Introduction

Urbanization continues to expand globally at an unprecedented rate1. The increasing urban populations and the pressures for urban development together with environmental challenges, such as climate change, air pollution, and natural disasters, are continually threatening the well-being of urban residents2,3,4. A critical nature-based solution to adapt to and mitigate these pressures and ensure human well-being is through proper planning, conservation, and management of urban green infrastructure. This urban ecological infrastructure is a system of interconnected ecosystems, ecological–technological hybrids, and building infrastructures that provide social, environmental, and technological functions and benefits5,6,7, as well as a wide range of ecosystem services8.

An ecosystem’s ability to provide ecosystem services is determined by its structure and functioning, that is, the ecological processes that control the fluxes of energy, nutrients, and organic matter through an environment9,10. These ecological functions rely on the existing biological diversity, particularly on functional diversity, the type and abundance of species, and traits present11. These traits are, in turn, the characteristics and mechanisms that operate at the individual level linked to different functions12,13.

Functional traits have been classified into response and effect traits14. The former are traits that potentially affect individuals’ performance or fitness and can influence environmental tolerances, habitat requirements, and responses to pressures (for example, traits related to fecundity, dispersal, and regeneration)9,15; the latter influence ecosystem structure and functions, and consequently the provision of ecosystem services (for example, traits related with nutrient cycling and storage)14,16,17. Response and effect traits can overlap to different degrees, from being closely correlated to being random. This means that the response to a disturbance and the effect on ecosystem functioning can be influenced by attributes that are not related, that somewhat overlap, or that are tightly correlated, leading to different consequences on ecological functions, ecosystem services, and ecological resilience14. For example, when response and effect are influenced by the same functional attributes, the risk of quickly losing important functions is reinforced, which highlights the negative consequences of changes in the environment14.

The ability of traits to be expressed as functional responses (i.e., the phenotypic expression of traits) is, therefore, often filtered by the environment. Thus, only a few effect traits may be available to provide core functions to support the provision of ecosystem services18,19. In urban areas, this filtering of traits’ phenotypic expression is of particular concern due to the diversity of interacting and dynamic filters that affect biodiversity in these systems, and the increasing need for a constant supply of trait-based ecosystem services20,21,22. Therefore, it is fundamental to fully understand the filtering processes impacting traits and their functional expression in such areas, and the possible consequences on ecosystem functioning and ecosystem services delivery.

However, to date, there are still fundamental theoretical and conceptual gaps in understanding these links, which are crucial to contemplate efficient and target-oriented strategies to maximize and multiply potential benefits for the well-being, quality of life, and health of increasing urban populations23. Here, we build upon a study by Andersson et al.19, to respond to the necessity of a conceptual framework centered around traits, that addresses the impacts of socio-ecological systems on traits’ phenotypic expression and ecosystem services delivery. With this framework, we intend to provide a practical way forward to interpret and compare the complexity of urban ecosystems at different scales and thus improve support for ecosystem services-based decision-making, with potential application in cities worldwide.

The Socio-Ecological Traits Framework

In this perspective, we propose a Socio-Ecological Traits Framework that joins and clarifies the linkages between traits’ dispersal to urban systems, the effects of social and ecological filters on the phenotypic expression of traits, possible feedback from filtered response traits to effect traits, and the consequences on the resulting filtered trait combinations, ecological functions, ecosystem services, and human well-being (Fig. 1). Since traits are not unique to individual species, our trait-based framework has the additional advantage of allowing the understanding of ecological processes and management of ecosystems in a more mechanistic and universal way than species-based approaches18,19,24.

