Beyond herbivory, plant sex effects on arthropod communities
By studying the effect of B. cordat a’s plant sex on specific groups of arthropods, we found that the only significant differences occurred on the carnivores: male plants hosted significantly more carnivores than female plants during June, while the opposite pattern was true during December (Figure 4). Observations of effects present only on higher trophic levels have been recorded before inBaccharis salicifolia whose female plants had 50% more predators than male plants, while there were no differences in herbivores (Nell et al. 2018). Consistent with these results, previous studies found examples of female plants hosting more natural enemies (Mooney et al. 2012, Petry et al. 2013) by increasing the availability of floral resources that provide nutritional benefits (i.e., nectar) to associated arthropod communities (Ashman and King 2005, Wäckers et al. 2005, Pacini and Nepi 2007). Such effects can increase indirect defenses from predators and parasitoids attracted to these floral resources (Cepeda-Cornejo and Dirzo 2010). Plants commonly use this strategy to mechanistically employ indirect plant defenses, increasing top–down effects (Kessler and Heil 2011). This dynamic is leveraged in agricultural systems, where they intercrop flowering plants as a biological control method (Letourneau et al. 2011, Bickerton and Hamilton 2012). Our findings, thus, lend support to the growing consensus that floral resources for natural enemies and predators are a key trait driving sexual dimorphism in the structure of plant-associated multitrophic communities.
Even if we did not find evidence for bottom forces occurring in B. cordata ’s associated arthropod community (i.e., no differences in plant traits and herbivory), there are many axes of variation that we did not capture in our study. In fact, a previous laboratory experiment reported that the looper Acronyctodes mexicanaria (Lepidoptera: Geometridae) preferred to feed from leaves of female B. cordataplants than from males and that caterpillars nourished with leaves ofB . cordata female plants were less susceptible to parasitoidism and developed faster, in comparison to caterpillars nourished in male leaves in the laboratory (García-García and Cano-Santana 2015). Because García-García and Cano-Santana’s study was conducted during October 2015, their results are consistent with a scenario in which female plants of B. cordata are preferred by herbivores during the fructification season, ultimately affecting the carnivore guild through bottom-up forces; consistent with the significant differences between carnivores we found in our study (Fig. 4). It is possible that our study didn’t capture the differences in the herbivore guild due to the expectation that, in terrestrial ecosystems, changes in the productivity of an ecosystem (i.e., the plant) will affect carnivores ten times more than herbivores according to the Lindeman rule (Lindeman 1942).
Not only the resources that dioecious plants offer to arthropods yearlong (like leaves) can vary in their quality, but the reproductive structures of plants are a resource in themselves (flower, pollen, fruits) and their availability is highly tied to time. Furthermore, pollen and fruits are resources produced exclusively in one of the two sexes and may favor different specific groups of arthropods such as pollinivores and fruit-eaters. For example, we noticed a higher abundance of thrips in male plants during August and October, probably associated with pollen consumption in male flowers. Pollen as a resource might be particularly important in plants with wind-dispersal strategies (Delph 1999), like B. cordata (dispersal type infered by floral morphology and personal observations). Unfortunately, the sample size for arthropod guilds, like pollinivores and fruit-eaters, in our study wasn’t big enough to conduct statistical analyses, but we hypothesize that the availability of flowers and fruits is a determining force of arthropod communities through bottom-up forces.