How air pollution prevents pollinators from finding their flowers

News Article: https://www.washingtonpost.com/climate-environment/2024/02/08/air-pollution-pollinator-moths/

Scientific Publication: https://www.science.org/doi/10.1126/science.adi0858

As humans, we typically rely on our eyes and ears to facilitate daily movement and acquire our basic needs. It is usually far less necessary for us to rely on our use of sense for such tasks. However, this is not the case for all types of wildlife - particularly plants and animals - that occupy our shared ecosystems. As air pollution becomes an increasing concern in our daily lives, it also poses a growing concern for both the flora and fauna that depend on scent for communication and to fulfill their ecological niches. While recent research has demonstrated connections between dirty air and conditions such as Alzheimer's and dementia in humans, systematic studying of organisms that primarily rely on scent as their main sensory mode is limited. I chose the article "How air pollution prevents pollinators from finding their flowers", which discusses recent research published in Science. It summarizes a study on nocturnal pollinators, particularly hawkmoths (Hyles lineata and Manduca sexta), and their ability to locate their flowers of choice at night despite increasing NO₃ concentrations.

It is known that human-caused pollution (anthropogenic pollution) is a major source of NO₃ in our atmosphere, sourced from vehicular combustion, carbon fuels, fertilizers, and industrial emissions to name a few. Despite the continued release into the troposphere, concentrations of NO₃ are lower throughout the day due to the breakdown of these molecules in the presence of light. However, during the nighttime hours, these concentrations rise and are not alleviated until sunrise, resulting in increased reactions between NO₃, O₃, and volatile naturally found chemicals in our ecosystems. Because of this concern, research was performed to identify areas of concern.

The news article provides a comprehensive overview of the study, emphasizing the need for sensory research, explaining how the research team reached their conclusions, and acknowledging the limitations of the current study. It effectively outlines the anthropogenic sources of NO₃, the importance of scent for various species, and the relationship between NO₃ concentration and hawkmoths’ inability to locate primroses for pollination, supported by control experiments and lab-derived mixtures. Additionally, the article mentions another study measuring ozone levels around bees during the day, which yielded similar findings. However, the article falls short in discussing where NO₃ and O₃ concentrations are highest and how these trends have changed over time—an important point covered in the scientific article. In conclusion, the authors note the study’s limitations, particularly the small sample size and species diversity, and emphasize the need for further research before any calls to action can be made. This also recognizes that this study is the first of its kind, and we are still in the early stages of understanding its broader implications.

 

Fig. 2. Sensitivity of floral odor to degradation by free radicals. (A) GC-EAD traces of male (top) Hyles lineata and (bottom) Manduca sexta antennal response to O. pallida night scent sample from the field. Responses to the monoterpenes β-pinene and cis-β-ocimene, 2-methylbutanal oxime (aldoxime), benzaldehyde (benz.), and eucalyptol (euca.) are highlighted in gray. (B) Chemical equations for the formation of the nitrate radical (NO3) from nitrogen dioxide (NO2) and ozone (O3). Dinitrogen pentoxide (N2O5) forms reversibly from NO3 and NO2 and acts as a reservoir of NO3. During the day, ultraviolet light causes the dissociation of NO3 to NO2 and O, preventing the buildup of NO3. NO3 reaction with floral volatile organic compounds (fVOCs) rapidly yields reaction products that are not detected by the moths. (C) Schematic of the setup for generating NO3 from NO2 and O3 and oxidizing the fVOCs in a flowtube. (D) Example traces of O3 followed by NOx oxidation of 2-methylbutanal oxime (orange line), β-pinene (blue line), and cis-β-ocimene (green line), measured with a Vocus-PTR-TOF mass spectrometer. (Inset) Table of measured volatiles and their degradation rates under NOx and O3 oxidation (all differences P < 0.001, Welch’s t test/Mann-Whitney U test) (table S9). The O3 and NO2 concentrations in the flowtube were 120 and 60 ppb, respectively, which correspond to the upper range of highly polluted urban environments (29, 30). The reaction time in the flowtube was 73 s, which simulates the impacts on odor transmission within 50 to 100 m from the odor source.

The scientific study through which this news article mentioned, published in February of 2024, offers a more insightful and systematic study of this relationship. The authors identified impactful VOCs (volatile organic compounds) impacted through NO₃ reactions with headspace gas chromatography (GC) to formulate lab and environmentally comparable tests. Through these comparisons, reactions with NO₃ and O₃ found that NO₃ was the dominant impactor in disrupting various monoterpenes which provide the hawkmoths direction in finding their flower host. Plant visitation was tracked for 300 plants for over 200 hours to provide ample data. The study concludes by looking over historically relevant trends in NO₃ and O₃ concentrations on a global scale through GEOS-Chem models, highlighting areas of notable NO₃ and O₃ concentrations that have shifted since pre-industrial times. Important in this area was an extrapolation of anticipated "pollution recognition distance", based on wind conditions and pollutant concentrations. This emphasized areas of urban growth were among those with the lowest recognition distance. This work concluded that NO₃ negatively impacts plant fitness and insect pollinators on a regional basis but more work was needed to conclude what other ecological processes may also be affected.

The most important issue shown in both sources is that an abundance of NO₃ radical chemistry is to blame for the negative impacts on plant and insect pollination. Through the scientific study, this was outlined as a clear causation rather than a correlation effect, a difference that is often overlooked in the media. With diminished nocturnal and potential diurnal pollination seen in increased O₃ and NO₃ radical chemistry, a relation is seen in pollutant concentration, which hasn't been outlined before. While this was demonstrated on a small scale with few insect and plant couplings, a correlation is seen even among bees and hawkmoths noted in the media article. While these processes may be seen regionally depending on industrial growth as well as human presence, a clear relation is pointed out emphasizing the need for future studies in both sources, leaving readers interested in learning more about this impact. Seeing this is the first major study of its kind, omitting a call to action in both sources was beneficial seeing the problem at hand is not yet studied enough to offer conclusive solutions. 

I rate this Washington Post article a 9/10. This effectively raises awareness of an ongoing known process (human-sourced pollution) impacting natural pollination processes without preemptively raising steep alarms about what to do about it. I find the scientific explanation on this in the media to be sufficient in explaining how the conclusions were made and therefore they are not likely to be scrutinized by the average reader. An area for improvement can be seen in the omission of information emphasizing how the NO₃ and O₃ landscape has changed over the years. Although only outlined in Science, this detail could provide better insight on how the individual could potentially alter their practices in aiding our ecosystems, even in the most polluted areas. For instance, the article could prompt readers to reconsider their use of fertilizers and pesticides, many of which contain nitrates, particularly in maintaining manicured lawns. This practice may come at the expense of pollinators, which are crucial for the ecosystem's health.