) may account for these results. The interaction lends some support to the idea that changes at the producer level can ‘cascade’ back and affect herbivory, although it is apparent that changes in plant and herbivore performance brought about by high diversity are not strong enough to produce similar patterns.