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Risk of bird predation and defoliating insect abundance are greater in urban forest fragments than street trees

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Predation by natural enemies is important for regulating herbivore abundance and herbivory. Theory predicts that complex habitats support more natural enemies, which exert top-down control over arthropods and therefore can reduce herbivory. However, it is unclear if theory developed in other more natural systems similarly apply to predation by vertebrate and invertebrate natural enemies across urban habitats of varying complexity. We used plasticine caterpillar models to assess risk of predation by birds and insects, collected leaf-feeding arthropods, and measured herbivory in willow oak trees (Quercus phellos) in two seasons to determine how predation influenced herbivory across urban forest fragments, street trees planted near forest fragments, and downtown street trees. Predation attempts by birds and abundance of chewing folivores were greater on trees growing in urban forest fragments than downtown street trees. Bird predation attempts and herbivory levels were inconsistent for near-forest trees. Predation attempts by arthropods did not statistically vary among the three urban tree habitats. Contrary to expectations based on theory, chewing folivore abundance and herbivory were generally highest on trees growing in urban forests, the most complex habitat we studied, and the habitat where risk of bird predation was greatest. We suggest that urban forest fragments provide better habitat than other urban landscapes for both urban birds and chewing folivores by having greater habitat complexity. Therefore, basal resources, such as availability of suitable habitat, mediate top-down effects on herbivores in cities.
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Risk of bird predation and defoliating insect abundance are greater
in urban forest fragments than street trees
Lawrence C. Long
&Steven D. Frank
Published online: 20 February 2020
#Springer Science+Business Media, LLC, part of Springer Nature 2020
Predation by natural enemies is important for regulating herbivore abundance and herbivory. Theory predicts that complex habitats
support more natural enemies, which exert top-down control over arthropods and therefore can reduce herbivory. However, it is
unclear if theory developed in other more natural systems similarly apply to predation by vertebrate and invertebrate natural enemies
across urban habitats of varying complexity. We used plasticine caterpillar models to assess risk of predation by birds and insects,
collected leaf-feeding arthropods, and measured herbivory in willow oak trees (Quercus phellos) in two seasons to determine how
predation influenced herbivory across urban forest fragments, street trees planted near forest fragments, and downtown street trees.
Predation attempts by birds and abundance of chewing folivores were greater on trees growing in urban forest fragments than
downtown street trees. Bird predation attempts and herbivory levels were inconsistent for near-forest trees. Predation attempts by
arthropods did not statistically vary among the three urban tree habitats. Contrary to expectations based on theory, chewing folivore
abundance and herbivory were generally highest on trees growing in urban forests, the most complex habitat we studied, and the
habitat where risk of bird predation was greatest. We suggest that urban forest fragments provide better habitat than other urban
landscapes for both urban birds and chewing folivores by having greater habitat complexity. Therefore, basal resources, such as
availability of suitable habitat, mediate top-down effects on herbivores in cities.
Keywords Forest fragments .Urban birds .Defoliators .Street trees .Top-down control
Urban trees provide ecosystem services for people and habitat
for species.Unfortunately, urban street trees often have greater
arthropod pest densities and herbivory than trees in rural for-
ests which can reduce these services (Frankie and Ehler 1978;
Dreistadt et al. 1990; Lunney and Burgin 2004; Christie and
Hochuli 2005; Raupp et al. 2010; Dale and Frank 2014).
However, Kozlov et al. (2017) found lower herbivory in
European cities than rural forests due to predation by birds
and ants suggesting that while common, the pattern of elevat-
ed herbivory in urban ecosystems in not universal (Hochuli
and Threlfall 2017). Urban trees grow in many different loca-
tions, such as along streets and in parking lots, in managed
ornamental landscapes, and in forest fragments. Trees in each
location likely encounter different herbivore communities
which inflict different amounts of herbivory (Hanks and
Denno 1993;Longetal.2019). Trees in forest fragments are
often surrounded by many types of vegetation, including other
trees and shrubs. Vegetation diversity and density could pro-
tect trees from herbivory through multiple ecological mecha-
nisms, such as enhanced natural enemy abundance (Hanks
and Denno 1993; Shrewsbury and Raupp 2000; Tooker and
Hanks 2000) or associational resistance (Barbosa et al. 2009)
where nearby plants help obscure host plants from their her-
bivores (Root 1973; Feeny 1976;Frank2014). On the other
hand, forest trees may be exposed to greater herbivore abun-
dance and diversity and thus more herbivory than trees
surrounded by pavement due to greater plant richness and
abundance (Haddad et al. 2001; Vehviläinen et al. 2007;
Borer et al. 2012; Meyer et al. 2017). Trees growing in a
downtown sidewalk pit or the median of a busy street are
isolated from other vegetation and exposed to abiotic stresses
which can increase herbivore abundance and herbivory in
Electronic supplementary material The online version of this article
( contains supplementary
material, which is available to authorized users.
