Figure 3 . Interactive effect of percentage of urbanization and of local canopy cover (measured as the cover of urban and local canopy cover within a buffer of 200 and 20 m radius, respectively) on the incidence of gall-inducing (A) and leaf-mining herbivores (B).
Leaf miners were present in 17.98 ± 0.01 % of the sampled leaves. Model selection retained the percentage of urbanization, local canopy cover, their interaction, year and spring precipitation as important predictors explaining variability in leaf-miner incidence (Figure 2C, Table A ). Specifically, leaf-miner incidence significantly decreased with increasing urbanization (from 21.33 to 1.00 % along the range of urbanization, Figure 2C, Figure B ). The effect of urbanization on leaf-miner incidence was, however, contingent on local canopy cover (significant urbanization × local canopy cover interaction;Figure 2C, Figure 3B ): the negative effect of urbanization on leaf-miner incidence was more pronounced when there was a lower canopy cover around focal oaks. It was significantly lower in 2019 and 2020 as compared to 2018 (Figure 2C ). Spring precipitation and local canopy cover had no consistent effect on leaf-miner incidence (Figure 2C , Figure B ).
Discussion
Our study revealed that urbanization can consistently reduce insect herbivory on the pedunculate oak throughout its geographic range. The effect of urbanization was partially modulated by the percentage of canopy cover around oaks, with differences among herbivore feeding guilds. Specifically, we found that urbanization and local canopy cover had independent and opposite effects on overall leaf damage. In contrast, effects of urbanization on gall-inducer and leaf-miner incidence depended on local canopy cover, with a more pronounced negative effect of urbanization on gall-inducer and leaf-miner incidence when there was a greater and lower local canopy cover, respectively. These results show the complexity of plant-herbivore interactions in urbanized areas, highlighting that local habitat characteristics within cities have the potential to attenuate or modify the effect of urbanization on biotic interactions.
Effect of urbanization on herbivory
We found that the incidence of both gall-inducing and leaf-mining herbivores decreased with increasing urbanization, suggesting that the abundance of these species decreased with urbanization. This result agrees with previous reports that have shown that urbanization reduces the abundance and diversity of several guilds of insect herbivores (Barr et al., 2021; Dobrosavljević et al., 2020; Fenoglio et al., 2020; Herrmann et al., 2012; Kozlov et al., 2017; Moreira et al., 2019) with the noticeable exception of sap-feeding herbivores (de Andrade and Rivkin, 2020; Parsons and Frank, 2019). For instance, Herrmann et al. (2012) found lower species richness of galler communities on the valley oak (Quercus lobata ) in urban areas than in natural areas. Similarly, Dobrosavljević et al. (2020) found lower richness, abundance and diversity of leaf-miner communities on the pedunculate oak in urban areas than in natural areas.
We also found that leaf damage decreased with increasing urbanization around focal oaks. Although this result aligns with the observation that the incidence of gall-inducing and leaf-mining herbivores decreased with increasing urbanization, it is important to consider that leaf damage (i.e., the amount of biomass consumed by chewing herbivores) may not scale proportionally with the abundance and diversity of insect herbivores. Several other ecological factors that are influenced by urbanization may determine the amount of damage caused by chewing herbivores. They include the top-down control of herbivore populations by their enemies (Kozlov et al., 2017; Planillo et al., 2021; Turrini et al., 2016) as well as plant defenses and quality (Moreira et al., 2019; Thompson et al., 2016). The correlative nature of our data does not allow further robust inferences on the mechanisms underlying the observed patterns.