Fig. 1: The Socio-Ecological Traits Framework.
figure 1

The ensemble of ecological (regional scale) and anthropogenic trait combinations disperse to urban systems (natural vs. human-mediated dispersions) making up the potential urban trait combinations (city scale). The potential urban traits are affected by socio-ecological filters that often interact. These filters comprehend the ecological filters of abiotic and biotic dimensions and the social filters of socio-cultural, economic, and governance dimensions. The filtering processes affect the phenotypic expression of traits (by affecting response traits which feedback to effect traits), resulting in the filtered trait combinations at the local scale. The socio-ecological outcomes, which comprise the ecological functions, ecosystem services, and human well-being, are provided by the filtered expression of effect traits. These outcomes can then influence the socio-ecological filters.

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This framework aims at: (i) facilitating the generalization of ecological knowledge on urban biodiversity through a functional approach, providing information on potential changes to ecosystem services, (ii) being potentially applied in exploratory models to explain biodiversity patterns and processes in urban landscapes, and (iii) serving as a basis to support informed management and planning decisions for urban biodiversity, traits, and services trade-offs, with opportunities to meld ecological management and landscape design to maximize the supply of desired services in specific contexts. More specifically, we also aim at categorizing the filters that can affect phenotypic trait expression in cities and the different trait combinations affected, which can influence their response to socio-ecological filters, with implications for planning and management decisions.

In our framework, we start by recognizing two specific trait combinations that represent the way different traits are dispersed to urban systems through species’ dispersal (traits’ arrival): the ecological and anthropogenic trait combinations (Figs. 1 and 2). Ecological traits represent the combinations of traits of native species in the bioregion, which naturally and spontaneously disperse to urban systems without direct human intervention25,26. For example, many traits associated with species of fungi and microbes, invertebrates, plants, and vertebrates naturally disperse to urban areas. These species often present highly adaptative characteristics such as opportunistic behaviors, that allow them to thrive in urban environments (for example, raccoons)26. Anthropogenic traits represent combinations of traits of species introduced to urban systems through human-mediated dispersion25,26. These species can be intentionally or unintentionally dispersed and can be sourced from anywhere in the world. Intentionally dispersed species are introduced by humans in urban areas voluntarily (for example, transportation of organisms for agriculture, biocontrol, ornamental use, and pet trade), usually due to their commercial viability and ornamental traits25; unintentionally dispersed species are introduced accidentally in urban areas from different sources (for example, species’ accidental attachment to dispersal vectors and escapees from pet trade). For example, species with dispersal traits such as small clinging seeds with appendages can easily attach to dispersal vectors and can be unintentionally dispersed to urban systems25. The natural or human-mediated dispersion influences species and traits’ ability to surpass filters. For example, native species that disperse from the bioregion to urban systems have traits that make them naturally adapted to the climatic conditions of the area, as opposed to species introduced from areas with different climatic conditions, that must surpass climatic filters to establish and thrive. Both the ecological and anthropogenic trait combinations make up the potential urban traits, representing the traits that could potentially thrive within a city if no filters affected them (Figs. 1 and 2). The filtered combinations of specific traits are the result of filtering processes at a local level (for example, parks and gardens)27. These correspond to both the response traits that allow species to establish and thrive despite the socio-ecological filters and the effect traits that will ultimately provide the socio-ecological outcomes, through their filtered phenotypic expression. These filtering processes occur due to a range of socio-ecological filters that affect the phenotypic expression of traits. In the subsection below we explore multiple filters previously identified as affecting biodiversity in urban areas, as well as the filters considered in our framework.

Fig. 2: The different trait combinations considered by the Socio-Ecological Traits Framework.
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The upper part of figure (a) represents the different combinations of traits that the Socio-Ecological Traits Framework considers: the ecological trait combinations are composed of the traits of native species, which naturally disperse to urban systems, and the anthropogenic trait combinations are associated with species introduced through human-mediated dispersion; these combinations constitute the potential urban trait combinations, which are affected by socio-ecological filters, leading to the filtered trait combinations at the local level. The lower part of figure (b) represents specific examples of these combinations of traits: the ecological trait combinations are represented by the traits of aphids, which can naturally disperse to urban areas due to morphologic, behavioral, and physiological traits, and the anthropogenic traits are represented by the traits of ladybugs and two species of plants with different traits (a, b). These anthropogenic traits arrive to urban systems through human-mediated dispersion due to morphologic, behavioral, and physiological traits. Both these combinations of traits compose the potential urban traits. Considering biotic (for example, predation), socio-cultural (for example, cultural preferences), and economic (for example, budget constraints) filters, the filtered trait combinations in a private garden represent species with traits that have predatory feeding habits (ladybugs towards aphids), are culturally preferred (due to size and color), and have the lowest maintenance requirements (lower need for irrigation) (trait combinations of plant a).

