Warming disrupts fitness trade-offs in Oenothera biennis caused by herbivory
Climate change will dramatically alter plant reproductive fitness. In particular, warming can reduce seed mass, increase seed output, and inhibit germination in numerous plant species. However, we know relatively little about the effects of warming on plant fitness outside of arctic and alpine habitats. Herbivory also influences plant fitness through indirect and direct effects on seed production. Although warming can increase herbivory pressure by increasing herbivore abundances or per capita herbivory rates, there is no information regarding the ways in which warming modifies how insect herbivores impact plant reproductive output. Here, we report the results of a study designed to assess the joint influence of climate warming and herbivory on reproductive fitness of a biannual herb, Oenothera biennis. We used in situ field warming combined with herbivore exclusions to factorially manipulate temperature and herbivory pressure over two growing seasons. We measured flowering phenology, fruit production, and seed mass at the end of the second growing season.
At ambient temperatures, herbivores consumed a large proportion of flowers, reducing the number of viable fruits. Herbivory simultaneously reduced individual seed mass and increased seed production per fruit, which offset declines in fruit numbers, yielding little difference in total seed production between ‘Herbivore’ and ‘No Herbivore’ treatments at ambient temperatures. Warming stimulated flower production, reduced individual seed mass, and increased seed production per fruit in both the presence and absence of herbivores. Total lifetime seed production was highest in plants exposed to herbivores under warmed conditions, suggesting that warming enhanced the ability of O. biennis to compensate for herbivory. Oenothera biennis grown at ambient temperatures compensated for reduction in fruit numbers by produced fewer, bigger fruits packed with smaller seeds. Warming increased the number of fruits regardless of herbivore treatment, but herbivores still increased the number of seeds per fruit. Thus, warming led to increased reproductive output of plants exposed to herbivores compared to herbivore-free plants. Thus, climate warming may dramatically alter the ways in which insect herbivores influence plant community dynamics and evolution.