How does urbanization affect herbivory?

Urban environments are expanding at a rapid rate with over half of the human population residing now in cities. This fast development promises dramatic changes on physical environment, ecological communities and local and global ecosystems in the coming decades. For instance, urbanization usually leads to significant changes in environmental conditions (e.g., atmospheric CO2 increase, urban heat island) which can dramatically alter the quantity, quality, temporal and spatial arrangement of resources (e.g., water, sunlight, nutrients) needed to support plant and animal communities. While recent advances on urban ecology has primarily focused on how urbanization impacts plant and animal assemblages, its effects on ecological interactions remain largely unexplored. In this study, we investigated the effects of urbanization on insect herbivory associated with the English oak Quercus robur (Fagaceae) by comparing damage on trees in urban vs. adjacent rural areas in 18 cities of Western Europe with different population size.

To assess the potential causes behind patterns of herbivory, we further measured changes in leaf chemical defences and nutritional traits, as well as CO2 emissions. Our results showed that trees in urban locations exhibited lower levels of leaf herbivory and chemical defences, and increased levels of leaf nutrients compared with trees in natural forest locations. Importantly, while increased CO2 emissions were associated with lower levels of chemical defences, changes in leaf herbivory were not associated with either leaf chemical defences or CO2 levels.

These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here.

The authors through: Xoaquín Moreira Tomé

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