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Socio-ecological filters affect the phenotypic expression of traits

The idea of filters affecting biodiversity has its roots in the concept of environmental filtering in natural ecosystems, particularly in the study of plant community assembly and dynamics28. In this concept, the environment acts as a selective force that favors or disfavors species, either by broadening, narrowing, or shifting the distribution and abundance of traits or by affecting their expression29,30, influencing species composition, functional diversity, and phylogenetic distribution20,31. Therefore, only species with favorable traits can establish and persist in a particular environment32, and those community members have a high probability of sharing response traits that confer that environmental tolerance30.

In urban areas, several conceptual frameworks have been developed to analyze the filters that affect species’ traits: in 2009, Williams and colleagues33 explored habitat transformation, fragmentation, urban environment, and human preferences as ecological and social filters of urban vegetation that have consequences on floristic composition, functional traits, and phylogenetic distributions. According to the authors, ecological filters related to the urban environment (for example, atmospheric pollution and high temperatures), can lead to traits narrowing or shifts to traits that are able to withstand those environmental conditions. In addition, human preferences can also lead to functional shifts, due to the introduction of new species, or traits narrowing, due to species eradication; in 2016, Aronson and colleagues27 introduced a hierarchical filters approach, where environmental and social filters successively remove species based on their traits, affecting species distribution and community assemblages; other authors have studied how social filters related to management decisions, preferences, and residents’ socioeconomic and cultural backgrounds influence trait distribution in urban areas34,35,36; more recently, Avolio et al.26 introduced a framework where species selection, local environmental conditions, biotic interactions, and overall human behavior affect the traits of urban biodiversity, with varying degrees of intensity and consequences.

These frameworks are important works that exemplify how urban biodiversity is affected by social and ecological filters. However, we find that it is still necessary to deeply explore the consequences of filtering processes on response traits, the feedback between altered response traits and effect traits, and the consequences of trait-based ecosystem services delivery. Since arrival to urban systems does not guarantee traits’ establishment and thrive, by understanding what filters influence traits in urban areas and how these processes can take place, it may be possible to prevent unwanted changes in the phenotypic expression of traits and services supply.

In the Socio-Ecological Traits Framework, we consider both ecological and social filters due to their demonstrated influence on biodiversity in urban areas. Moreover, we consider that ecological filters are derived from the abiotic and biotic dimensions of any city and social filters are derived from direct pressures on biodiversity by humans through the socio-cultural, economic, and governance dimensions of any city (Fig. 1). This means that drivers such as the urban heat island effect, a climatic phenomenon that creates warmer and drier conditions in cities than their surroundings37, despite being derived by driving forces related to social factors (for example, built environment), is here considered as an ecological filter since the direct pressure on biodiversity is ecological (increased temperatures and lower relative humidity).

The ecological filters of abiotic dimensions consist of local abiotic conditions such as microclimate, air pollution, and soil composition, due to their direct or indirect influence on the phenotypic expression of traits38,39. For example, high temperatures can lead to shifts in the functional response of traits, where only species with traits that fit ecological niches of warmer and drier areas can survive and thrive, which often leads to narrower suites of traits33,40. These high temperatures can affect response traits and modify their phenotypic expression by, for example, inducing leaf shedding, decreasing leaf area development, and reducing photosynthesis rate41, or by extending the length of the growing season of plants42, accelerating budburst and flowering43.

