How Do Females Persist? Life History, Environment, and Female Advantage in Gynodioecious Cirsium

Gynodioecy-the coexistence of females and hermaphrodites-raises a fundamental question in evolutionary ecology: how do females persist when hermaphrodites can both sire and set seed? We addressed this by quantifying how life-history strategy (polycarpic vs. monocarpic), trait-specific components of female advantage, and environmental context shape female frequencies in two gynodioecious thistles with contrasting life histories: the polycarpic Cirsium heterophyllum and the monocarpic C. palustre. Female frequencies differed sharply between species (mean 53.3% vs. 19.3%), and this divergence closely matched species-level female advantage, calculated as the ratio of viable achene production in females relative to hermaphrodites, which was strong in C. heterophyllum (3.77) but weak in C. palustre (1.26). Across species, female advantage arose from reproductive rather than vegetative traits: females produced more florets per plant, had higher achene viability, and experienced lower predispersal granivory than hermaphrodites. However, the dominant trait component differed with life history: achene viability was the primary contributor to female advantage in polycarpic C. heterophyllum, whereas enhanced floret production contributed most strongly in monocarpic C. palustre. Environmental predictors had limited explanatory power, with temperature seasonality (BIO4) accounting for only a small fraction (6.5%) of variation in female frequency in C. palustre and no significant effects in C. heterophyllum. Overall, our findings suggest that female persistence in the two studied Cirsium species partly reflects differences between polycarpic and monocarpic life-history strategies, with reduced susceptibility to granivory providing modest reinforcement and environmental filtering playing a minimal role.