*Lawrence C. Long
Department of Entomology and Plant Pathology, North Carolina
State University, Raleigh, NC 27607, USA
Urban Ecosystems (2020) 23:519531
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Arthropods are experiencing global declines, which can disrupt energy transfer in food webs and affect ecosystem services (Hallmann et al., 2017;Lister & Garcia, 2018;Sánchez-Bayo & Wyckhuys, 2019). Arthropods are a particularly important food resources for birds, providing calcium and amino acids, and arthropods in urban forests are often crucial to bird success in urban environments (Long & Frank, 2020;Nagy & Holmes, 2005;Seress et al., 2018). The phenology of strict herbivores like caterpillars, the larval form of butterflies and moths in the order Lepidoptera, are reliant on particular plant communities (Koricheva & Hayes, 2018). ...
... These results suggest urban forests invaded by nonnative plants can support some arthropod populations which in turn provide food resources for other wildlife, including birds, that depend on calcium and amino acid-rich foods to feed their young Long & Frank, 2020;Nagy & Holmes, 2005;Parsons et al., 2020). ...
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Aim Ecological theory and empirical evidence indicate that greater structural complexity and diversity in plant communities increases arthropod abundance and diversity. Nonnative plants are typically associated with low arthropod abundance and diversity due to lack of evolutionary history. However, nonnative plants increase the structural complexity of forests, as is common in urban forests. Therefore, urban forests are ideal ecosystems to determine whether structural complexity associated with nonnative plants will increase abundance and diversity of arthropods, as predicted by complexity literature, or whether structural complexity associated with nonnative plants will be depauperate of arthropods, as predicted by nonnative plant literature. Location We sampled 24 urban temperate deciduous and mixed forests in two cites, Raleigh, North Carolina and Newark, Delaware, in the eastern United States. Methods We quantified ground cover vegetation and shrub layer vegetation in each forest and created structural complexity metrics to represent total, nonnative and native understory vegetation structural complexity. We vacuum sampled arthropods from vegetation and quantified the abundance, biomass, richness and diversity of spiders and non‐spider arthropods. Results Nonnative plants increase understory vegetation complexity in urban forests. In Raleigh and Newark, we found support for the hypotheses that dense vegetation will increase arthropod abundance and biomass, and against the hypothesis that nonnative vegetation will decrease arthropods. Urban forest arthropod abundance and biomass, but not diversity, increased with greater nonnative and native structural complexity. Main Conclusions Invaded urban forests may provide adequate food in the form of arthropod biomass to transfer energy to the next trophic level, but likely fail to provide ecological services and functions offered by diverse species, like forest specialists. Urban land managers should survey urban forests for nonnative and native plant communities and prioritize replacing dense nonnative plants with native species when allocating vegetation maintenance resources.
... Therefore, the number of studies examining how urban landscapes and urbanization processes (defined here as reduced vegetation cover and increasing proportion of impervious surfaces) affect arthropods has increased considerably in recent decades (Dale and Frank, 2018;Fenoglio et al., 2020). However, observations report inconsistent (positive, negative or neutral) effects of urbanization on the abundances of herbivores (Bergerot et al., 2010;Dale and Frank, 2014;Herrmann et al., 2012;Korányi et al., 2021;Liu et al., 2016;Long and Science of the Total Environment 834 (2022) 155396 Frank, 2020) and their natural enemies (Alaruikka et al., 2002;Burks and Philpott, 2017;Dale and Frank, 2014;Rocha and Fellowes, 2020;Tamburini et al., 2016;Zolotarev and Belskaya, 2015). ...