The biotic dimensions of ecological filters include interactions such as competition, facilitation, predation, parasitism, or mutualism33,39. For example, interactions between plants and pathogens or pests can lead to functional responses such as the evolution of traits associated with defense mechanisms (for example, leaves with trichomes, higher toughness, and chemical defenses)44,45. Moreover, biotic interactions between pollinator type and floral trait variation can also influence the selection of certain floral traits over others (for example, a specific odor and color)46.

The social filters considered in the Socio-Ecological Traits Framework reflect bottom-up individual choices (household scale) and top-down decisions and rules of stakeholders (non-governmental and municipalities) that affect the selection and phenotypic expression of traits, often influenced by interacting socio-cultural, economic, and governance factors26,47,48 (Fig. 1). The social filters of socio-cultural dimensions include cultural background, traditions, and religion, since these factors influence the perception of what constitutes meaningful cultural heritage in a landscape49, and therefore often influence human needs, uses and preferences towards biodiversity23,48,50. For example, Fraser and Kenney51 showed that British communities favor shade trees, Chinese communities prefer landscapes without trees and Mediterranean communities choose vegetable gardens over shade trees. The economic dimensions of cities that influence human actions toward urban biodiversity include economic benefits, incentives, constraints, and management costs, which are often limited by budget availability52. For example, with reduced budgets, species with lower management costs, such as slow root growth, are often selected53. The government dimensions that influence human actions concern top-down (for example, local government, public authorities, non-governmental stakeholders) informal and formal mechanisms, arrangements, strategies and decisions, compromises, impositions, and processes towards urban biodiversity48,52,54. For example, landscape planners, professional arborists, and municipal foresters can drive plant composition in urban areas due to trends in landscape architecture55. One example of this filter can be often seen in Mediterranean cities, where landscape architects and planners have traditionally selected palm trees as street trees due to their simple and superficial system of frail roots, lack of branches, arboreal bearing, and leaf arrangements56.

Unlike ecological filters, social filters often do not directly translate into different responses in the phenotypic expression of traits. For example, socio-cultural preferences or budget constraints do not affect physiological or morphological traits directly as ecological filters. However, social filters can lead to human actions that directly affect traits’ morphology or physiology. For example, top-down impositions or economic incentives to irrigate vegetation can increase the resistance of response traits to disturbances and guarantee the functional expression of effect traits.

With the categorization into the different dimensions of ecological and social filters, we intend to contribute to management purposes since social filters are often more easily managed than most ecological filters. These socio-ecological filters often interact and may affect trait expression with different degrees of consistency and predictability, depending upon the nature of the filter, the temporal and spatial scales considered40, organizational level, environmental context23, and species functional ecology14.

Socio-ecological outcomes as reflections of the altered expression of effect traits

In urban systems, social-ecological filtering processes influence the establishment of particular traits and thus determine the resulting filtered traits at the local scale. The effect traits within the filtered trait combinations will deliver the socio-ecological outcomes, that is, the ecosystem services derived by ecological functions, which will ultimately contribute to human well-being18 (Fig. 1). The resulting outcomes can then affect the presence and intensity of the socio-ecological filters occurring in an urban area (for example, temperature regulation contributes to the mitigation of the urban heat island effect; Grilo et al.57). However, when filters directly or indirectly affect effect traits (by affecting overlapping response traits), important functions can be lost, affecting the socio-ecological outcomes. To better understand the consequences of traits’ filtering processes on the socio-ecological outcomes, we further explore the examples provided in the previous section.

