The effect of plant sex on arthropod communities through time
Although in recent years it has become evident that there is not a
universal pattern in terms of plant-sex preference by herbivores (i.e.,
male vs female plants), many studies show that sexual dimorphism can
modify the availability and quality of resources for arthropod
herbivores, ultimately determining herbivore density on plants (Danell
et al. 1985, Hjaltén 1992, Boecklen et al.1990, Agren et al. 1999,
Carneiro et al. 2006). These effects on herbivores have the potential to
affect the rest of the arthropod community (i.e., carnivores) through
bottom-up forces (Chen y Wise 1999, Utsumi et al. 2009, Gruner 2004),
but most studies have only focused on understanding pairwise
interactions between a dioecious plant and one herbivore. In this study,
not only we evaluated the effect of B. cordata ’s sex on the whole
community of arthropods, but we also accounted for the potential
temporal variation of this interaction by studying the community every
two months during a year. Although we did not detect differences in the
total richness of species (0D ) between
plant-sexes at any point in time, our results showed that B.
cordata ’s sex affects the diversity of all the species
(1D ) and the diversity of the most abundant
species (2D ) (Fig. 3). We also found that the
sex-associated differences in arthropod communities have a strong
temporal component; depending on the date of observation, male or female
plants hosted more diverse arthropod communities (Fig. 3). During the
highest peak of the flowering season (June-July), the male plants showed
a more diverse arthropod community (1D and2D ), and during the peak fructification season
in October-November the female plants had higher diversity of arthropods
(1D and 2D ). These two
periods of plant-sex related differential diversity are separated by a
period of no statistical differences between the arthropod communities
(August-September). Our results highlight the dynamism of arthropod
communities associated with B. cordata , most likely driven by the
high seasonality of the whole PSAER ecosystem.
This finding exemplifies the importance of accounting for seasonal
effects when trying to understand the ecological interactions among
species that inhabit seasonal ecosystems. In fact, other studies have
recorded seasonal variation in metabolite contents in nutrients and
metabolite contents in plants inhabiting seasonal regions like the
savannas in South Africa (Scogings et al. 2015), the Atlantic rainforest
of Brazil (Suguiyama et al, 2014), and alpine regions in China (Dai et
al. 2014). Furthermore, Dai et al. (2014) showed that this variation
influences the preference of one herbivore. In fact, Rabska et al.
(2020) observed that differences in carbohydrates levels between male
and female plants of Juniperus communis varied through time, thus
constituting an example of the potential seasonal variation of sexual
dimorphism expression. Based on our results, we hypothesize that
dioecious species that experience seasonal environments might offer
different resources and conditions to the resident arthropods at
different times, which might translate into time-dependent diversity
patterns. For example, if chemical defense correlates with reproduction,
we might expect different peaks of chemical defense for males and
females; males having the higher concentration of defenses during
pollination, and females having the highest defense during
fructification season. This hypothesis is difficult to evaluate given
the current state of knowledge, which highlights the need of studies
that address the temporal component of sex-plant effect on arthropods.