... First, feeding guild of arthropods provides important information about ecological traits related to resource-acquisition, behavior and life-cycle (Raupp et al., 2010) and thus might influence their response to urbanization [e.g. positive effects on sap-feeders (Dale and Frank, 2014;Korányi et al., 2021), adverse effects on chewing folivores (Long and Frank, 2020;Moreira et al., 2019), and inconsistent effects on leaf miners (Denys and Schmidt, 1998;Dobrosavljević et al., 2020;Moreira et al., 2019)]. Second, arthropods from different taxonomic and functional groups have distinct sensitivities to the altered abiotic conditions of urban environments (e.g. ...
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Biological control is a major ecosystem service provided by pest natural enemies, even in densely populated areas where the use of pesticides poses severe risks to human and environmental health. However, the impact of urbanization on this service and the abundance patterns of relevant functional groups of arthropods (herbivores, predators, and parasitoids) remain contested. Here, we synthesize current evidence through three hierarchical meta-analyses and show that advancing urbanization leads to outbreaks of sap-feeding insects, declining numbers of predators with low dispersal abilities, and weakened overall biological pest control delivered by arthropods. Our results suggest that sedentary predators may have the potential to effectively regulate sap-feeders, that are one of the most important pests in urban environments. A well-connected network of structurally diverse and rich green spaces with less intensive management practices is needed to promote natural plant protection in urban landscapes and sustainable cities.
... This method has been successfully used to estimate predation pressure on caterpillars (Ferrante et al. 2014, Howe et al. 2009, Loiselle & Farji-Brener 2002, Richards & Coley 2007, Tvardikova & Novotny 2012. The majority of these studies were carried out in wooded areas with a different level of succession, whereas only a few studies have evaluated predation pressure in urban and suburban environments (Eötvös et al. 2018, Ferrante et al. 2014, Kozlov et al. 2017, Long & Frank 2020, Roels et al. 2018. Although it has been suggested that generalist predators are similarly attracted by chemical cues of artificial and real caterpillars (Ferrante et al. 2017b, Richards & Coley 2007, this method does not measure actual predation rates . ...
Growing urban expansion can alter ecological processes within trophic networks. Predation on herbivores is known to vary with the size of the area covered by vegetation, successional stage, altitude and predator community structure; however there are gaps in understanding how this occurs in urban and suburban environments. The purpose of this study was to determine whether predation pressure on artificial models of caterpillars varied with the degree of urbanisation and type of substrate. Artificial caterpillars were placed on two types of substrates (leaf vs . stem) in two areas of the city (urban vs . suburban). Total predation was measured as the number of models with evidence of attack by predators, with the predation rate estimated on a weekly basis. Predation was affected by the degree of urbanisation, being higher in urban ( x̄ = 9.88%; SD = 4.09%, n = 8) than suburban areas ( x̄ = 5.75%, SD = 4.21%, n = 8). Attack marks were observed in 23.8% ( n = 125) of artificial caterpillars. The weekly predation rate on leaves ( x̄ = 9.63%, SD = 5.95%, n = 8) was higher than that on stems ( x̄ = 6%, SD = 4.2%, n = 8). These results suggest that the incidence of predation might vary with the degree of urbanisation and by the type of substrate on which prey organisms are found.
... In some respects, this pattern may match demand for such services, with a higher need for litter removal in population-dense areas. However, invertebrate pest outbreaks often occur in urban areas and isolated green spaces within a high impervious surface context may be more vulnerable to such outbreaks (Long & Frank, 2020). Furthermore, despite particularly high demand for cultural ecosystem services in areas with more impervious surface cover (Goodness et al., 2016;Valente-Neto et al., 2021), they are likely most scarce there due to the absence of colorful species. ...