As explained, photosynthetic rate, leaf shedding, and leaf area are response traits that respond to high temperatures. However, these traits are also effect traits that influence ecological functions, such as shading and evapotranspiration, influencing, therefore, the service of temperature mitigation10,44. This ecosystem service is of particular importance in urban systems since it contributes to the mitigation of the urban heat island effect57. Therefore, if the phenotypic expression of these traits is altered due to high temperatures, causing, for example, earlier leaf shedding and reduced leaf area, the potential for temperature mitigation can be reduced. This can then lead to an increased risk of heat stress during uncomfortable thermal conditions, with negative consequences for human well-being57. High temperatures can also affect other response and effect traits such as budburst and flowering, duration of the flowering period, and amount of nectar volume per flower, with consequences on functions and services such as primary production, pest control, and pollination10,50,58. For example, flowers with higher volumes of nectar are often more attractive to pollinators, contributing to higher pollination services44. This service is essential for human well-being since it contributes to the reproduction of plant species and the development of flowers, fruits, and vegetables59, and can be very significant in cities due to their high diversity of native and non-native flowering plants60.

Moreover, the human selection of traits associated with socio-cultural, economic, and governance drivers, often has consequences on the socio-ecological outcomes. For example, the selection of palm trees as street trees is known to enhance the service of esthetic experiences, since many of their traits are often considered visually pleasing (for example, their arboreal bearing and leaf arrangement)61. This service is very important in urban areas since it promotes the connection between people and nature, contributing to human well-being62,63,64.

Applying the Socio-Ecological Traits Framework to specific scenarios

The presented framework can help stakeholders by serving as a guiding tool for green infrastructure planning and management, to ensure that biodiversity is resilient to disturbances and is able to supply desired ecosystem services. However, it is necessary to understand the traits linked with the desired ecosystem services and the filters that can affect those traits. To have a clearer view of how the framework has the potential to guide urban planning decisions, we explore the consequences for the phenotypic expression of traits and socio-ecological outcomes of planting different species of trees with different traits in a highly polluted location within an urban area. To do so we consider two scenarios with the same ecological traits, different anthropogenic traits (species available in a nursery), the same ecological filter of abiotic dimensions (air pollution), and explore the consequences of the same socio-ecological outcome (air quality mitigation). To simplify the scenarios provided we do not specify the ecological traits and limit the anthropogenic traits to a community of single species of Eucalyptus citriodora in scenario A, and to the traits of Caesalpinia sappan and Dalbergia sissoo in scenario B (Fig. 3). We have considered these tree species since their resistance to air pollution has been previously studied at the functional level65.

As explained in Fig. 3, air pollutants affect the phenotypic response of traits such as leaf phenology, shape and temperature, canopy size and shape, the transmission of photosynthetically active radiation, mechanical functioning of stomata, and formation of reactive oxygen species65,66,67. For example, tall evergreen trees (such as Eucalyptus citriodora) often respond phenotypically to air pollutants by reducing the transmission of photosynthetically active radiation, closing stomata, increasing leaf temperature, and forming reactive oxygen species65. These effects can feedback and disturb photosynthesis, respiration, and transpiration, causing damage to the physiology of the tree, and therefore, can affect effect traits linked to pollutants scavenging68. On the other hand, the phenotypic response of deciduous trees with compound leaf form, and a round/oval small to medium canopy (such as Caesalpinia sappan and Dalbergia sissoo) is often less altered65 (Fig. 3).

Fig. 3: Theoretical application of the Socio-Ecological Traits Framework.
figure 3

Scenarios A and B represent the same ecological trait combinations and different anthropogenic trait combinations. In scenario A, the anthropogenic trait combinations are composed of the traits of Eucaliptus citriodora—an evergreen tree with an oval canopy and simple leaves. In scenario B, the anthropogenic trait combinations are composed of the traits of Caesalpinia sappan—deciduous tree with round canopy and compound bipinnate leaves—and Dalbergia sisso—deciduous tree with oval canopy and compound bipinnate leaves. Therefore, both scenarios have different potential urban trait combinations, which are then affected by the same ecological filter of abiotic dimensions—high levels of air pollutants. This filter will affect the phenotypic expression of traits which will lead to contrasting resulting filtered trait combinations at the local scale, with consequences on ecological functions, ecosystem services, and human well-being.

