Urban tree diversity reduces invertebrate leaf herbivory

Urban tree diversity reduces invertebrate leaf herbivory | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Urban tree diversity reduces invertebrate leaf herbivory Juan Antonio Hernández-Agüero, Velin Ivaylov Velichkov, Bas Krijnen, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7907916/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Global rise of urbanization and climate change are strongly affecting biodiversity, species interactions and species survival. Urban areas are socio-ecological ecosystems which often result in climate novelty and unique artificial species assemblages through tree plantation. This unique tree species pool can be used as a tool to counterbalance the aforementioned impacts. Enemies hypothesis and Host Dilution hypothesis post that higher tree biodiversity can reduce herbivory by reducing host detectability or by having a positive effect on natural enemies of herbivores. Oppositely, the Resource-Concentration hypothesis claims that higher plant diversity increases overall herbivory. The aim of this study was to test the effect of urban tree diversity in trophic interactions. This was assessed by measuring herbivory rates in 3153 leaves of 30 Quercus robur trees surrounded by different tree diversity levels. Urbanization level was included to consider local warming due to urban heat island effect. Estimation of the surrounding vegetation or water was also considered as a reductor of this artificial heating. Herbivory was negatively affected by local tree diversity, with Shannon’s index having the strongest effect. Host Dilution hypothesis supports these results, as more even tree communities suffered from less herbivory. Higher levels of tree diversity can be used as a nature based solution to avoid the ecosystem disservice of herbivory in urban areas. Our results suggest that urban managers need to avoid monocultures and consider relative abundance in urban plantations in order to increase urban tree fitness by reducing herbivory. Urban ecology Biodiversity Nature based Solutions Herbivory Quercus robur Figures Figure 1 Figure 2 1. Introduction More than half of the world population is currently living in cities with more than 300.000 inhabitants. With urbanization increasing globally, this number is expected to rise to 70 % by 2050 (United Nations, 2018). Urban environments suffer from numerous environmental impacts, such as water and air pollution, or drastic changes in land use, which results in devastating effects for ecosystems and climate (Bai et al., 2017). Due to these impacts, urbanization is considered one of the main contributors to local biodiversity extinction rates in these areas (McKinney, 2002). Moreover, global climate has experienced a change in the last decades with a pace never previously recorded (Diffenbaugh & Field, 2013). The global temperature increase (NOAA, 2023) is affecting ecosystem functioning at all levels. This is exacerbated locally in cities, as they experience the so-called urban heat island (UHI) (Kim, 1992). Because of this, cities are valuable places to study ecological networks, as the temperatures today resemble future temperature scenarios outside urban areas in the same latitudes due to climate change (Youngsteadt et al., 2015). One of the most interesting ecological networks nowadays are trophic interactions. They are particularly relevant for biological communities, as they shape community dynamics, drive important aspects of ecosystem functioning, and provide essential ecosystem services (Gaüzère et al. 2022). These services include nutrient cycling (DeAngelis, 1992), pest control (Whelan, 2008), or those related with the adaptation to the effects of climate change, which are especially relevant in urban environments (Sirakaya et al., 2018). At the same time, these trophic interactions can result as well in ecosystem disservices to humans, which are likewise of considerable interest. There are some examples, such as the apparition of pests that damage cultivars that also can harm people (Moreira & Abdala-Roberts, 2023) or the processionary oak moth, Thaumetopoea processionea , which base their diet on oak leaves. This moth releases barbed hairs that cause dermatitis among humans, posing a threat to society (Rahlenbeck & Utikal, 2015), and constituting an additional ecosystem disservice. Yet, non-consensus has been established on how urbanization affects pests and its herbivory interactions (Meineke et al., 2013). In some cases, a disruption has been observed (Miles et al., 2019; Meineke et al., 2019; Moreira et al., 2019; Nuckols & Connor, 1995; Valdés-Correcher et al., 2022), while in others it may be beneficial (Christie & Hochuli, 2009; Dale & Frank, 2014; Parsons, & Frank, 2019; Rivkin & de Andrade, 2023). These discrepancies have been explained in the past by regional (Hernández-Agüero et al., 2024a) or latitudinal differences (Alonso-Crespo & Hernández-Agüero, 2023; Hernández-Agüero et al., 2024b), and are mainly explained by artificial abiotic factors produced in urban areas, such as impervious surfaces or temperature increment. Artificial biotic factors could be also used to explain these discrepancies. However, to date, this approach has not been widely applied (but see Stemmelen et al., 2025a). Urban environments differ from natural environments in numerous ways. The main ones are the reduced size and composition of habitats due to land use change (Liu et al., 2018). Urban habitats tend to be more fragmented, which leads to lower connectivity between natural areas which entails a significant difference in biodiversity, ecosystem functioning and provision of ecosystem services (Liu et al., 2018). Also, urban areas usually are modified in a way that species pool, specially of plants, are artificially altered by plantations and management. Thus, the combination of vegetation management and the local novel climates due to urban heating ensemble an unseen niche for native and exotic species. This can exacerbate the disappearance of species, while favoring the suitability of some urban exploiters ( sensu Blair, 1996), increasing ecosystem disservices, with fatal consequences for environment and people. However, these unique urban tree species pools, often more diverse than the ones found in natural settings (Augustinus et al., 2024), can be used as a tool to counterbalance the aforementioned impacts. More diverse communities should be more resistant to biological invasions, based on the associational resistance (Barbosa et al., 2009). This has been explained in the past by the Enemies hypothesis (Letourneau, 1987) which states that higher tree biodiversity can have a positive effect on natural enemies of herbivores. Neighboring plants can help to reduce damage to a focal plant by providing resources (e.g., nectar) or harboring alternative prey that attract herbivore predators or parasitoids. In line with that is the Host Dilution hypothesis (Underwood et al., 2014). This hypothesis explains how herbivory is reduced where tree neighbor biodiversity is higher, since abundance of predators per host gets reduced. In contrast, the Resource Concentration hypothesis (Root, 1973) posits that herbivores could benefit from the presence of different species, as alternative resources could increase their surveillance. This hypothesis is of special interest in urban areas, as it affects specially generalist species. Altogether, these hypotheses established a solid framework to explore