Land use intensification in urban areas can have profound effects on biological communities that provide valuable ecosystem services to urban residents. We used a response-and-effect functional trait approach to determine how bird species’ responses to local and landscape-scale habitat of urban green spaces affects the supply of cultural and regulating ecosystem services. We sampled bird communities and habitat variables in urban green spaces that varied in local- and landscape-scale habitat composition and compiled a dataset of species’ response and effect traits related to nesting, foraging, diet, and visual and acoustic aesthetic appeal. Overall, the landscape-scale context of a green space had a stronger influence on species’ abundances than local- scale habitat. Landscape-scale impervious surface surrounding our study sites interacted with response traits related to nesting in human-built structures, clutch size, wing length, canopy foraging, and consumption of seeds and invertebrates to drive bird species’ abundances. Because correlations between response and effect traits can influence the effect traits available to provide ecosystem services at a site, we explored the correlation of these three response traits to a suite of effect traits. We determined that the response traits were correlated with several effect traits related to diet and regulating services but correlated with few of the plumage and acoustic traits that produce cultural services. Finally, we found that effect traits associated with cultural and regulating ecosystem services varied strongly along the landscape-scale gradient of urbanization. Sites with high impervious surface cover are expected to have low levels of invertebrate pest control and visual appeal but high levels of acoustic appeal, diet evenness (generalism), and granivory. Overall, our study highlights the key role of landscape-scale habitat in driving bird-mediated ecosystem services and underscores the importance of regional urban planning to create healthy and livable cities.
... Forest remnants in the city are scattered and surrounded by high buildings, resulting in extreme cases of forest fragmentation (Dobbs, Nitschke, & Kendal, 2017). These forests become "green islands" in a matrix of urban land uses, helping to maintain biodiversity and ecosystem services (Canedoli, Manenti, & Padoa-Schioppa, 2018;Long & Frank, 2020). Reforestation is one of the direct approaches to support the maintenance of ecological functions and services of urban forests (Williams et al., 2009;Aronson et al., 2016). ...
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Urban forests are highly fragmented in mega-cites, acting as islands in terms of preserving species diversity. To maintain the ecological services of urban forests, management measures such as reforestation have been implemented, which might have a long-term effect on biodiversity. To understand how fragmentation and reforestation affect the natural regeneration of urban forests, we investigated the relationship between forest area, isolation and seed dispersal mode and the diversity and composition of woody species at the tree, seedling, and sapling stages in 28 secondary forests of the mega-city of Wuhan, China. We found that the alpha diversity of woody species was positively correlated with forest area, while their beta diversity was negatively correlated with forest area. The beta diversity of nonanimal-dispersed species significantly correlated with isolation. Animal-dispersed plants had consistently a higher alpha diversity from trees to seedlings and saplings, while their beta diversity was lower than nonanimal-dispersed plants at the seedling and sapling stage. The community composition of woody plants in urban forests was largely congruent among the three life stages. However, only the communities of animal-dispersed plants were consistent across life stages in small or highly-isolated forest patches. The results show that the woody plant diversity of urban forests is largely similar to that expected by island theory. Animal-dispersed trees are more likely to regenerate successfully due to a more diverse set of species used in reforestation and their higher tolerance to urban forest fragmentation. More management measures for nonanimal-dispersed species, such as enriching and repeating reforestation, will be required to maintain their high biodiversity in urban forests.
... It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for this this version posted January 20, 2023. ; doi: bioRxiv preprint proved to be a good predictor of biodiversity, including acoustic diversity, likely due to the complex 170 architecture of trees and diversity of resources and trophic interactions between trees, insect 171 herbivores and their natural enemies (Barbaro et al., 2022;Long & Frank, 2020 Here, we addressed the LBIH through the lens of predation by simultaneously characterizing changes 194 in insectivorous bird community diversity, bird predation, and the biodiversity-function relationship 195 while controlling for local factors throughout the European distribution range of the pedunculate oak 196 (Quercus robur L., 1753), a major forest tree species. Specifically, we predict the following (Fig. 1): (i) 197 bird diversity (including bird acoustic diversity, insectivorous bird species richness and functional 198 diversity) and predation rates increase toward lower latitudes; (ii) bird predation rates increase with 199 bird acoustic activity, species richness and greater functional diversity of insectivorous birds; (iii) bird 200 diversity, acoustic activity and bird predation rates increase with increasing forest cover at both local 201 (neighborhood) and larger spatial scales; (iv) large-scale variability in bird predation rates is primarily 202 driven by local changes in the diversity and activity of birds. ...