Effect of local canopy cover on herbivory
Herbivory varied with canopy cover in the immediate vicinity of oaks, but this effect was guild specific: there was an independent positive effect of local canopy cover on herbivory, a negative effect on gall-inducing herbivores, and no effect on leaf-mining herbivores. Whereas several studies have compared herbivore abundance or diversity, and sometimes herbivory, between urban and forested environments (Herrmann et al., 2012; Kozlov et al., 2017; Moreira et al., 2019), only a handful of them have addressed the effect of urban tree density on insect herbivores (Barr et al., 2021; Christie et al., 2010; Christie and Hochuli, 2005; Herrmann et al., 2012; Long and Frank, 2020; Meyer et al., 2020; Raupp et al., 2010). Their findings were contradictory with reports of both higher (Christie and Hochuli, 2005) and lower (Herrmann et al., 2012; Long and Frank, 2020) herbivory in isolated trees as compared to trees growing in larger forest patches. This effect of local canopy cover also mirrors variability in the response of herbivory to the size of forest fragments (De La Vega et al., 2012; Kaartinen and Roslin, 2011; Simonetti et al., 2007; Valdés-Correcher et al., 2019). We therefore refrain from putting forth any particular mechanism that may underlay the patterns we observed. However, we speculate that denser tree canopies may have buffered micro-climatic variations (Coley and Barone, 1996; Dale and Frank, 2014; Yamasaki and Kikuzawa, 2003), which may have been particularly favourable to chewing herbivores that are external feeders (Savilaakso et al., 2009) and at the same time unfavorable to leaf-galling herbivores, as they benefit from high temperatures (Valdés-Correcher et al., 2021). Alternatively, top-down forces also vary with local canopy cover and may consequently influence insect herbivory. For instance, predation activity of birds (Stemmelen et al., 2020) and the abundance of birds (Valdés-Correcher et al., 2019) increase with increasing local canopy cover. However, if the observed negative association between herbivory and forest cover would have been mediated by bird predation, it would have displayed the opposite pattern. Deciphering the relative importance of the mechanisms acting upon insect herbivores will represent a major step forward in our understanding of insect herbivory in urban environments.
Interactive effect of urbanization and local canopy cover on herbivory
Cities are heterogeneous environments where greenness may vary widely. The design of our study allowed us to partially disentangle the response of herbivory to the joint variation in urbanization and local canopy cover. We found that increasing local canopy cover modulated the effect of urbanization on some herbivores. Specifically, the negative effect of urbanization on gall-inducing herbivores strengthened with increasing local canopy cover, whereas increasing local canopy cover annulled the effect of urbanization on leaf-mining herbivores. Urbanization and local canopy cover have antagonistic effects on the microclimate and enemy pressure. Cities are warmer than the surrounding rural areas as a result of the “heat island effect” (Kalnay and Cai, 2003; Parker, 2010; Roth et al., 1989), which is buffered by the presence of trees (Loughner et al., 2012; Nuruzzaman, 2015). Likewise, forest patches in urban environments serve as habitats for both herbivores and predators, which is likely to modify the strength of horizontal (herbivores-herbivores) and vertical (herbivores-predators) interactions in urban trees (Long et al., 2019; Long and Frank, 2020). Endophagous herbivores such as gall-inducers and leaf-miners are more sheltered from the environment than ectophagous herbivores. Thus, ectophagous herbivores may be more sensitive to local environmental conditions than endophagous herbivores. For instance, we found a positive relationship between the incidence of gall-inducing herbivores and temperature. It is possible that by buffering the heat island effect, the presence of a denser canopy reduced the incidence of gall-inducers on oaks. On the contrary, leaf-mining herbivores were found to be favoured by lower (Gaston et al., 2004) or intermediate temperatures (Valdés-Correcher et al., 2021). For these herbivores, a denser canopy could have negated the heat island effects, creating more favourable habitats. We cannot exclude that the interactive effect of urbanization and canopy cover was partially dependent on differential predation rates, but this could not be investigated in the present study.
Effect of climate on insect herbivory
Climatic variables were included in the analyses to take into account that oaks were sampled along a latitudinal gradient. They had a significant effect on insect herbivory and this effect varied among feeding guilds. Consistently with previous studies (Kozlov et al., 2016; Valdés-Correcher et al., 2021), we found that precipitation had a negative effect on leaf damage; temperature had a positive effect on gall-inducer incidence, whereas leaf-miner incidence did not vary with climate. The differences in the effect of climate among feeding guilds may be due to differences in insect herbivore strategies to survive different climatic conditions, which was discussed extensively in a previous paper (see Valdés-Correcher et al., 2021).
Conclusions
Our simultaneous consideration of the effect of urbanization and local canopy cover on insect herbivory provides novel insights into plant-herbivore interactions. We found that insect herbivory responds simultaneously to both urbanization and local canopy cover in the pedunculate oak in the major part of its geographic range. Importantly, our results highlight that urbanization has a negative effect on insect herbivory across the three feeding guilds. However, local canopy cover as well as its interaction with urbanization influenced insect herbivory of different feeding guilds differently. Thus, local canopy cover within cities has the capacity to mitigate or modify the effect of urbanization on biotic interactions, as it influences the effect of urbanization on herbivores differently. Therefore, this study supports the importance of maintaining trees in urban areas as it may consequently contribute to the preservation of biodiversity of insect herbivores in urban areas. Important insights will be gained by investigating the mechanisms driving these patterns, in particular by deciphering the interactive effects of urbanization and canopy cover on the microclimate and enemy pressures herbivores are exposed to.