the effect of diversity on species interactions in urban areas. Nevertheless, they have mostly been applied in experimental tree plantations, with manipulative diversity patterns restricted to combinations of one to five species (Muiruri et al. 2019). However the level of complexity that urban areas can have in terms of species composition make these environments unique. For example, it is known that diversity effects on herbivory are greater when mixed forests comprise taxonomically more distant tree species (Jactell et al., 2007), thus in urban areas where species pools are the result of a human induced configuration, without any common evolutionary history between species, and therein representing a higher taxonomically distant species, we expect the diversity effects on herbivory of great relevance. The present study aims to test the effect of urban tree diversity in species trophic interactions. If we find that higher levels of herbivory are detected in low tree diversity areas, the results will be supported by the Enemies hypothesis (Letourneau, 1987)/ Host Dilution hypotheses (Underwood et al., 2014). Contrary, if we find higher levels of herbivory in high tree diversity areas, the results will be supported by the Resource Concentration hypothesis (Root, 1973). 2. Methodology The city of Amsterdam (The Netherlands) was selected to study how tree diversity affects herbivory in urban areas, as it has certain characteristics that make it the ideal location for this purpose. First, Amsterdam is relatively flat and herbivory can be influenced by elevation (Galmán et al., 2021). Secondly, it is a highly populated city. The metropolitan area of Amsterdam has a size of 219.3 km 2 and a population of 1,174,000 in 2023 (United Nations, 2022). Lastly, Amsterdam municipality has available a database of all trees of the city and it is georeferenced, which makes the diversity characterization easier. In order to examine the relationship between insect herbivory and local tree biodiversity, leaves were collected and examined from a set of Quercus robur L. trees (Common or English oak). The percentage of leaf area eaten by herbivores was analyzed by collecting and scanning leaves of 30 Q. robur specimens (Figure 1). The selection of the trees was done in the following way: a list of all the Quercus robur trees in Amsterdam were downloaded from the website of the Amsterdam municipality (Klaas-Bindert de Haan, 2023), which amounted to 8,654 specimens. Those were then filtered to show only trees smaller than 12 meters tall (to allow for accessible leaf collection), resulting in 2,882 trees. For these ones, human population density, NDVI and water cover percentage around each tree were calculated using R. The rankings on these three factors were spread in 5 quantiles and only one tree was selected from each quantile so a set as diverse as possible was created (but see Krijnen & Hernández-Agüero, 2025; for more details). The set contained 30 trees (Appendix 1). From each tree, 20 leaves were collected monthly during all the growing season (May to November), making for a total of 3,153 leaves. Not all trees could be sampled every month due to various unforeseen circumstances. Trees were excluded through time due to loss of reachable branches and gardening works out of control from the authors of this paper (16 trees with 7 reviews, 2 with 6 reviews, 3 with 5 reviews and 9 with only 2 reviews). The leaves were picked randomly, ensuring local orientation variability, at 1.5 to 2 meters high. In case that the lower reachable leaves were in branches higher than 2 meters, the lowest leaves were collected. Leaves were scanned with a high resolution scanner (Epson Perfection V39 scanner) using a consistent methodology. Leaf scans were analyzed using the processing software ImageJ (Schneider et al. 2012), to measure the eaten area of the leaf (Meineke et al., 2018, Alonso-Crespo & Hernández-Agüero, 2023). The percentage of eaten area was calculated by delineating the shape of the leaf without herbivory and the real perimeter of each leaf. The real shape of the leaves was drawn out by hand in ImageJ. A biodiversity assessment of the area around every of the 30 selected trees was conducted. Using the same tree database as above, each tree name was harmonized using the ´tol_resolve´ function from the ´taxize´ package (version 0.10.0; Chamberlain & Szocs, 2013). Buffers of 200 meters were created around each Q. robur tree used to estimate herbivory with the ‘st_buffer’ function from ‘sf’ package (version 1.0.17; Pebesma, 2018) using EPGS:3857 as a coordinate reference system (CRS). A 200 meter buffer was chosen for this study based on previous research on herbivorous insect dispersal on Q. robur in northern Europe, where the majority of dispersing happens in a couple hundred meters (Gripenberg et al., 2008, Zheng et al., 2015, Barr et al., 2021, Valdés-Correcher et al., 2022). List of trees from the complete list of trees of the study area inside each buffer were extracted with the ‘st_intersection’ function of the ‘sf’ package. Species richness was estimated as the total number of different species present around each Q. robur . Shannon and inverse of Simpson were obtained with the ‘diversity’ function of the ‘vegan’ package (version ‘2.6.6.1’; Oksanen et al., 2024). Shannon index (Shannon, & Weaver, 1949) shows the weighted geometric mean of the proportional abundance of species, and the inverse of Simpson index (Simpson, 1949) contains the number of species present and their relative abundance. Data on percentage of water, NDVI and Human Population Density in each radius was extracted from Krijnen & Hernández-Agüero (2025) with the following methodology. Satellite imagery from Sentinel-II (ESA, 2023) was used for remote sensing of vegetation and water via NDVI and NDWI, following strict selection criteria (low cloud cover, same satellite, close sensing dates). Predictors were reclassified and inverted to isolate features. Population data from WorldPop (2024) was used to assess density in Amsterdam at 100×100 m resolution. Generalized Linear Mixed Models (GLMM) were created to explore how herbivory was affected by each biodiversity index with the ‘glmmTMB’ function of ‘glmmTMB’ package (version ‘1.1.9’; Mollie et al., 2017). This package was used to include a beta family of errors with logit link. Following package authors, and considering that herbivory data has a zero inflated nature, which could produce errors when adjusting models, a transformation of data was made. Transformation was made using the formula: y' = (y × (n - 1) + 0.5) / n where y is the original herbivory (defoliation) value, n is the total number of observations and y' is the transformed value. This data transformation is necessary to correct any existing bias in the data and avoid issues with extreme values like 0 and 1. Normalized Difference Vegetation Index (NDVI), Human Population Density (HPD), Water cover percentage in the proximity of every tree and their interaction were included as fixed factors in the models, along with the additive effect of each diversity index. One model was fitted for each diversity index. Additionally, an extra model was fitted including the number of conspecific ( Q. robur ) individuals around each studied tree. In all cases, an ID including the tree and month of review was added as a random effect. Alternative models were compared using the Akaike Information Criterion (AIC) to assess the effects of explanatory variables (i.e. fixed effects). Models with a difference in AIC >2 suggested that the less favorable model could be omitted. All analyses were done in the R environment (version 4.4.0; R Core Team, 2024). 