The functional microclimate of an urban arthropod pest: Urban heat island temperatures in webs of the western black widow spider

• Published in: Journal of thermal biology 2024-02, Vol.120, p.103814-103814, Article 103814
• Main Authors: Clark, Ryan C., Johnson, J. Chadwick
• Format: Article
• Language: English
• Subjects: Black widow spider; Thermal microclimate; Urban heat island; Urban pest
• ISSN: 0306-4565; 1879-0992
• DOI: 10.1016/j.jtherbio.2024.103814

Urbanization alters natural landscapes and creates unique challenges for urban wildlife. Similarly, the Urban Heat Island (UHI) effect can produce significantly elevated temperatures in urban areas, and we have a relatively poor understanding of how this will impact urban biodiversity. In particular, most studies quantify the UHI using broad-scale climate data rather than assessing microclimate temperatures actually experienced by organisms. In addition, studies often fail to address spatial and temporal complexities of the UHI. Here we examine the thermal microclimate and UHI experienced in the web of Western black widow spiders (Latrodectus hesperus), a medically-important, superabundant urban pest species found in cities across the Western region of North America. We do this using replicate urban and desert populations across an entire year to account for seasonal variation in the UHI, both within and between habitats. Our findings reveal a strong nighttime, but no daytime, UHI effect, with urban spider webs being 2–5 °C warmer than desert webs at night. This UHI effect is most prominent during the spring and least prominent in winter, suggesting that the UHI need not be most pronounced when temperatures are most elevated. Urban web temperatures varied among urban sites in the daytime, whereas desert web temperatures varied among desert sites in the nighttime. Finally, web temperature was significantly positively correlated with a spider's boldness, but showed no relationship with voracity towards prey, web size, or body condition. Understanding the complexities of each organism's thermal challenges, the “functional microclimate”, is crucial for predicting the impacts of urbanization and climate change on urban biodiversity and ecosystem functioning. •Urban heat island (UHI) effects are often estimated from broad-scale temperature estimates, and finer scale studies of thermal microclimates experienced by urban biota are necessary.•We find a pronounced nighttime UHI in the webs of Western black widow spiders (Latrodectus hesperus).•We also found spatial complexity (thermal patchiness) for urban spiders during the day and desert spiders at night.•Lastly, we found hotter webs to house bolder spiders, but found no relation between web temperature and web size, body condition, or voracity towards prey.