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According to the Latitudinal Biotic Interaction Hypothesis (LBIH), the general increase in biodiversity towards lower latitudes can be partially explained by an increase in the intensity of biotic interactions. While LBIH received some support for plant-herbivores interactions, much less is known about how higher trophic levels may contribute to shape biotic interactions across latitudinal gradients. We hypothesized that the diversity of insectivorous birds increases towards lower latitude, leading to higher predation rates on insect herbivores. We deployed plasticine caterpillars in 138 oak trees in 47 sites along a 19 degree latitudinal gradient in Europe to quantify bird insectivory through predation attempts. In addition, we used passive acoustic monitoring to (i) characterize the acoustic diversity of surrounding soundscapes; and (ii) infer both taxonomic and functional diversity of insectivorous birds from recordings. The functional diversity of insectivorous birds increased towards lower latitude. Bird predation increased with latitude, forest cover and bird acoustic diversity but decreased with mean annual temperature and functional richness of insectivorous birds. Contrary to our predictions, latitudinal clines in bird predation attempts were not directly mediated by changes in insectivorous bird diversity or acoustic diversity, but latitude and habitat still had independent effects on predation attempts. Our study does not fully support the predictions of the LBIH of more biotic interactions southwards and advocates for better accounting for activity and abundance of insectivorous birds when studying the large-scale variation in insect-tree interactions.
... Land cover diversity in the surrounding landscape may further provide complementary habitats and alternative prey for natural enemies, especially the generalist ones, which in turn would promote pest topdown regulation. Still, how the spatial pattern of urban trees influences the relative importance of bottom-up and top-down forces controlling pest dynamics remains an understudied question (Long & Frank, 2020). ...
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Urban tree isolation affects the abundance of pests and their enemies. • Urban tree isolation affects the top-down regulation of pests by their enemies. • Monospecific Eucalyptus patches had more herbivore insects than isolated trees. • Pest parasitism rate was 30-fold greater in urban forest patches than isolated trees. • Pest-predator-parasitoid abundance interdependence differed by tree arrangement. A R T I C L E I N F O Keywords: Anthocoris nemoralis Glycaspis brimblecombei Psyllaephagus bliteus Urban forest Top-down regulation Resource concentration A B S T R A C T The proximity to transportation hubs and the large movement of goods and people in cities make trees especially prone to invasive insect pests. Urban tree arrangements, in isolated tree sites or in forest patches, may alter their susceptibility to insect herbivory. Surprisingly little is known about how urban tree spatial arrangements influence pest abundance and top-down regulation by natural enemies. We sampled the abundance of an invasive pest of Eucalyptus, the psyllid Glycaspis brimblecombei, and of two of its natural enemies, the specialist exotic parasitoid Psyllaephagus bliteus and the generalist native predator Anthocoris nemoralis over two years. We measured insect abundances in isolated trees versus monospecific planted forest patches in 17 pairs of sites, each within a Portuguese city. The abundance of the exotic pest was threefold higher in forest patches than in isolated trees. The parasitism rate was ~ 30 times higher in forest patches (11.5 %) than in isolated trees (0.4 %). The interdependence among insect species abundances also differed between spatial arrangements. In isolated trees, predator and parasitoid abundances depended markedly on prey abundance. In forest patches, we found a marked dependence of the predator on the composition of the surrounding landscape. Our study adds empirical data indicating that urban tree isolation matters for pest abundance and regulation by its enemies. In similar systems, avoiding dense monospecific patches of exotic trees and increasing the compositional heterogeneity of the landscape are promising paths to maintain the sustainability of urban trees and their environmental and societal benefits.
... The stability of this clay product was proven under summer temperatures (Roels et al. 2018). The green-colored clay models effectively captured predatory interactions, and as a result, the impressions were visible on the clay surface (Low et al. 2014, Sam et al. 2015, Roels et al. 2018, Long and Frank 2020, Khan and Joseph 2021a. A 30 × 4 mm (length × diameter) model, simulating the late instar caterpillar pest, such as fifth instar of S. frugiperda larva, was prepared by rolling the clay on a smooth wooden surface using a piece of a 10 cm × 5 cm clear acrylic sheet (Khan and Joseph 2021a). ...
