While many primates seek shelter at night, we humans rely on artificial light to stay awake. Our methods of generating light at night have always created consequences for other species. Prior to electricity, street lamps were fueled by whale fat, which created severe hunting pressure on whales. Electricity reduced the demand for whale fat, while increasing the fraction of the Earth’s area that received a continuous source of light. Today, satellite images (Fig 1) demonstrate how we’ve illuminated the planet at night. Extended periods of light provide many advantages to people, but how does artificial light affect other species?
You don’t need eyes to benefit from light. Plants rely on sunlight to grow and develop, both of which are influenced by different amounts and types of light. Prior to our extensive use of artificial light, a normal day for a plant included periods of sunlight followed by darkness. However, many plants in urban habitats are now exposed to 24 hours of light every day. Researchers know that some plants grow faster and larger when exposed to extended periods of light. While enhanced growth benefits certain plants, such as our crops, increased growth becomes problematic when unwanted plants are the beneficiaries. If problematic plants thrive in environments with continuous light cycles, then these habitats deserve inclusion in strategies for managing plant populations and artificial light environments.
Species introduced into new regions become problematic when they replace species that we desire while also creating hazards to our economy and health. Cheatgrass is an Eurasian plant introduced into North America, where it often outcompetes North American plants. In addition, cheatgrass is highly flammable and poses a serious fire hazard for wilderness, agricultural, and residential areas. The majority of cheatgrass research occurs in wild and agricultural
environments, yet little is known about this species within urban habitats where artificial light sources might create environments favoring cheatgrass.
To fill this knowledge gap, we investigated cheatgrass growing in residential alleys within the Denver metropolitan area. Our goal was to determine whether the presence of cheatgrass is associated with streetlights. The 54 residential alleys we surveyed were paved and had 81 poles with or 192 poles without streetlights, which allowed us to isolate the effects of artificial light from the effects of the disturbance created when a pole is installed. To determine whether the disturbance from pole installation rather than streetlights affected plants, we also included 239 sites that lacked any poles. At the base of the poles were several plants species, including cheatgrass. We found that cheatgrass was more likely to occur at sites with streetlights than at sites with only poles or sites without any poles.
Our results highlight an often overlooked relationship between urban habitats and unwanted plant species. We’ve always known that disturbance from human activities, such as construction, can promote the growth of introduced species along roadsides and development zones. However, our new results show that artificial light, common in all cities, can also enhance the occurrence of unwanted plants. If the extra light exposure gives cheatgrass an advantage over other plants, then urban habitats might unintentionally promote cheatgrass populations. Future research directions should investigate whether cheatgrass is evolving differently in urban areas compared to cheatgrass in more wild environments. We know that plants can evolve traits to deal with heavy metals and air pollution. Artificial light may also cause plants, such as cheatgrass, to change in a manner that makes the species more difficult to manage. Overall, our study creates several intriguing questions regarding the relationship between urban environments and introduced problem species. Urban areas are increasing in size to match the growing human population. As a result, urban habitats should be considered important and unique ecological regions.
Rarely are members of the urban community involved in studies on urban ecology. In addition to being one of the first to investigate how artificial light affects cheatgrass, our study is unique in that 9 of its authors are middle school girls representing diverse racial and economic backgrounds from 8 local middle schools. Our teenage co-authors participated in the University of Denver’s (DU) SciTech Summer Camp (https://science.du.edu/du-sci-tech). DU SciTech is a weeklong summer camp that three of us (Jennifer Hoffman, Shannon Murphy, Robin Tinghitella) designed to increase gender and racial diversity in STEM (science, technology, engineering, and mathematics) fields. The camp is fully funded by the National Science Foundation and DU, therefore attending the camp is free for the students. During their DU SciTech experience, our middle school co-authors built and took home their own telescopes and Raspberry Pi computers, learned to use microscopes, and created an insect collection (Fig. 2). In 2019, the students also contributed to the design, fieldwork, and editorial input for this study.
Although our local community includes a wide range of racial, ethnic, and socioeconomic backgrounds, the demographics of STEM fields do not reflect this diversity. A lack of belonging is one factor that prevents members of traditionally underrepresented groups from persisting in STEM disciplines. To increase diversity in these fields, we must embrace recruitment strategies that enable youth to experience and practice STEM. By conducting research in an environment familiar to our student co-authors, we show that science is not restricted to a lab or campus but can be successful in our backyards. Bringing science closer to home also removes the barrier of accessibility, which often limits participation in STEM activities. The experiential learning environment of DU SciTech offers opportunities for middle school girls to work alongside female scientists, gain meaningful hands-on STEM experiences, and form mentorship and professional networks. We are excited to see our young co-authors grow their curiosity in STEM, and we hope they gain the capacity to invite us to be their collaborators in the near future.
Blog written by Dhaval Vyas, Jennifer Hoffman, Shannon Murphy & Robin Tinghitella. Read the full paper here.