3. Results An area of 200 m was established around the 30 trees selected for herbivory analysis to measure diversity, finding a total of 205 different tree species, with the most common species being Quercus robur with 799 specimens. Species richness for the 30 analyzed trees ranged from 1 to 75 with a mean of 22.3 species. Shannon’s index ranged from 0 to 3.27 with a mean of 2.19. Simpson’s index ranged from 0 to 0.95 with a mean of 0.77. NDVI ranged from 0.1 to 0.49 with a mean of 0.3. Water cover ranged from 0 to 0.3 with a mean of 0.04. HPD ranged from 2.9 to 43.1 with a mean of 24.68 people/hectare. A total of 3,153 leaves were analyzed for herbivory. They had a mean herbivory of 0.063, with the data ranging from 0 to 0.74. Herbivory data had a standard deviation of 0.096. The best model for each diversity index includes all environmental variables and the additive effect of diversity (Table 1 ). Among all the diversity variables (species richness, Shannon index, and Simpson's inverse index), the best model was the one that included the Shannon index. The estimates for each diversity index were negative in the case of species richness (p-value = 0.002267; Fig. 2 a), the Shannon index (p-value < 2.80e-08; Fig. 2 b) and the Simpson index (p-value = 1.32e-07; Fig. 2 c). Table 1 AIC results of each studied model including degrees of freedom (df) and Akaike Information Criterion values. Model df AIC Null 3 -14092.44 Without diversity 10 -14146.00 Species richness 11 -14153.09 Shannon index 11 -14172.13 Simpson index 11 -14169.59 R 2 m and R 2 c were 0.097 and 0.219 for Species Richness, 0.115 and 0.220 for Shannon index, and 0.113 and 0.220 for Simpson index. Finally, the inclusion of the number of conspecific individuals did not increase model performance, but the proportion of conspecifics between the overall number of trees did. 4. Discussion Our findings showed that urban tree diversity affected invertebrate herbivory in the city of Amsterdam, independently of the diversity index metric used. The trees studied that were surrounded by higher diversity (for any of the diversity indices: species richness, Shannon diversity index, or Simpson's inverse diversity index) suffered lower levels of herbivory compared to those surrounded by lower diversity. Ours is not the first study to detect these effects of urban tree diversity. A similar study in the city of Montreal (Canada) also showed a reduction in insect herbivory with the increase of tree diversity (Stemmelen et al., 2022 ), and despite not being able to prove it, they explained their results by predation pressure. In a parallel study in Burdeaux (France), Stemmelen et al. ( 2025 ) demonstrated that the neighboring diversity increased predation on a chestnut herbivore, but did not decrease overall herbivory, that was more mediated by tree density. In line with this result, we found better model performance when considering Shannon diversity index. One of the novelties of this study is the use of different taxonomic diversity metrics apart from species richness, that is a very poor biodiversity indicator (Roswell et al., 2021 ), as it only shows the amount of species, not taking into consideration their proportional abundance or abundance at all. Shannon diversity not only takes into account the number of species in a community, but also their relative abundance, including rare species. In contrast, the Simpson index gives more weight to dominant species. Therefore, our results indicate that tree density influences herbivory, as reflected in the Shannon diversity index. Another study, in North Carolina (USA), showed comparable results, finding less pest density in more diverse tree urban forests, but this increment was not explained by an increase in pest regulation (Wilson et al., 2023 ). They did explore the relation between pest abundance and impervious surfaces, finding higher abundance in more urbanized areas. Environmental differences (e.g. urban warming) between urban and forested settings can simplify enemies communities, making it difficult to predict how the effects of tree diversity on enemies found in forests translate to cities (Dale & Frank, 2018 ). In our case, this was controlled in the models by the inclusion of abiotic factors, thus results shown here can be considered independent of the urban abiotic configuration. Our results support the idea that more diverse plant communities are more resistant to biological invasions. This is consistent with the associational resistance hypothesis (Barbosa et al., 2009 ) in which it is stated that specific plant associations reduce the likelihood of a plant being detected by herbivores compared to where the plant grows alone or with certain neighbors. This pattern aligns with the Enemies hypothesis (Letourneau, 1987 ), which suggests that higher tree diversity can enhance populations of natural enemies that regulate herbivores. In this same context, neighboring plant species may reduce damage to focal plants by providing alternative resources such as nectar or by harboring alternative prey that attract predators and parasitoids of herbivores. In the same line, the Host Dilution hypothesis (Underwood et al., 2014 ), proposes that high number of non-target species reduces the proportion of herbivorous host, as in a more diverse community each particular plant species is relatively less abundant and its distribution is likely to be more fragmented, making it less available to specialist herbivores (Jactel & Brockerhoff, 2007 ). This host dilution can also reduce the overall proportion of plants attacked or damaged by herbivores by providing various chemical and physical traits that reduce the overall herbivory (Ruttan & Lorite, 2014). It is possible that plants in close proximity to the focal Q. robur trees released chemicals that repelled herbivorous insects or that those insects were not able to chemically detect their host by the presence of ´chemical barriers' to host location. This is known to occur when signals from non-host trees disrupt olfactory host tree recognition, thus also resulting in a decrease of herbivory (Jactel & Brockerhoff, 2007 ). Host Dilution in cities could be of greater intensity than in nature, since the proportion of non-host species can reach higher levels due to a higher proportion of exotic species (Avolio et al., 2018 ). These exotic species usually have a different evolutionary history that could result in signals less easily recognizable by herbivores (Castagneyrol et al., 2014 ). In addition to the host dilution effect, the presence of exotic plants can further reduce overall herbivory. This is because, when introduced, many exotic species lack natural enemies specialized in the new environment, which is known as the Enemy Release hypothesis (Blossey & Notzold, 1995 ). Consequently, exotic species not only contribute to diluting the herbivorous pressure on native species, but also decrease their own susceptibility to being attacked. This would make urban areas even more unsuitable for native herbivores as the difficulties in host location by the presence of different tree species are not counterbalanced by the presence of alternative food sources. We found support for the host dilution hypothesis, opposite to previous studies (Stemmelen et al., 2025 ). The inclusion of host numbers did not improve model