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Arthropod predators are abundant in turfgrass systems, and they play an important role in managing pests. Understanding the vertical distribution of predation is critical to developing cultural strategies that enhance and conserve predatory services. However, little is known on how the predation is vertically distributed within the turfgrass canopy. Thus, the objective of this study was to determine the vertical distribution of predation within the turfgrass canopy. Clay models were used to emulate the general appearance of Noctuidae caterpillars, to estimate the predatory activity. The choice and no-choice experiments were conducted by placing clay models at 2.54, 5.08, and 7.62 cm from the thatch surface and denoted as lower, intermediate, and upper levels, respectively, within turfgrass canopy. The predator-mediated impressions, paired mark, scratch, deep cut mark, deep distortion, prick, dent, stacked surface impression, scooped mark, granulation, and U-shaped mark, were identified on clay models. The incidence and severity of impressions were significantly greater on clay models placed at the lower canopy level than on those placed at the intermediate and upper canopy levels in the choice and no-choice experiments (P < 0.05). Thus, predators are more likely to find their prey at the soil level. This information can be used to refine management strategies, such as mowing height and insecticide use for effectively managing soil-borne and foliar-feeding arthropod pests and beneficial arthropods.
... In built-up areas, habitats for animals are very scarce. Tree crowns can attract birds, insects, and other animals to settle or forage [42,43], thus facilitating the colonization of animal-dispersed plants. In our study, the species richness and proportion of animal-dispersed plants were significantly higher in the habitats with tree cover than in those without tree cover. ...
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A large number of trees have been planted in built-up areas to improve the urban environment, but the effects of tree cover on spontaneous understory herbs are not yet well understood. This study surveyed spontaneous herbs in two kinds of habitats (habitats with and without tree cover) in the built-up area of the small city Junlian in Sichuan Province, China. A total of 222 species of spontaneous herbaceous plants in 180 genera of 71 families were recorded, including a vulnerable species and six species endemic to China. Although the overall species richness values were similar in the two kinds of habitat, the average species richness per quadrat of all plants, perennials, plants with the dwarf growth form, and animal-dispersed plants was significantly higher in the habitats with tree cover than in those without tree cover. The overall species association was significantly positive in the habitats with tree cover (VR = 1.51, p < 0.05) and neural (VR = 0.86) in the habitats without tree cover. Among the top 25 frequently recorded species in each kind of habitat, the species association of plants with the same trait combination type differed greatly in the two kinds of habitats. For the species association between annuals, only 13.33% of species pairs were significantly associated in the habitats with tree cover, while 22.22% of the species pairs were significantly negatively associated in the habitats without tree cover. For the species association between plants with tall growth forms, the proportion of significant positive associations in the habitats with tree cover was approximately twice than in the habitats without tree cover. For the species association between plants with the dwarf growth form, the proportion of negative associations in the habitats without tree cover was approximately twice that in the habitats with tree cover. Species with the same dispersal mode generally had a very low proportion of negative interspecific associations or a high proportion of positive interspecific associations in habitats unfavorable to their establishment. Our findings suggest that tree cover can improve the species richness of the spontaneous herbaceous species beneath them and profoundly influence interspecific coexistence relationships in a built-up area.
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Urban trees often host greater insect pest abundance than trees in rural forests. This may be due, in part, to differences in tree diversity and canopy cover between these settings. Urban trees are often planted in isolation or monoculture, which favors pest accumulation. Gloomy scale, Melanaspis tenebricosa Comstock, is a pest of urban red maples (Acer rubrum L.) that is abundant where impervious surfaces dominate the local landscape. Increasing tree diversity and canopy cover around urban red maples may reduce gloomy scale abundance by supporting natural enemy communities. We investigated the effect that surrounding tree species richness and tree canopy cover had on gloomy scale abundance, natural enemy abundance, and biological control in red maple trees in Raleigh, NC, USA. We collected scales and natural enemies from red maples that spanned a gradient of tree species richness, canopy cover, and impervious surface values. We also measured gloomy scale parasitism and predation of sentinel prey in red maple canopies. Greater tree species richness and canopy cover were associated with lower gloomy scale density. Red maples in diverse settings also hosted fewer scales per natural enemy. Parasitoids were less common in maples in diverse settings, but generalist predator abundance was unaffected by tree diversity. Finally, tree species richness and canopy cover did not increase biological control of scales or sentinel prey. Our findings suggest that higher tree diversity and greater canopy cover may reduce gloomy scale density, but this is not entirely explained by the effects of natural enemies and biological control.
