Sex-specific variation in species interactions matters in ecological communities

Understanding how natural communities and ecosystems are structured and respond to anthropogenic pressures in a rapidly changing world is key to successful management and conservation. A fundamental but often overlooked biological characteristic of organisms is sex. Sex-based responses are often considered when conducting studies at organismal and population levels, but are rarely investigated in community ecology. Focusing on kelp forests as a model system, and through a review of other marine and terrestrial ecosystems, we found evidence of widespread sex-based variation in species interactions. Sex-based variation in species interactions is expected to affect ecosystem structure and functioning via multiple trophic and nontrophic pathways. Understanding the drivers and consequences of sex-based variation in species interactions can inform more effective management and restoration.

Sex-driven neighborhood effects on herbivory in the dioecious Mediterranean palm Chamaerops humilis L

Although it is well recognized that the strength of plant-herbivore interactions can vary with the plant sex, the distance, and the density of conspecific neighbors, no study has yet assessed their combined influence. Here, we filled this knowledge gap by focusing on the dioecious palm Chamaerops humilis L., and its two main herbivores, the invasive moth Paysandisia archon Burmeister and the feral goat Capra hircus L. We evaluated levels and spatial patterns of herbivory, as well as those of plant size and number of inflorescences in two palm populations in Mallorca (Balearic Islands, Spain). Our spatial point pattern analyses revealed that palms not affected by moth herbivory or goat florivory were spatially aggregated, goats fed more strongly upon inflorescences in palms with more neighbors, but they consumed more leaves in isolated palms. Interestingly, we could reveal for the first time that plant sex is a key plant trait modulating neighborhood effects. For instance, whereas aggregated female palms experienced lower intensity of goat florivory than isolated ones, male palms showed the opposite pattern. Palm size and number of inflorescences also showed sex-related differences, suggesting that sexual dimorphism is a key driver of the observed neighborhood effects on herbivory. Our study highlights the importance of considering relevant plant traits such as sex when investigating plant neighborhood effects, calling for further research to fully understand the dynamics governing plant-herbivore interactions in dioecious systems.

Salix myrtillacea Female Cuttings Performed Better Than Males under Nitrogen Deposition on Leaves and Drought Conditions

Drought and nitrogen (N) deposition are major threats to global forests under climate change. However, investigation into how dioecious woody species acclimate to drought and N deposition and how this is influenced by gender has, so far, been unexplored. We examined the phenotypic and physiological changes in Salix myrtillacea females and males under 60 d drought, and wet N deposition on leaves’ treatments. Drought inhibited their growth by limiting water acquisition, photosynthesis, and increasing oxidative stress, especially in males. However, females exhibited greater drought resistance than males due to their better water acquisition ability and instantaneous water use efficiency (WUEleaf), higher foliar abscisic acid (ABA) and auxin (IAA) levels and greater antioxidase activities. N deposition increased foliar ABA, H2O2 accumulation, and reduced N distribution to the leaves, causing restricted photosynthesis and aerial growth in males. Interestingly, N deposition improved biomass accumulation in both the genders under drought, with greater positive effects on drought-stressed males by increasing their radial growth and causing greater N distribution to the leaves, increased foliar IAA and reduced oxidative stress. Regardless, S. myrtillacea females still showed better growth and drought resistance than males under both drought and N deposition. The females’ superior performance indicated that they are more appropriate for forestation, thus supporting the dominant gender’s selection in the afforestation of unisexual S. myrtillacea in drought and severe N deposition regions.

