Understanding spontaneous biodiversity in informal urban green spaces: A local-landscape filtering framework with a test on wall plants

https://doi.org/10.1016/j.ufug.2023.127996Get rights and content

Highlights

  • Spontaneous plants present rich diversity on the vertical surfaces of ancient city wall of Nanjing.

  • Taxonomic and functional diversity are correlated with local wall habitat conditions and surrounding landscape attributes.

  • The local-landscape environmental filtering framework helps to understand biodiversity in informal urban greenspaces.

  • Neighboring landscape context plays an important role in shaping local-scale biodiversity in informal urban green spaces.

Abstract

In densely urbanized areas, small pockets of vegetated areas such as street verges, vacant lots, and walls can be rich in biodiversity. In spite of their small size, these ‘informal urban greenspaces’ can provide critical ecosystem services to urban residents. Maintaining and enhancing the provisioning of ecosystem services requires a systematic understanding of biodiversity patterns and drivers in informal urban green spaces. The ‘environmental filtering’ (a process of certain species selected by specific environmental conditions) concept in community ecology theory may serve as a useful tool for this goal. We tested a multi-scale filtering framework by examining the spontaneous plant diversity patterns (from 83 surveyed sites) on the vertical surfaces of the ancient city wall of Nanjing, China. We found that the variables representing local-habitat filtering (e.g., wall substrates and aspect) and landscape filtering (including spatial configuration of urban land cover, and nighttime light intensity surrounding the local habitats) can jointly explain substantial fractions of variations in taxonomic diversity (up to ca. 60%) and functional diversity (up to ca. 40%). The explanatory power was stronger in the repaired wall habitats than in the unrepaired counterparts, in line with the prediction that environmental filtering is more pronounced during the early stages of community assembly. While the strength of landscape filtering showed clear scale-dependency, its relative importance consistently outweighs local-habitat filtering across all study scales of 200–1600 m, suggesting that configuration of neighboring landscape context can play an important role in shaping local-scale biodiversity of informal urban green spaces. Our results have useful implications for the study, design, and management of informal urban green spaces. Well-tailored multi-scale filtering frameworks may contribute to understanding urban biodiversity patterns in a systematic way.

Introduction

Urbanization has profoundly changed our biosphere (Ellis, 2015, Grimm et al., 2008, Johnson et al., 2017). Nowadays, urbanized areas, characterized by built-up environments and intensive human activities, have taken up ∼3% of the Earth’s land surfaces (Herold et al., 2003). These areas create a ‘novel theater’ where the ecological and evolutionary plays become substantially different than in natural systems (Hulme-Beaman et al., 2016, Johnson and Munshi-South, 2017). Ecologists have traditionally viewed cities as strongly disturbed systems, and tended to avoid them, especially when it comes to developing and testing ecological theories (Grimm et al., 2000, Young, 2009). However, in recent decades there is an increasing recognition of the importance of building rigorous links between urbanization and fundamental theories of ecology (Aronson et al., 2016, Avolio et al., 2021). This may be partly due to that there are few ‘pristine’ ecosystems left on Earth (Kowarik, 2011). Also, it is realized that our knowledge about urban ecosystems, such as their composition (Dunn, 2021) and assembly mechanisms, is disproportionately insufficient, despite that the well-being of urban residents as over half of the world’s populations are now relying on urban ecosystem services. Understanding the drivers of species assemblages in urban settings can help us understand how species respond to the unique urban environments and identify which species are most likely to persist in such environments. This knowledge is key to urban planning and design for the purpose of optimizing urban ecosystem services.

Of great interest to urban ecologists is the linkage between urbanization and biodiversity. Previous studies have used the urbanization gradient approach (McDonnell and Hahs, 2008) to demonstrate that urbanized areas, compared with natural habitats, have lower local-scale biodiversity (i.e., lower α-diversity, McKinney, 2008; Vakhlamova et al., 2014), as well as higher similarity of species composition (‘biotic homogenization’, as indicated by lower β-diversity, Kühn and Klotz, 2006; McKinney, 2006). In contrast to the extensively studied urbanization gradient patterns, how biodiversity is shaped within densely urbanized areas (central business districts for instance) remains largely underexplored. In these highly artificial areas, high biodiversity is often found in small pockets of ‘informal urban green spaces’, including spontaneously vegetated areas such as street verges, vacant lots, walls, and roofs (Kwok, 2018, Rupprecht and Byrne, 2014). Despite their small size, informal urban green spaces rich in biodiversity provide many important ecosystem services that benefit urban residents’ physical and mental health (Rupprecht and Byrne, 2014, Sikorska et al., 2020, Threlfall and Kendal, 2018). This importance of biodiversity is not only in the taxonomic sense (‘species diversity’), but also hinges on the functional aspect (‘functional diversity’). For example, many animals can use informal urban green spaces as critical habitats if the plants therein can provide forage, shelter, and other necessary resources for their life histories (Chiquet et al., 2013, Madre et al., 2015). This provisioning ability is closely linked to plant functional traits – previous work suggests that plant functional diversity often support animal diversity (Vollstädt et al., 2017). Plant functional traits also determines the provisioning of many key ecosystem services to humans, ranging from air quality improvement, urban heat island amelioration, to aesthetic value (Burn et al., 2017). Existing studies have focused on the role of management practices in shaping biodiversity in informal urban green spaces (Kim et al., 2018, Phillips and Lindquist, 2021). However, systematic frameworks are still crucially needed to connect these urban ecosystems to general ecological theories for a better understanding and effective management of biodiversity therein (Aronson et al., 2016).