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These air pollutants can be removed by effect traits related to leaf phenology, shape, surface area and roughness, epicuticular wax amount, and stomatal density69,70. For example, trees with high stomatal densities, rough leaf surfaces (trichomes and grooves), or evergreens (thick epicuticular wax, great leaf area per plant, and year-round foliage longevity) can potentially remove larger amounts of air pollutants than trees without these characteristics68,70. Therefore, there is a significative overlap between the traits that allow species to be resilient to pollution and the traits that allow species to remove pollutants from a location. This means that E. citriodora is potentially able to remove larger amounts of pollutants than Caesalpinia sappan and Dalbergia sissoo due to its effect traits, being, at the same time, the species with lower tolerance to traffic-induced pollutants. Due to this response and effect, a community of E. citriodora is not viable to be placed near roads with high traffic levels, since this species is not able to thrive in the long term, and therefore, the effect traits related to air quality mitigation will not be expressed lastingly. In scenario B, the socio-ecological outcomes, namely particle retention, and air quality mitigation are potentially secured over time, improving human well-being in the long term (Fig. 3).

We acknowledge that these scenarios represent an oversimplification of the real complexity of interactions and feedback that affect biodiversity in urban systems (for example, social filters such as top-down norms to irrigate vegetation can alter traits’ functional responses to ecological filters and increase their tolerance to several disturbances). However, we intended to, not only present a first step in the application of this framework in complex socio-ecological systems, but also, show that it is imperative to strengthen efforts on easily available information on trait profiles, on filters that can interact with those traits, and on links between traits and desired/undesired functions, to evaluate trade-offs between contrasting planning and management scenarios.

Given the many goals we have for urban biodiversity, urban ecosystems need to be resilient to a diversity of disturbances, and to be able, at the same time, to deliver a constant supply of ecosystem services. We believe the Socio-Ecological Traits Framework can help urban planners and managers to achieve these goals. However, to be effective in management, we emphasize the necessity of applying the framework to the supply of different ecosystem services, but also to different urban contexts, since a driver may be considered a strong filter for biodiversity in one particular context, but not at another. For example, increasing global temperatures may serve differently as an impactful filter for biodiversity in Global North versus Global South cities. Therefore, this framework can be used to compare studies in different regions empirically throughout the world, particularly comparing Global North and Global South contexts, to understand the similarities and differences more deeply between these regions. We encourage the broad application of the Socio-Ecological Traits Framework, particularly in Global South cities, to understand the main socio-ecological filters acting on specific urban areas and how to improve the supply of ecosystem services.

Conclusion

The ecosystem services provided by urban green infrastructure and its associated biodiversity are fundamental for the livelihoods and well-being of increasing urban populations. Therefore, it is fundamental to understand the linkages between ecosystem services, traits, and filters in urban systems. By explicitly linking these concepts, our proposed Socio-Ecological Traits Framework contributes to the emerging literature on traits in urban areas, can help to explain patterns of functional diversity, species distributions, and evolutionary and ecological processes in these systems, and support the development of scenarios to predict how the supply of ecosystem services under global change and restoration strategies could be improved.

The broad application of the proposed framework is a critically important next step for better planning and management of urban biodiversity and, overall, a more robust understanding of urban ecological dynamics and the social-ecological filters that affect them. However, to do so, information on generic and site-specific trait profiles need to be easily available, either in databases or through monitoring efforts. Even so, we encourage the scientific community and stakeholders to apply our framework in diverse urban regions and for different taxa, not only to assess the relative strength of the different social–ecological filters acting on urban biodiversity, but also for effective management and planning strategies for an enduring supply of ecosystem services. By doing so, it can be possible to prioritize landscape features that need to be conserved, protected, or restored to ensure the resilience of urban ecosystems and the sustainable supply of ecosystem services to meet expanding human well-being needs in urban areas.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.