Bottom-up and top-down drivers influence urbanization effects on insect herbivory in oaks

Urban forests provide essential ecosystem services. Herbivory is a
widespread biotic interaction that shapes ecosystem functions, which
underpin these services. Urbanization disrupts plant-herbivore
interactions by altering plant traits (bottom-up factors) or by changing
the abundance of herbivore natural enemies (top-down control), potentially
threatening the services provided by urban forests. Disentangling these
drivers of herbivory is crucial for designing and managing urban forests
to enhance resilience. We examined insect leaf herbivory on Quercus robur
trees in urban and rural forest stands across 13 European cities (N = 104
trees). To assess the effects of top-down forces on herbivory, we excluded
vertebrate (e.g., birds, bats), invertebrate (e.g., ants), or both groups
of predators from branches on each tree. We then measured insect damage on
the different treatments. We also assessed bottom-up drivers by measuring
leaf traits, specifically nutrients and phenolic compounds, and tested for
correlations with leaf damage. Additionally, we recorded temperature
within stands, an abiotic factor that may modulate both top-down and
bottom-up forces on herbivory. Herbivory was 24% lower on urban trees
compared to rural trees. Excluding vertebrate predators increased
herbivory by 40%, with effects being stronger in urban stands.
Additionally, urban trees had higher leaf quality, with higher nutrient
and lower phenolic concentrations; however, the measured traits did not
correlate with herbivory. Finally, temperature was positively associated
with urbanization and correlated with predation (positively), but not with
herbivory. Temperature also did not mediate the bottom-up or top-down
effects of urbanization on herbivory. Urbanization affects herbivory
through both bottom-up and top-down processes, independent of local
temperature. Despite stronger predator effects and better leaf quality,
urban trees experience lower herbivory, suggesting that unmeasured
factors, such as changes in herbivore behaviour or community structure,
may play an important role. Further studies are needed to deepen our
understanding and inform urban forest management.