Wildfire and floral herbivory alter reproduction and pollinator mutualisms of Yuccas and Yucca moths

Abstract

Aims Wildfire and ungulate herbivore pressure are increasing globally due to human activities, including arid ecosystems that are sensitive to disturbance, and are highly dependent on pollinator mutualisms. We evaluated how wildfire and ungulate herbivore pressure influence plant reproductive success and pollinator mutualisms. Methods We evaluated flower production, floral herbivory, pollinator visita-tion and fruit set of two Yucca species with highly specialized polli-nator mutualisms (Yucca baccata and Yucca brevifolia) in unburned and burned landscapes created by fires that occurred in the Mojave Desert in 2005. Important Findings Yucca baccata plants in burned landscapes had a greater proportion of flowering individuals than plants in unburned areas (23 versus 12%). All Y. brevifolia individuals in burned and unburned landscapes produced flowers. Sixty-four percent of Y. baccata inflorescences were removed due to herbivory by cattle in burned areas, compared to 51% in unburned locations. Forty-two percent of Y. brevifolia inflorescences on branches ≤2 m in height were removed in burned areas due to cattle herbivory compared to 39% in unburned locations. However, we did not observe any inflores-cence removal on Y. brevifolia branches >2 m where the majority of Y. brevifolia inflorescences are produced. Yucca moths, the only known pollinators of these two species, visited Y. brevifolia uniformly in burned and unburned areas but failed to visit Y. bac-cata altogether. Yucca brevifolia had strong fruit production, but not a single fruit was produced among the thousands of flowering Y. baccata plants surveyed. Floral herbivory and the loss of obligate pollinator mutualisms appear to be critical threats to the long-term viability of Y. baccata populations in this landscape. the reproductive resilience of Y. brevifolia and the maintenance of its pollinator populations appear to be due to its taller stature, which allows vertical escape of flowers from ungulate herbivores. Removal of livestock during the flowering period appears to be a critical first step to restoring Y. baccata's pollinator mutualisms and capacity for sexual reproduction.