Atmospheric nitrate radicals degrade floral scents, disrupting pollinator-plant interactions

Air pollutants reduce nocturnal hawkmoth pollination of evening primrose flowers by altering the flowers’ appealing scents, according to a new study that involved field experiments in Washington state. The findings illustrate the impact of anthropogenic airborne pollutants on an animal’s olfactory ability and suggest that such pollutants may limit global pollination. Human activities have drastically altered the environment. Sensory pollutants – human-introduced noise, artificial light, and chemical pollutants – can change animal behavior and fitness by introducing new stimuli or modifying naturally occurring stimuli used by animals’ sensory systems. Airborne pollutants, such as oxidants like ozone (O₃) and nitrate radicals (NO₃), are known to degrade the chemical compounds that produce floral scents. Many plant pollinators navigate long distances, drawn by the flowers’ scents while foraging for food. It’s thought that plant-pollinator interactions may be particularly susceptible to the effects of these airborne pollutants by making it difficult for insects to locate and pollinate flowers. However, little is known about how degradation of natural scents affects pollinator olfactory behaviors and plant fitness.

Jeremy Chan and colleagues investigated the effects of O₃ and NO₃ oxidants on nocturnal hawkmoth pollination of the evening primrose (Oenothera pallida). These desert flowers release a strong floral scent that attracts a rich diversity of pollinators. Through field observations in eastern Washington state, and laboratory experiments, Chan et al. discovered that NO₃ – the dominant oxidant at night in some polluted regions – rapidly degrades specific floral scent compounds, making flowers undetectable by nocturnally foraging hawkmoths. According to the findings, NO₃ was much more reactive than O₃ and selectively oxidized a specific subset of monoterpenes in the floral odor that hawkmoths use to recognize the flower. Oxidized scents resulted in a 70% (+/-20%) drop in flower visitation by hawkmoths, likely reducing plant fruiting and fitness. Using a global model, Chan et al. demonstrate that many urban areas have sufficient atmospheric O₃ and NO₃ pollution levels to significantly reduce the distances at which pollinators can sense flowers.