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.