1. Urban ecosystems are formed by pronounced socio-ecological gradients, which are distinct from other ecosystems and can simultaneously filter and promote taxa, ultimately affecting their interactions. However, the strength of the effect of filtering and facilitation across the different trophic levels could vary among biotic and abiotic factors.
2. Here, we investigate the effects of habitat amount, temperature and host-enemy biotic interactions in shaping communities of cavity-nesting bees and wasps and their natural enemies. We installed trap-nests in 80 sites distributed along urban intensity gradients in five European cities (Antwerp, Paris, Poznan, Tartu and Zurich). We quantified the species richness and abundance of hosts and their natural enemies, as well as two performance traits (survival and parasitism) and two lifehistory traits (sex ratio and number of offspring per nest for the hosts). We analysed the importance of the abiotic and biotic variables using generalised linear models and multi-model inference.
3. We found that habitat amount was the main driver of multiple host responses, with larger habitat amounts resulting in higher species richness and abundance for hosts and natural enemies, as well as a larger probability of survival and a larger number of brood cells for hosts.
4. Local resources proxies shaped both bees and wasps and indicate different uses of existing vegetation between bees and wasps. Temperature proxies had a minor role in shaping host and natural enemies.
5. Biotic interactions were a main driver of host and enemy community structure, with natural enemies being strongly affected by host availability, that is, with direct density-dependence between hosts and their natural enemies. 6. Overall, our study highlights the importance of habitat amount and temperature in shaping urban food webs, as well as on biotic interactions through direct effects on hosts responses and the subsequent consequences for their natural enemies. As cities prepare to tackle the consequences of global change, strategies that make it possible to maintain habitat and mitigate urban overheating emerge as a key urban adaptation for biodiversity conservation.