performance, but their relative abundance did. However, the best model always included the diversity metric. At the same time, the availability of different food sources for herbivores could indirectly increase their predators and parasitoids, explained by the Enemies hypothesis (Letourneau, 1987 ). As in higher plant diversity areas predators, specially generalist (Jactel & Brockerhoff, 2007 ), can have access to different preys that typically spans temporally different, predators and parasitoids can have higher variety of preys, thus increasing predation (Shao et al., 2021 ; Schillé et al., 2024 ). The presence of generalist species is greater in urban than in natural areas (Lefcheck et al., 2013 ), this could have affected the overall detected herbivory, since generalist species are not strongly affected as specialist species from a higher plant diversity (Moreira et al 2016 ), as explained by the Resource Concentration hypothesis (Root, 1973 ), but this was not the case in Amsterdam. The reason could be the high specificity of herbivorous species (Hernández-Agüero et al., 2022 ) that explain a lower proportion of generalist species than other trophic groups such as predators and parasitoids. In this context, although results suggest that predators could be responsible for the reduction in herbivory in more diverse urban forests, it is important to consider that higher trophic levels tend to be more sensitive to urbanization than lower trophic levels (Burkman & Gardiner, 2014 ). Therefore, without a direct analysis of the predator community, it is not possible to fully confirm the role of the Enemies hypothesis. Further studies must explore the predation role in explaining the effect of plant diversity in herbivory to be able to definitively point to Enemies hypothesis as the one explaining the reduction of herbivory with increased diversity. Moreover, because our study was observational, we could not separate the effects explained by non-evaluated predictors of herbivory, such as tree age, or landscape configuration, among others. Despite it, recent calls advocate for complement experimental designs in the effect of diversity on ecosystem functioning with observational studies (Dee et al., 2023 ). 5. Conclusion Townhalls spend millions in pest eradication in urban areas, to maintain otherwise typically maladapted tree species to enhance aesthetics of a human dominated environment (Fleck & Hernández-Agüero, 2025 ). The strategic plantation of diverse species of trees also taking into account their relative abundance, as we detected in this study and many others has reported previously, will reduce pest damage. More research is needed to discern if this reduction is mediated by host dilution, enemies release or both. In any case, increased diversity plantation in urban areas will enhance plant protection mediated by association resistance, and thus reducing costs in urban management. Declarations Conflict of interest The authors declare no competing interests. Acknowledgements Velin Ivaylov Velichkov and Bas Krijnen were supported by the MSc program Environment and Resource Management at the Vrije Universiteit (VU) in Amsterdam and J. A. H.-A. was supported by the INTEGRADIV project from Biodiversa+. We acknowledge Dr. Pablo Castro Sánchez-Bermejo for his assistance in data collection. Funding This study has been conducted with the financial support of the Dobberke Grant 2023, granted by the Dr J. L. Dobberke Foundation for Comparative Psychology and the KNAW Ecology Fund 2023. Contributions Juan Antonio Hernández-Agüero: Conceptualization, Methodology, Formal analysis and investigation, Writing - original draft preparation, Funding acquisition, Supervision. 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11:16:49","extension":"html","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":160965,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/5490d5bfb80e0903efd86d61.html"},{"id":95378240,"identity":"57ae29f9-7277-4e17-b262-660d8beef5f6","added_by":"auto","created_at":"2025-11-07 11:16:49","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":434401,"visible":true,"origin":"","legend":"\u003cp\u003eMap of study locations. The dots represent all selected \u003cem\u003eQuercus robur\u003c/em\u003e trees (n = 30) in Amsterdam. Orange lines represent the paths followed for the monthly tree visits. Background map is the satellite image obtained from Google Earth.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/07a482b50c65d84bef5fe051.png"},{"id":95526476,"identity":"f3b81354-0fff-4c9a-a445-e80f52e26e43","added_by":"auto","created_at":"2025-11-10 10:07:02","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":66108,"visible":true,"origin":"","legend":"\u003cp\u003ePredictions of generalized linear mixed models showing the proportion of herbivory with 95% confidence intervals along a gradient of a) tree species richness, b) Shannon index and, c) Inverse of Simpson index. Mean observed values of the proportion of herbivory per plant individual and review are shown by dots.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/becbcd016f797a778c0ecb84.png"},{"id":97250908,"identity":"edecf565-27b4-480b-95f1-bb2507906a32","added_by":"auto","created_at":"2025-12-02 13:15:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":958724,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/535d0539-ff6b-4d37-b76c-5f85d256bc6c.pdf"},{"id":95378254,"identity":"51b698d7-d1ae-4c8b-bb97-48c97f925d75","added_by":"auto","created_at":"2025-11-07 11:16:49","extension":"zip","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":8734060,"visible":true,"origin":"","legend":"","description":"","filename":"DataHernandezAguero2025APIS.zip","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/b1cb48c409eaf12876d28beb.zip"},{"id":95378244,"identity":"78e1c53b-9739-4b3c-8833-903ca035be18","added_by":"auto","created_at":"2025-11-07 11:16:49","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":531396,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-7907916/v1/da77bbbd3120855513f082ef.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Urban tree diversity reduces invertebrate leaf herbivory","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMore than half of the world population is currently living in cities with more than 300.000 inhabitants. With urbanization increasing globally, this number is expected to rise to 70 % by 2050 (United Nations, 2018). Urban environments suffer from numerous environmental impacts, such as water and air pollution, or drastic changes in land use, which results in devastating effects for ecosystems and climate (Bai et al., 2017). Due to these impacts, urbanization is considered one of the main contributors to local biodiversity extinction rates in these areas (McKinney, 2002). Moreover, global climate has experienced a change in the last decades with a pace never previously recorded (Diffenbaugh \u0026amp; Field, 2013). The global temperature increase (NOAA, 2023) is affecting ecosystem functioning at all levels. This is exacerbated locally in cities, as they experience the so-called urban heat island (UHI) (Kim, 1992). Because of this, cities are valuable places to study ecological networks, as the temperatures today resemble future temperature scenarios outside urban areas in the same latitudes due to climate change (Youngsteadt et al., 2015).