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Trees provide many ecosystem services in our urban environments. However, city trees are often stressed by pests that are typically higher than those in nearby natural areas. Our research highlights a potential mismatch in scale between the habitat elements that affect the densities of pests and their natural enemies on city trees. We tested a well-known ecological concept, the enemies hypothesis, in the city, where relationships of pests and their enemies have not been thoroughly studied. To test our hypothesis that natural enemies and aphid predation services on urban trees increase with more local structural complexity around trees, we collected data on crape myrtle trees on NC State University’s campus from 2016 to 2017. We measured local structural complexity of vegetation around study trees, quantified impervious cover among other urban habitat elements, collected crape myrtle aphids (Tinocallis kahawaluokalani) and their natural enemies on trees, and performed predation experiments. We found that aphid abundance was positively correlated with more impervious cover within 100 m of crape myrtle trees. Alternatively, greater local structural complexity within the 10 × 10 m area around crape myrtles correlated with a higher abundance of natural enemies. Aphid predation was mostly predicted by local structural complexity and impervious cover within 20 m of crape myrtle trees. Together, these findings suggest that although the impervious nature of our cities may mean higher densities of some pests, local landscapes around trees can play an important role in maintaining natural enemies and predation services that help regulate pest populations.
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1. Habitat loss and modification are hallmarks of anthropogenic ecosystems, but the consequences for ecosystem functions and services often remain unclear. Understanding these links in cities is complicated by strong but fine‐scale differences in habitat structure among green space patches, and a high variance in habitat amount across urban landscapes. 2. We used airborne laser scanning (ALS) data to disentangle the effects of 3D woody habitat heterogeneity of urban home gardens, and woody habitat amount at four landscape spatial scales (50, 100, 250, 500 m), on the predation risk of artificial sentinel prey by birds and arthropods. 3. In both predator groups, and at all the investigated spatial scales, cross‐scale interactive effects between garden habitat heterogeneity and habitat amount in the urban landscape were the main drivers of predation. Risk of predation by birds was highest in heterogeneous garden habitats, but only in densely built urban landscapes where habitat amount was low to intermediate (10‐20%) at large spatial scales (250‐500 m). It dropped independently of garden habitat heterogeneity when habitat amount became too low (<10%) at small (50‐100 m) spatial scales. In contrast, risk of predation by arthropods mostly peaked in homogenous garden habitats when habitat amount was intermediate (20%) at large spatial scales. 4. Our findings show that the ability of urban green space patches to sustain ecosystem functions in cities mainly depends on cross‐scale interactive effects with larger‐scale habitat amount. In birds, predation risk can increase when high patch‐scale habitat heterogeneity contrasts with reduced larger‐scale habitat amount, suggesting concentration effects. Yet thresholds exist under which ecosystem functioning drops independently of habitat structure. 5. Synthesis and applications: The potential of small‐scale interventions to enhance habitat heterogeneity (e.g. by planting native trees with understory shrubs) for restoring ecosystem functions such as bird predation in urban areas is dependent on wider landscape habitat structure. Urban planning should therefore adopt a multi‐scale approach to sustain and restore ecosystem functions and services; a simple but still not broadly recognized finding. Airborne laser scanning is a useful tool to infer habitat structure across a hierarchy of scales in spatially heterogeneous anthropogenic ecosystems. This article is protected by copyright. All rights reserved.
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Research on the functional importance of biodiversity, motivated by global species loss, has documented that plant species richness affects many plant-related ecosystem functions. Less is known about the effects of plant species richness on functions related to higher trophic levels, such as the consumption of biomass by animals, that is, herbivory. Previous studies have shown positive, neutral, or negative effects of plant species richness on herbivory. In the framework of a grassland biodiversity experiment (the Jena Experiment), we investigated herbivory (the proportion of leaf area damaged and the amount of leaf biomass consumed by arthropod herbivores) along two experimental gradients of plant species richness ranging from 1 to 60 species (Main Experiment) and from 1 to 8 species (Trait-Based Experiment) biannually for five and three years, respectively. Additionally, plant functional diversity, based on traits related to plant growth, was manipulated as the number of functional groups in a community (Main Experiment) or a gradient of functional trait dissimilarity (Trait-Based Experiment). Herbivory at the level of plant communities ranged from 0% to 31% (0 and 33.8 g/m2) in the Main Experiment and 0% to 8% (0 and 13.7 g/m2) in the Trait-Based Experiment, and it was on average higher in summer than in spring. For both experimental gradients and all years investigated, we found a consistent increase in damaged leaf area and consumed biomass with increasing plant species richness. As mechanistic explanations for effects of plant species richness, we propose changes in plant quality and herbivore communities. The presence of specific plant functional groups significantly affected herbivory, likely related to traits affecting plant defense and nutritional value, but we found little evidence for effects of plant functional diversity. The general positive relationship between plant species richness and herbivory might contribute to effects of plant species richness on other ecosystem functions such as productivity and nutrient mineralization and can cascade up the food web also affecting higher trophic levels.
