Abstract
There is growing evidence for the influence of plant intraspecific variation on associated multi-trophic communities, but the traits driving such effects are largely unknown. We conducted a field experiment with selected genetic lines of the dioecious shrub Baceharis salicifolia to investigate the effects of plant growth rate (two-fold variation) and gender (males vs. females of the same growth rate) on above- and belowground insect and fungal associates. We documented variation in associate density to test for effects occurring through plant-based habitat quality (controlling for effects of plant size) as well as variation in associate abundance to test for effects occurring through both habitat quality and abundance (including effects of plant size). Whereas the dietary specialist aphid Uroleucon macaolai was unaffected by plant sex and growth rate, the generalist aphid Aphis gossypii and its tending ants (Linepithema humile) had higher abundances and densities on male (vs. female) plants, suggesting males provide greater habitat quality. In contrast, Aphis and ant abundance and density were unaffected by plant growth rate, while Aphis parasitoids were unaffected by either plant sex or growth rate. Arbuscular mycorrhizal fungi had higher abundance and density (both marginally significant) on females (vs. males), suggesting females provide greater habitat quality, but lower abundances (marginally significant) and higher densities on slow- (vs. fast-) growing genotypes, suggesting slow-growing genotypes provided lower resource abundance but greater habitat quality. Overall, plant sex and growth rate effects on associates acted independently (i.e., no interactive effects), and these effects were of a greater magnitude than those coming from other axes of plant genetic variation. These findings thus demonstrate that plant genetic effects on associated communities may be driven by a small number of trait-specific mechanisms.
Highlights
There is mounting evidence for the influence of plant intra-specific genetic variation on associated multi-trophic communities
We found a significant effect of growth rate on total plant biomass (Table 1), with fast-growing genotypes (547.39 ± 32.42 g) having 1.9x more biomass than slow-growing genotypes (287.60 ± 31.75 g) (Fig. 1), a result that was consistent with the differences observed in our source common garden (Fig. A1)
We found a significant sex × growth rate interaction (Table 1), where fast males (599.05 ± 42.77 g) had more biomass than fast females (495.74 ± 40.21 g), but slow females (350.15 ± 41.37 g) had more biomass than slow males (225.05 ± 39.68 g) (Fig. 1); as a result, the difference between fast and slow genotypes was greater for males (2.7x) than for females (1.4x), i.e. for the particular genotypes selected for this experiment there was more variation in growth rate for males than females
Summary
There is mounting evidence for the influence of plant intra-specific genetic variation on associated multi-trophic communities Studies have found that plant genotypes vary in arthropod community structure (Dungey et al 2000, Johnson and Agrawal 2005; reviewed by Bailey et al 2009), and that plant genetic variation influences antagonistic (Fritz 1995, Hare 2002, Mooney and Singer 2012) and mutualistic (Mooney and Agrawal 2008, Abdala-Roberts et al 2012) interactions associated with plant canopies. Past studies testing for plant intra-specific genetic variation on associated communities have sampled genotypes randomly and without regard to any particular trait of importance to consumers (Hughes et al 2008). Plant genetic effects are likely mediated by multiple plant traits, yet little is known of the relative importance of variation in different traits or whether the effects of such traits on associated communities act additively or synergistically
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