\u003c/p\u003e\n\u003cp\u003eOne of the most interesting ecological networks nowadays are trophic interactions. They are particularly relevant for biological communities, as they shape community dynamics, drive important aspects of ecosystem functioning, and provide essential ecosystem services (Gaüzère et al. 2022). These services include nutrient cycling (DeAngelis, 1992), pest control (Whelan, 2008), or those related with the adaptation to the effects of climate change, which are especially relevant in urban environments (Sirakaya et al., 2018). At the same time, these trophic interactions can result as well in ecosystem disservices to humans, which are likewise of considerable interest. There are some examples, such as the apparition of pests that damage cultivars that also can harm people (Moreira \u0026amp; Abdala-Roberts, 2023) or the processionary oak moth, \u003cem\u003eThaumetopoea processionea\u003c/em\u003e, which base their diet on oak leaves. This moth releases barbed hairs that cause dermatitis among humans, posing a threat to society (Rahlenbeck \u0026amp; Utikal, 2015), and constituting an additional ecosystem disservice. Yet, non-consensus has been established on how urbanization affects pests and its herbivory interactions (Meineke et al., 2013). In some cases, a disruption has been observed (Miles et al., 2019; Meineke et al., 2019; Moreira et al., 2019; Nuckols \u0026amp; Connor, 1995; Valdés-Correcher et al., 2022), while in others it may be beneficial (Christie \u0026amp; Hochuli, 2009; Dale \u0026amp; Frank, 2014; Parsons, \u0026amp; Frank, 2019; Rivkin \u0026amp; de Andrade, 2023). These discrepancies have been explained in the past by regional (Hernández-Agüero et al., 2024a) or latitudinal differences (Alonso-Crespo \u0026amp; Hernández-Agüero, 2023; Hernández-Agüero et al., 2024b), and are mainly explained by artificial abiotic factors produced in urban areas, such as impervious surfaces or temperature increment. Artificial biotic factors could be also used to explain these discrepancies. However, to date, this approach has not been widely applied (but see Stemmelen et al., 2025a).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eUrban environments differ from natural environments in numerous ways. The main ones are the reduced size and composition of habitats due to land use change (Liu et al., 2018). Urban habitats tend to be more fragmented, which leads to lower connectivity between natural areas which entails a significant difference in biodiversity, ecosystem functioning and provision of ecosystem services (Liu et al., 2018). Also, urban areas usually are modified in a way that species pool, specially of plants, are artificially altered by plantations and management. Thus, the combination of vegetation management and the local novel climates due to urban heating ensemble an unseen niche for native and exotic species. This can exacerbate the disappearance of species, while favoring the suitability of some urban exploiters (\u003cem\u003esensu\u003c/em\u003e Blair, 1996), increasing ecosystem disservices, with fatal consequences for environment and people.\u003c/p\u003e\n\u003cp\u003eHowever, these unique urban tree species pools, often more diverse than the ones found in natural settings (Augustinus et al., 2024), can be used as a tool to counterbalance the aforementioned impacts. More diverse communities should be more resistant to biological invasions, based on the associational resistance (Barbosa et al., 2009). This has been explained in the past by the Enemies hypothesis (Letourneau, 1987) which states that higher tree biodiversity can have a positive effect on natural enemies of herbivores. Neighboring plants can help to reduce damage to a focal plant by providing resources (e.g., nectar) or harboring alternative prey that attract herbivore predators or parasitoids. In line with that is the Host Dilution hypothesis (Underwood et al., 2014). This hypothesis explains how herbivory is reduced where tree neighbor biodiversity is higher, since abundance of predators per host gets reduced. In contrast, the Resource Concentration hypothesis (Root, 1973) posits that herbivores could benefit from the presence of different species, as alternative resources could increase their surveillance. This hypothesis is of special interest in urban areas, as it affects specially generalist species. Altogether, these hypotheses established a solid framework to explore the effect of diversity on species interactions in urban areas. Nevertheless, they have mostly been applied in experimental tree plantations, with manipulative diversity patterns restricted to combinations of one to five species (Muiruri et al. 2019). However the level of complexity that urban areas can have in terms of species composition make these environments unique. For example, it is known that diversity effects on herbivory are greater when mixed forests comprise taxonomically more distant tree species (Jactell et al., 2007), thus in urban areas where species pools are the result of a human induced configuration, without any common evolutionary history between species, and therein representing a higher taxonomically distant species, we expect the diversity effects on herbivory of great relevance.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe present study aims to test the effect of urban tree diversity in species trophic interactions. If we find that higher levels of herbivory are detected in low tree diversity areas, the results will be supported by the Enemies hypothesis (Letourneau, 1987)/ Host Dilution hypotheses (Underwood et al., 2014). Contrary, if we find higher levels of herbivory in high tree diversity areas, the results will be supported by the Resource Concentration hypothesis (Root, 1973).\u003c/p\u003e"},{"header":"2. Methodology","content":"\u003cp\u003eThe city of Amsterdam (The Netherlands) was selected to study how tree diversity affects herbivory in urban areas, as it has certain characteristics that make it the ideal location for this purpose. First, Amsterdam is relatively flat and herbivory can be influenced by elevation (Galm\u0026aacute;n et al., 2021). Secondly, it is a highly populated city. The metropolitan area of Amsterdam has a size of 219.3 km\u003csup\u003e2\u003c/sup\u003e and a population of 1,174,000 in 2023 (United Nations, 2022). Lastly, Amsterdam municipality has available a database of all trees of the city and it is georeferenced, which makes the diversity characterization easier.\u003c/p\u003e\n\u003cp\u003eIn order to examine the relationship between insect herbivory and local tree biodiversity, leaves were collected and examined from a set of \u003cem\u003eQuercus robur\u0026nbsp;\u003c/em\u003eL. trees (Common or English oak). The percentage of leaf area eaten by herbivores was analyzed by collecting and scanning leaves of 30 \u003cem\u003eQ. robur\u003c/em\u003e specimens (Figure 1). The selection of the trees was done in the following way: a list of all the \u003cem\u003eQuercus robur\u003c/em\u003e trees in Amsterdam were downloaded from the website of the Amsterdam municipality (Klaas-Bindert de Haan, 2023), which amounted to 8,654 specimens. Those were then filtered to show only trees smaller than 12 meters tall (to allow for accessible leaf collection), resulting in 2,882 trees. For these ones, human population density, NDVI and water cover percentage around each tree were calculated using R. The rankings on these three factors were spread in 5 quantiles and only one tree was selected from each quantile so a set as diverse as possible was created (but see Krijnen \u0026amp; Hern\u0026aacute;ndez-Ag\u0026uuml;ero, 2025; for more details). The set contained 30 trees (Appendix 1). From each tree, 20 leaves were collected monthly during all the growing season (May to November), making for a total of 3,153 leaves. Not all trees could be sampled every month due to various unforeseen circumstances. Trees were excluded through time due to loss of reachable branches and gardening works out of control from the authors of this paper (16 trees with 7 reviews, 2 with 6 reviews, 3 with 5 reviews and 9 with only 2 reviews).