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.