The Urban Heat Island and its spatial scale dependent impact on survival and development in butterflies of different thermal sensitivity

Abstract

Climate alteration is one of the most cited ecological consequences of urbanization. However, the magnitude of this impact is likely to vary with spatial scale. We investigated how this alteration affects the biological fitness of insects, which are especially sensitive to ambient conditions and well‐suited organisms to study urbanization‐related changes in phenotypic traits. We monitored temperature and relative air humidity in wooded sites characterized by different levels of urbanization in the surroundings. Using a split‐brood design experiment, we investigated the effect of urbanization at the local (i.e., 200 × 200 m) and landscape (i.e., 3 × 3 km) scale on two key traits of biological fitness in two closely related butterfly species that differ in thermal sensitivity. In line with the Urban Heat Island concept, urbanization led to a 1°C increase in daytime temperature and an 8% decrease in daytime relative humidity at the local scale. The thermophilous species Lasiommata megera responded at the local scale: larval survival increased twofold in urban compared to rural sites. Urbanized sites tended to produce bigger adults, although this was the case for males only. In the woodland species Pararge aegeria, which has recently expanded its ecological niche, we did not observe such a response, neither at the local, nor at the landscape scale. These results demonstrate interspecific differences in urbanization‐related phenotypic plasticity and larval survival. We discuss larval pre‐adaptations in species of different ecological profiles to urban conditions. Our results also highlight the significance of considering fine‐grained spatial scales in urban ecology.

The heat is on: Genetic adaptation to urbanization mediated by thermal tolerance and body size

“…Non-significant pathways are given in dashed grey lines. These results are in line with recent results on community responses to urbanization in invertebrates (Kaiser et al, 2016;Piano et al, 2017) and suggest that the studied D. magna populations are quite well equipped to cope with the observed urban-driven warming of habitats in Flanders. (b) Proportional contribution of significant indirect (via size) and direct pathways to the estimated total effect of urbanization (i.e.…”

Intraspecific variation shapes community‐level behavioral responses to urbanization in spiders

“…(), the changes in temperature/humidity needed to observe changes in mesh width of the same magnitude as in our study are much greater than the average climate differences observed between urban and non‐urban sites in our study region (12°C against 1–2°C; Kaiser et al. ). This indicates that climate alone is unlikely to explain observed mesh width differences across urbanization gradients.…”

“…However, all environmental characteristics observed in this study either varied similarly at both scales (prey size, built‐up surfaces) or more prominently at the local scale (urban heat island effect; Kaiser et al. ). While other environmental effects cannot be ruled out, trait differences between landscapes also reflect dispersal effects.…”

“…We used the same site selection procedure, and many of the same sites, as previous studies realized in this region (Kaiser et al. , Brans et al. , Piano et al.…”

“…S2 and average temperature (predominantly at the local scale, average temperature increased as urbanization increased; Kaiser et al. ).…”