Plants that depend on pollinator mutualisms may be at risk of experiencing reproductive failure if interactions with pollinators are hindered or lost. Currently the size, frequency and severity of wildfires and ungulate herbivory pressure are increasing at a global scale, including arid ecosystems that exhibit great sensitivity to disturbance. To understand how fire and ungulate herbivory affect reproductive success of specialist pollinated desert plants, we quantified the sexual reproductive effort, success, pollinator visitation, and proportion of floral consumption by ungulate herbivores for two yucca species (Yucca baccata, and Yucca brevifolia) in burned and unburned areas of the Mojave Desert. Wildfire and herbivore effects on plant reproduction were assessed comparing the proportion of flowering plants and floral herbivory, pollinator collections, and the number of flowers and fruits produced per plant in burned and unburned areas. These were assessed in the spring of 2013 and 2014 along paired burned-unburned transects and a grid of survey transects positioned across multiple independent fires that occurred in the Beaver Dam Wash of the Mojave Desert in 2005.
Results/Conclusions
Yucca baccata plants in burned landscapes had a greater proportion of flowering individuals than plants in unburned areas (23% vs. 12%). All Y. brevifolia individuals in burned and unburned landscapes produced flowers. Fire had no effect on the number of flowers or fruits produced per plant for Y. baccata or Y. brevifolia. No fruits were produced among the thousands of flowering Y. baccata plants surveyed. Ungulate herbivores removed 64% of Y. baccata inflorescences in burned areas, compared to 51% in unburned locations. Forty-two percent of inflorescences produced on Y. Brevifolia branches ≤ 2m in height were removed in burned areas, compared to 39% in unburned areas. However, the majority of Y. Brevifolia inflorescences are produced > 2m where no inflorescence removal was observed. Yucca moths visited Y. brevifolia uniformly in burned and unburned areas but failed to visit Y. baccata altogether. Floral herbivory and a loss of obligate pollinator mutualisms appear to be critical threats to the long-term viability of Y. baccata populations in this landscape. Removal of livestock during peak flowering periods may be a critical first step to restoring Y. baccata’s pollinator mutualisms and capacity for sexual reproduction.