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Population and community consequences of ecological differences between the sexes

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Males and females commonly differ in ecologically important traits. These traits mediate the acquisition and allocation of resources, altering individual interactions with the abiotic and biotic environment. Environmental change may therefore affect the performance of one sex more than the other, biasing the sex ratio. Despite this ecological divergence, the sexes remain co-dependent for reproduction. Low frequencies of one sex limit the mating opportunities for the more common sex, potentially reducing population growth rate. Currently very little is known about how demographic differences between the sexes scale up to produce widely documented patterns of skewed sex ratios. Moreover, the consequences of sexual variation for the dynamics of populations and communities are poorly understood. This dissertation aims to bridge these gaps by mechanistically decomposing the origins of biased sex ratios, tracing the consequences of biased sex ratios for population dynamics, and the effects of this form of intraspecific variation on the structure of associated communities. Population sex ratios in the long-lived dioecious plant Valeriana edulis (Caprifoliaceae) are shown to respond to climatic variation across the species elevation range and to recent anthropogenic climate change because the sexes differ in a key physiological trait, water use efficiency. Biased sex ratios arise because of sex differences in multiple demographic processes including differential mortality and altered schedules of reproduction. Seed production is strongly limited when males are rare. However, population growth is not strongly affected by this mating failure, as this species’ longevity buffers against perturbations in reproductive rates. Finally, the sex-specific patterns of association in a multi-trophic arthropod community are disentangled through a series of field experiments that show that arthropod preferences for female plants are driven by direct plant-arthropod interactions. Here even higher trophic levels respond more strongly to trait variation between plant sexes rather than to numerical or qualitative changes in their prey and mutualist partners. Overall, this work explores a poorly studied but widespread axis of intraspecific variation, showing that ecological difference between the sexes are powerful drivers of ecological pattern and process.

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