Substantial urbanization-driven declines of larval and adult moths in a subtropical environment

Abstract

Recent work has shown the decline of insect abundance, diversity, and biomass, with potential implications for ecosystem services. These declines are especially pronounced in regions with high human activity, and urbanization is emerging as a significant contributing factor. However, the scale of these declines and the traits that determine variation in species-specific responses remain less well understood, especially in subtropical and tropical regions, where insect diversity is high and urban footprints are rapidly expanding. Here, we surveyed moths across an entire year in protected forested sites across an urbanization gradient to test how caterpillar and adult life stages of subtropical moths (Lepidoptera) are impacted by urbanization. Specifically, we assess how urban development affects the total abundance of caterpillars and adult moths, and quantify how richness and phylogenetic diversity of macro-moths are impacted by urban development. Additionally, we determine the effects of urban warming on species-specific adult macro-moth abundance while accounting for urban development and explore how life-history traits condition species’ responses to urban stressors. At the community level, we find that urban development decreases caterpillar biomass and adult moth abundance. We also find sharp declines of adults in response to urban development across the moth phylogeny, leading to a decrease in species richness and phylogenetic diversity in more urban sites. Finally, our study found that smaller macro-moths are less impacted by urban development than larger macro- moths in subtropical environments, perhaps highlighting the tradeoffs of metabolic costs of urban heat island effects favoring smaller moths over the relative benefits of dispersal for larger moths. In summary, our research underscores the far-reaching consequences of urbanization on moths and provides compelling evidence that urban forests alone may not be sufficient to safeguard biodiversity in cities.

Large uncertainty in trait responses across insects among overall declines in a subtropical city

“…Another complicating factor is that trait‐environment responses may differ in different climate contexts. For example, urbanisation appears to favour larger moths in colder environments (Merckx & Van Dyck, 2019) but may select for smaller moths in warmer environments (Belitz et al, 2023). One trait that might be expected to generalise across all taxonomic groups and environments is diet generalism, since this has been demonstrated to be an important strategy for insect success in urban areas (Callaghan et al, 2021; Geslin et al, 2016; Lizée et al, 2011).…”

“…This approach potentially underestimates the negative impacts of urban development on insect abundance and richness. For example, comprehensive sampling of Lepidoptera communities at the same sites revealed significant declines in overall abundance, species‐specific abundance and species richness due to urban development (Belitz et al, 2023). Consequently, our study suggests that the selection criteria constraining which species are included in a community abundance analysis can influence outcomes.…”

“…Our study utilised adult insect by-catch collected at traps intended to target moths (see Belitz et al, 2023 for details of moth study). These insects were collected at a single light trap per site that was built following the blueprint of White et al (2016).…”