Nitrogen-influenced competition between the genders of Salix rehderiana

Male and female willow plants show spatial segregation of genders along the environmental gradients. The skewed gender ratio of willows is related not only to altitude, but also to nutrient status and sexual competition, which can affect their growth and defense by altering secondary metabolite production. The relationship between metabolites and nutrients in the two genders of Salix rehderiana was explored in the Gongga Mountain. We found that the gender ratio was altered with a change in soil nitrogen (N) status; in the high N habitats, secondary metabolites accumulated in males. Furthermore, a pot experiment was conducted to test the effect of N supply on gender competition in S. rehderiana. Sufficient N supply stimulated females to produce amino acids and carbon (C)-containing secondary metabolites for maintaining their C-N balance, but extra N for males was used for growth to occupy more space. Nitrogen supply induced foliar nutrient imbalances and growth of opportunistic species, allowing them to outcompete neighbors. Better C allocation and storage in male than female willows would benefit intersexual competitiveness of males if environment N increases. Competition between the genders has a significant correlation with skewed gender ratio, spatial separation and resource utilization. Female willows would suffer fiercer competition for space by males with the increased soil N, which would result in the gender ratio alteration. Therefore, gender ratio of willows is likely to convert to gender balance from female-biased with long-term N deposition in the future.

Relative effects of genetic variation sensu lato and sexual dimorphism on plant traits and associated arthropod communities

Intraspecific plant trait variation can have cascading effects on plant-associated biotic communities. Sexual dimorphism is an important axis of genetic variation in dioecious plants, but the strength of such effects and the underlying mechanisms relative to genetic variation are unknown. We established a common garden with 39 genotypes of Baccharis salicifolia sampled from a single population that included male and female genotypes and measured plant traits and quantified associated arthropod communities. Genetic variation sensu lato (genotypic variation) had strong effects on most plant traits (flower number, relative growth rate, specific leaf area, percent water content, carbon-nitrogen ratio, monoterpene but not sesquiterpene concentrations) and on herbivore and predator density, and on arthropod community composition (relative abundance of 14 orders). In contrast, sexual dimorphism had weaker effects on only a few plant traits (flower number and relative growth rate), on predator density, and on arthropod community composition, but had no effect on herbivore density. Variation in flower number drove genetic variation sensu lato and sex dimorphism in predator density and arthropod community composition. There was unique genetic variation sensu lato in herbivore density (positively) associated with monoterpene concentration and in arthropod community composition associated with specific leaf area and carbon-nitrogen ratio. There was unique sexual dimorphism in arthropod community composition associated with plant relative growth rate. Together, these results demonstrate that genetic variation sensu lato and sexual dimorphism can shape plant-associated arthropod communities via both parallel and unique mechanisms, with greater overall effects of the former.

Consumptive and nonconsumptive effect ratios depend on interaction between plant quality and hunting behavior of omnivorous predators

Predators not only consume prey but exert nonconsumptive effects in form of scaring, consequently disturbing feeding or reproduction. However, how alternative food sources and hunting mode interactively affect consumptive and nonconsumptive effects with implications for prey fitness have not been addressed, impending functional understanding of such tritrophic interactions. With a herbivorous beetle, two omnivorous predatory bugs (plant sap as alternative food, contrasting hunting modes), and four willow genotypes (contrasting suitability for beetle/omnivore), we investigated direct and indirect effects of plant quality on the beetles key reproductive traits (oviposition rate, clutch size). Using combinations of either or both omnivores on different plant genotypes, we calculated the contribution of consumptive (eggs predated) and nonconsumptive (fewer eggs laid) effect on beetle fitness, including a prey density‐independent measure (c:nc ratio). We found that larger clutches increase egg survival in presence of the omnivore not immediately consuming all eggs. However, rather than lowering mean, the beetles generally responded with a frequency shift toward smaller clutches. However, female beetles decreased mean and changed clutch size frequency with decreasing plant quality, therefore reducing intraspecific exploitative competition among larvae. More importantly, variation in host plant quality (to omnivore) led to nonconsumptive effects between one‐third and twice as strong as the consumptive effects. Increased egg consumption on plants less suitable to the omnivore may therefore be accompanied by less searching and disturbing the beetle, representing a “cost” to the indirect plant defense in the form of a lower nonconsumptive effect. Many predators are omnivores and altering c:nc ratios (with egg retention as the most direct link to prey fitness) via plant quality and hunting behavior should be fundamental to advance ecological theory and applications. Furthermore, exploring modulation of fitness traits by bottom‐up and top‐down effects will help to explain how and why species aggregate.