\u003c/p\u003e\n\u003cp\u003eThe leaves were picked randomly, ensuring local orientation variability, at 1.5 to 2 meters high. In case that the lower reachable leaves were in branches higher than 2 meters, the lowest leaves were collected. Leaves were scanned with a high resolution scanner (Epson Perfection V39 scanner) using a consistent methodology. Leaf scans were analyzed using the processing software ImageJ (Schneider et al. 2012), to measure the eaten area of the leaf (Meineke et al., 2018, Alonso-Crespo \u0026amp; Hern\u0026aacute;ndez-Ag\u0026uuml;ero, 2023). The percentage of eaten area was calculated by delineating the shape of the leaf without herbivory and the real perimeter of each leaf. The real shape of the leaves was drawn out by hand in ImageJ.\u003c/p\u003e\n\u003cp\u003eA biodiversity assessment of the area around every of the 30 selected trees was conducted. Using the same tree database as above, each tree name was harmonized using the \u0026acute;tol_resolve\u0026acute; function from the \u0026acute;taxize\u0026acute; package (version 0.10.0; Chamberlain \u0026amp; Szocs, 2013). Buffers of 200 meters were created around each \u003cem\u003eQ. robur\u003c/em\u003e tree used to estimate herbivory with the \u0026lsquo;st_buffer\u0026rsquo; function from \u0026lsquo;sf\u0026rsquo; package (version 1.0.17; Pebesma, 2018) using EPGS:3857 as a coordinate reference system (CRS). A 200 meter buffer was chosen for this study based on previous research on herbivorous insect dispersal on \u003cem\u003eQ. robur\u003c/em\u003e in northern Europe, where the majority of dispersing happens in a couple hundred meters (Gripenberg et al., 2008, Zheng et al., 2015, Barr et al., 2021, Vald\u0026eacute;s-Correcher et al., 2022).\u003c/p\u003e\n\u003cp\u003eList of trees from the complete list of trees of the study area inside each buffer were extracted with the \u0026lsquo;st_intersection\u0026rsquo; function of the \u0026lsquo;sf\u0026rsquo; package. Species richness was estimated as the total number of different species present around each \u003cem\u003eQ. robur\u003c/em\u003e. Shannon and inverse of Simpson were obtained with the \u0026lsquo;diversity\u0026rsquo; function of the \u0026lsquo;vegan\u0026rsquo; package (version \u0026lsquo;2.6.6.1\u0026rsquo;; Oksanen et al., 2024). Shannon index (Shannon, \u0026amp; Weaver, 1949) shows the weighted geometric mean of the proportional abundance of species, and the inverse of Simpson index (Simpson, 1949) contains the number of species present and their relative abundance. Data on percentage of water, NDVI and Human Population Density in each radius was extracted from Krijnen \u0026amp; Hern\u0026aacute;ndez-Ag\u0026uuml;ero (2025) with the following methodology. Satellite imagery from Sentinel-II (ESA, 2023) was used for remote sensing of vegetation and water via NDVI and NDWI, following strict selection criteria (low cloud cover, same satellite, close sensing dates). Predictors were reclassified and inverted to isolate features. Population data from WorldPop (2024) was used to assess density in Amsterdam at 100\u0026times;100 m resolution.\u003c/p\u003e\n\u003cp\u003eGeneralized Linear Mixed Models (GLMM) were created to explore how herbivory was affected by each biodiversity index with the \u0026lsquo;glmmTMB\u0026rsquo; function of \u0026lsquo;glmmTMB\u0026rsquo; package (version \u0026lsquo;1.1.9\u0026rsquo;; Mollie et al., 2017). This package was used to include a beta family of errors with logit link. Following package authors, and considering that herbivory data has a zero inflated nature, which could produce errors when adjusting models, a transformation of data was made. Transformation was made using the formula:\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u0026nbsp;y\u0026apos; = (y \u0026times; (n - 1) + 0.5) / n\u003c/em\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ewhere y is the original herbivory (defoliation) value, n is the total number of observations and y\u0026apos; is the transformed value. This data transformation is necessary to correct any existing bias in the data and avoid issues with extreme values like 0 and 1. Normalized Difference Vegetation Index (NDVI), Human Population Density (HPD), Water cover percentage in the proximity of every tree and their interaction were included as fixed factors in the models, along with the additive effect of each diversity index. One model was fitted for each diversity index. Additionally, an extra model was fitted including the number of conspecific (\u003cem\u003eQ. robur\u003c/em\u003e) individuals around each studied tree. In all cases, an ID including the tree and month of review was added as a random effect. Alternative models were compared using the Akaike Information Criterion (AIC) to assess the effects of explanatory variables (i.e. fixed effects). Models with a difference in AIC\u0026thinsp;\u0026gt;2 suggested that the less favorable model could be omitted. All analyses were done in the R environment (version 4.4.0; R Core Team, 2024).\u003c/p\u003e"},{"header":"3. Results","content":"\u003cp\u003eAn area of 200 m was established around the 30 trees selected for herbivory analysis to measure diversity, finding a total of 205 different tree species, with the most common species being \u003cem\u003eQuercus robur\u003c/em\u003e with 799 specimens. Species richness for the 30 analyzed trees ranged from 1 to 75 with a mean of 22.3 species. Shannon\u0026rsquo;s index ranged from 0 to 3.27 with a mean of 2.19. Simpson\u0026rsquo;s index ranged from 0 to 0.95 with a mean of 0.77. NDVI ranged from 0.1 to 0.49 with a mean of 0.3. Water cover ranged from 0 to 0.3 with a mean of 0.04. HPD ranged from 2.9 to 43.1 with a mean of 24.68 people/hectare. A total of 3,153 leaves were analyzed for herbivory. They had a mean herbivory of 0.063, with the data ranging from 0 to 0.74. Herbivory data had a standard deviation of 0.096.\u003c/p\u003e\u003cp\u003eThe best model for each diversity index includes all environmental variables and the additive effect of diversity (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Among all the diversity variables (species richness, Shannon index, and Simpson's inverse index), the best model was the one that included the Shannon index. The estimates for each diversity index were negative in the case of species richness (p-value\u0026thinsp;=\u0026thinsp;0.002267; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), the Shannon index (p-value\u0026thinsp;\u0026lt;\u0026thinsp;2.80e-08; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) and the Simpson index (p-value\u0026thinsp;=\u0026thinsp;1.32e-07; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eAIC results of each studied model including degrees of freedom (df) and Akaike Information Criterion values.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eModel\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003edf\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAIC\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNull\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-14092.