Trees are important components of urban landscapes because of the ecosystem services they provide. However, the effects of urbanization, particularly high temperatures, can benefit chronic insect pests and threaten ecosystem services offered by urban trees. Urban forest fragments are an often-overlooked component of the greater urban forest which may help to mitigate the damaging effects of urbanization. Melanaspis tenebricosa (gloomy scale) is a common pest of Acer rubrum (red maple) that becomes more abundant because of the urban heat island effect. We conducted observational and manipulative field experiments to test the hypothesis that trees in urban forest fragments would be cooler than those in surrounding ornamental landscapes and would thus have fewer M. tenebricosa, particularly in a hot mid-latitude city. Trees in forest fragments were 1.3° cooler and had three orders of magnitude fewer M. tenebricosa than trees in ornamental landscapes in Raleigh, NC USA. However, there was no difference in M. tenebricosa density between forest and landscape trees in Newark, DE and Philadelphia, PA USA which are 3.95 degrees of latitude higher, and nearer to the northern range extent. Trees in landscapes and forest fragments did not differ in predawn water potential, a measure of water stress, but likely differed in soil composition and moisture. We used potted trees to control for these differences and found that M. tenebricosa density still increased three times more in landscapes than forests suggesting temperature and not tree stress is the dominant factor. Taken together our results indicate two things. First, that trees growing in urban forest fragments are buffered from a chronic urban tree pest due to lower temperatures. Second, that temperature-driven differences in M. tenebricosa density which we saw in Raleigh could predict future density of the pest in higher latitude cities as the climate warms.
Urban areas, a rapidly expanding land cover type, are composed of a mix of impervious surfaces, ornamental plants, and remnant habitat, which alters abiotic conditions and affects arthropod community assemblages and trophic interactions. Importantly, these effects often reduce arthropod diversity and may increase, reduce, or not change individual species or trophic interactions, which affects human and environmental health. Despite the pace of urbanization, drivers and consequences of change in urban arthropod communities remains poorly understood. Here, we review recent findings that shed light on the effects of urbanization on plants and abiotic conditions that drive arthropod community composition and trophic interactions, with discussion of how these effects conflict with human values and can be mitigated for future urbanization.
Aim Urbanization broadly affects the phylogenetic and functional diversity of natural communities through a variety of processes including habitat loss and the introduction of non‐native species. Due to the challenge of acquiring direct measurements, these effects have been studied primarily using “space‐for‐time” substitution where spatial urbanization gradients are used to infer the consequences of urbanization occurring across time. The ability of alternative sampling designs to replicate the findings derived using space‐for‐time substitution has not been tested. Location Global. Methods We contrasted the phylogenetic and functional diversity of breeding bird assemblages in 58 cities worldwide with the corresponding regional breeding bird assemblages estimated using geographic range maps. Results Compared to regional assemblages, urban assemblages contained lower phylogenetic diversity, lower phylogenetic beta diversity, a reduction in the least evolutionary distinct species and the loss of the most evolutionarily distinct species. We found no evidence that these effects were related to the presence of non‐native species. Urban assemblages contained fewer aquatic species and fewer aquatic foraging species. The distribution of body size and range size narrowed for urban assemblages with the loss of species at both tails of the distribution, especially large bodied and broadly distributed species. Urban assemblages contained a greater proportion of species classified as passerines, doves or pigeons; species identified as granivores; species that forage within vegetation or in the air; and species with more generalized associations with foraging strata. Main conclusions Urbanization is associated with the overall reduction and constriction of phylogenetic and functional diversity, results that largely replicate those generated using space‐for‐time substitution, increasing our confidence in the quality of the combined inferences. When direct measurements are unavailable, our findings emphasize the value of developing independent sampling methods that broaden and reinforce our understanding of the ecological implications of urbanization.