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWithout diversity\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-14146.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpecies richness\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-14153.09\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eShannon index\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-14172.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSimpson index\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-14169.59\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eR\u003csup\u003e2\u003c/sup\u003em and R\u003csup\u003e2\u003c/sup\u003ec were 0.097 and 0.219 for Species Richness, 0.115 and 0.220 for Shannon index, and 0.113 and 0.220 for Simpson index. Finally, the inclusion of the number of conspecific individuals did not increase model performance, but the proportion of conspecifics between the overall number of trees did.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eOur findings showed that urban tree diversity affected invertebrate herbivory in the city of Amsterdam, independently of the diversity index metric used. The trees studied that were surrounded by higher diversity (for any of the diversity indices: species richness, Shannon diversity index, or Simpson's inverse diversity index) suffered lower levels of herbivory compared to those surrounded by lower diversity. Ours is not the first study to detect these effects of urban tree diversity. A similar study in the city of Montreal (Canada) also showed a reduction in insect herbivory with the increase of tree diversity (Stemmelen et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), and despite not being able to prove it, they explained their results by predation pressure. In a parallel study in Burdeaux (France), Stemmelen et al. (\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) demonstrated that the neighboring diversity increased predation on a chestnut herbivore, but did not decrease overall herbivory, that was more mediated by tree density. In line with this result, we found better model performance when considering Shannon diversity index. One of the novelties of this study is the use of different taxonomic diversity metrics apart from species richness, that is a very poor biodiversity indicator (Roswell et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), as it only shows the amount of species, not taking into consideration their proportional abundance or abundance at all. Shannon diversity not only takes into account the number of species in a community, but also their relative abundance, including rare species. In contrast, the Simpson index gives more weight to dominant species. Therefore, our results indicate that tree density influences herbivory, as reflected in the Shannon diversity index. Another study, in North Carolina (USA), showed comparable results, finding less pest density in more diverse tree urban forests, but this increment was not explained by an increase in pest regulation (Wilson et al., \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). They did explore the relation between pest abundance and impervious surfaces, finding higher abundance in more urbanized areas. Environmental differences (e.g. urban warming) between urban and forested settings can simplify enemies communities, making it difficult to predict how the effects of tree diversity on enemies found in forests translate to cities (Dale \u0026amp; Frank, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In our case, this was controlled in the models by the inclusion of abiotic factors, thus results shown here can be considered independent of the urban abiotic configuration.\u003c/p\u003e\u003cp\u003eOur results support the idea that more diverse plant communities are more resistant to biological invasions. This is consistent with the associational resistance hypothesis (Barbosa et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) in which it is stated that specific plant associations reduce the likelihood of a plant being detected by herbivores compared to where the plant grows alone or with certain neighbors. This pattern aligns with the Enemies hypothesis (Letourneau, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1987\u003c/span\u003e), which suggests that higher tree diversity can enhance populations of natural enemies that regulate herbivores. In this same context, neighboring plant species may reduce damage to focal plants by providing alternative resources such as nectar or by harboring alternative prey that attract predators and parasitoids of herbivores. In the same line, the Host Dilution hypothesis (Underwood et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), proposes that high number of non-target species reduces the proportion of herbivorous host, as in a more diverse community each particular plant species is relatively less abundant and its distribution is likely to be more fragmented, making it less available to specialist herbivores (Jactel \u0026amp; Brockerhoff, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). This host dilution can also reduce the overall proportion of plants attacked or damaged by herbivores by providing various chemical and physical traits that reduce the overall herbivory (Ruttan \u0026amp; Lorite, 2014). It is possible that plants in close proximity to the focal \u003cem\u003eQ. robur\u003c/em\u003e trees released chemicals that repelled herbivorous insects or that those insects were not able to chemically detect their host by the presence of \u0026acute;chemical barriers' to host location. This is known to occur when signals from non-host trees disrupt olfactory host tree recognition, thus also resulting in a decrease of herbivory (Jactel \u0026amp; Brockerhoff, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHost Dilution in cities could be of greater intensity than in nature, since the proportion of non-host species can reach higher levels due to a higher proportion of exotic species (Avolio et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). These exotic species usually have a different evolutionary history that could result in signals less easily recognizable by herbivores (Castagneyrol et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In addition to the host dilution effect, the presence of exotic plants can further reduce overall herbivory. This is because, when introduced, many exotic species lack natural enemies specialized in the new environment, which is known as the Enemy Release hypothesis (Blossey \u0026amp; Notzold, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). Consequently, exotic species not only contribute to diluting the herbivorous pressure on native species, but also decrease their own susceptibility to being attacked. This would make urban areas even more unsuitable for native herbivores as the difficulties in host location by the presence of different tree species are not counterbalanced by the presence of alternative food sources. We found support for the host dilution hypothesis, opposite to previous studies (Stemmelen et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The inclusion of host numbers did not improve model performance, but their relative abundance did. However, the best model always included the diversity metric.\u003c/p\u003e\u003cp\u003eAt the same time, the availability of different food sources for herbivores could indirectly increase their predators and parasitoids, explained by the Enemies hypothesis (Letourneau, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). As in higher plant diversity areas predators, specially generalist (Jactel \u0026amp; Brockerhoff, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), can have access to different preys that typically spans temporally different, predators and parasitoids can have higher variety of preys, thus increasing predation (Shao et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Schill\u0026eacute; et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The presence of generalist species is greater in urban than in natural areas (Lefcheck et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), this could have affected the overall detected herbivory, since generalist species are not strongly affected as specialist species from a higher plant diversity (Moreira et al \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), as explained by the Resource Concentration hypothesis (Root, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e1973\u003c/span\u003e), but this was not the case in Amsterdam. The reason could be the high specificity of herbivorous species (Hern\u0026aacute;ndez-Ag\u0026uuml;ero et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) that explain a lower proportion of generalist species than other trophic groups such as predators and parasitoids.\u003c/p\u003e\u003cp\u003eIn this context, although results suggest that predators could be responsible for the reduction in herbivory in more diverse urban forests, it is important to consider that higher trophic levels tend to be more sensitive to urbanization than lower trophic levels (Burkman \u0026amp; Gardiner, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Therefore, without a direct analysis of the predator community, it is not possible to fully confirm the role of the Enemies hypothesis. Further studies must explore the predation role in explaining the effect of plant diversity in herbivory to be able to definitively point to Enemies hypothesis as the one explaining the reduction of herbivory with increased diversity. Moreover, because our study was observational, we could not separate the effects explained by non-evaluated predictors of herbivory, such as tree age, or landscape configuration, among others. Despite it, recent calls advocate for complement experimental designs in the effect of diversity on ecosystem functioning with observational studies (Dee et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eTownhalls spend millions in pest eradication in urban areas, to maintain otherwise typically maladapted tree species to enhance aesthetics of a human dominated environment (Fleck \u0026amp; Hern\u0026aacute;ndez-Ag\u0026uuml;ero, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The strategic plantation of diverse species of trees also taking into account their relative abundance, as we detected in this study and many others has reported previously, will reduce pest damage. More research is needed to discern if this reduction is mediated by host dilution, enemies release or both. In any case, increased diversity plantation in urban areas will enhance plant protection mediated by association resistance, and thus reducing costs in urban management.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eVelin Ivaylov Velichkov and Bas Krijnen were supported by the MSc program Environment and Resource Management at the Vrije Universiteit (VU) in Amsterdam and J. A. H.-A. was supported by the INTEGRADIV project from Biodiversa+. We acknowledge Dr. Pablo Castro Sánchez-Bermejo for his assistance in data collection.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study has been conducted with the financial support of the Dobberke Grant 2023, granted by the Dr J. L. Dobberke Foundation for Comparative Psychology and the KNAW Ecology Fund 2023.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJuan Antonio Hernández-Agüero: Conceptualization, Methodology, Formal analysis and investigation, Writing - original draft preparation, Funding acquisition, Supervision. Velin Ivaylov Velichkov: Methodology, Formal analysis and investigation, Writing - original draft preparation. Bas Krijnen: Methodology, Formal analysis and investigation, Writing - review and editing. Inés María Alonso-Crespo: Conceptualization, Formal analysis and investigation, Writing - review and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data and script used for this paper is available as supplementary material.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAugustinus, B. 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Beyond metacommunity paradigms: habitat configuration, life history, and movement shape an herbivore community on oak. \u003cem\u003eEcology\u003c/em\u003e, 96(12), 3175\u0026ndash;3185. https://doi.org/10.1890/15-0180.1\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Urban ecology, Biodiversity, Nature based Solutions, Herbivory, Quercus robur","lastPublishedDoi":"10.21203/rs.3.rs-7907916/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7907916/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGlobal rise of urbanization and climate change are strongly affecting biodiversity, species interactions and species survival. Urban areas are socio-ecological ecosystems which often result in climate novelty and unique artificial species assemblages through tree plantation. This unique tree species pool can be used as a tool to counterbalance the aforementioned impacts. Enemies hypothesis and Host Dilution hypothesis post that higher tree biodiversity can reduce herbivory by reducing host detectability or by having a positive effect on natural enemies of herbivores. Oppositely, the Resource-Concentration hypothesis claims that higher plant diversity increases overall herbivory. The aim of this study was to test the effect of urban tree diversity in trophic interactions. This was assessed by measuring herbivory rates in 3153 leaves of 30 \u003cem\u003eQuercus robur\u003c/em\u003e trees surrounded by different tree diversity levels. Urbanization level was included to consider local warming due to urban heat island effect. Estimation of the surrounding vegetation or water was also considered as a reductor of this artificial heating. Herbivory was negatively affected by local tree diversity, with Shannon’s index having the strongest effect. Host Dilution hypothesis supports these results, as more even tree communities suffered from less herbivory. Higher levels of tree diversity can be used as a nature based solution to avoid the ecosystem disservice of herbivory in urban areas. Our results suggest that urban managers need to avoid monocultures and consider relative abundance in urban plantations in order to increase urban tree fitness by reducing herbivory.\u003c/p\u003e","manuscriptTitle":"Urban tree diversity reduces invertebrate leaf herbivory","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-07 11:16:44","doi":"10.21203/rs.3.rs-7907916/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"8a54cca9-62e4-4169-b301-496d1d536e93","owner":[],"postedDate":"November 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-01T16:38:42+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-07 11:16:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7907916","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7907916","identity":"rs-7907916","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}