Here we are interested in artificial vertical surfaces, a particular class of informal urban green spaces. Urban vertical habitats such as masonry walls and embankments are normally unfavorable for plants (Francis and Hoggart, 2009). Recent studies, however, found that some of these vertical surfaces can support much higher biodiversity than previously thought (Larson, 2004; Lundholm and Richardson, 2010). Ancient city walls are especially striking single bond these cultural heritages are often well protected, thus providing important refuges for rare and endemic species in urban environments (Dahmani et al., 2020, Itani et al., 2020, Kantsa et al., 2013). Apart from this conservation value, the protected conditions make ancient city walls unique systems for understanding how urban biodiversity patterns are shaped, as most direct human disturbances that are difficult to quantify (e.g., trampling) are absent in the wall habitats.

Keddy and Laughlin (2021) provide a synthesized framework to address common questions of community ecology such as assembly rules and biodiversity patterns. This framework presents a simple, logical structure by grouping influencing factors of (especially plant) communities into three categories: (1) species pools (i.e., the set containing all species that are possibly present in local areas), (2) environmental filters (i.e., the environmental conditions that determine the occurrence/survival of organisms in a given local area), and (3) functional traits (i.e., the characteristics of organisms related to their ecological functioning of how they interact with the environments, Violle et al., 2017). Tailoring this framework leads to a simplified view of how spontaneous plant diversity in informal urban green spaces is shaped: in a given regional species pool, a fraction of species can be dispersed into urban areas, and only a part of these species can eventually survive if they can adapt to the particular environmental conditions (with their particular functional traits). In line with this idea, Aronson et al. (2016) proposed a hierarchical series of filters influencing urban species distributions including regional climatic and biogeographical factors, human facilitation, urban form and development history, socioeconomic and cultural factors, and species interactions.

Dispersal limitation from regional species pools (for instance measured by the distance to species pools) often plays an important role at broad geographic scales (Germain et al., 2017, Mittelbach and Schemske, 2015), however, its effect on the presence of most plant species may be less pronounced at smaller scales (Gelmi-Candusso and Hämäläinen, 2019, Uriarte et al., 2011). For informal urban green spaces in a typical urban landscape, one may focus on environmental filters that are expected to play a ‘bottleneck’ role in shaping spontaneous plant diversity (Gabrych et al., 2016, Vanstockem et al., 2019).

We propose that a two-scale filtering framework can serve as a ‘minimum model’ for considering the urban environmental filtering effect in a conceptual sense. Among the various urban factors as potential filters of biodiversity, we distinguish two classes in terms of spatial scale, i.e., ‘local filters’ and ‘landscape filters’. We elaborate them in the specific context of spontaneous plants on vertical artificial surfaces, using city wall plants as an example.

  • (1)

    At the local scale, wall attributes such as surface substrate may act as a direct filter by affecting seed capture, germination, and growth of spontaneous plants (Segal, 2013). Previous studies in European (Nedelcheva, 2011, Reis et al., 2006, Yalcinalp and Meral, 2017) and Chinese cities (Huang et al., 2019, Jim, 2010, Jim, 2013, Jim and Chen, 2010) have shown that many wall-attached plants have similar life-history traits with respect to these aspects. If such similarity can be explained by that only plants with a narrow set of traits can survive in the wall habitats, it can serve as supportive evidence to the existence of strong local filtering underlying wall plant communities.

  • (2)

    At the landscape scale(s), the composition and spatial configuration of land use/cover surrounding the local wall habitats may act as a filter by affecting the type and intensity of human activities, as well as other factors that potentially influence the presence of wall plants. For instance, dense transportation and sparse vegetation cover may lead to the lack of frugivorous birds, thus lowering the colonization/regeneration success of fruited plant species (a landscape-scale dispersal process, note that this differs from the large-scale dispersal from species pools) (Fujita and Koike, 2009).

In this study, we aim at testing if and to what extent this simple ‘local-landscape filtering’ framework could capture some essential spontaneous plant diversity patterns on artificial vertical surfaces across densely urbanized landscapes. Our study system is the ancient city wall of Nanjing (ACWN), China, which was originally built around 900 years ago, and spans across the main urban district of Nanjing City (Fig. 1). We conducted detailed field survey of the plant species existing on the vertical surfaces of the ACWN, and then correlated species diversity indices to a set of local- and landscape-scale variables representing potential effects of environmental filtering. We are interested in answering two specific questions: 1) To what extent is plant diversity on the ACWN determined by the local and landscape filters? and 2) what is the relative importance of the local vs. landscape filters? By addressing these questions, we expect that our multi-scale filtering approach can go beyond ancient city walls and generate useful implications for understanding and management of vertical urban habitats as well as other informal urban green spaces in general.

Section snippets

Study system

Nanjing is the capital of Jiangsu Province in Eastern China. As one of the largest cities in China, Nanjing accommodates 9.4 million urban residents. Located in the subtropical climatic zone, Nanjing presents a mean temperature of 15.7 ℃ and a mean annual precipitation of 1106 mm. Most precipitation occurs between June and September.

The ACWN (32°0′N - 32°8′N, 118°44′E - 118°50′N) is one of the most important cultural heritage buildings in China (Fig. 1). The construction of the ACWN started

Results

In our field survey, we found 93 plant species belonging to 84 genera and 50 families in the 83 study sites on the vertical surfaces of the ACWN (see Supplement S1 for the species list, Fig. 2). The most species-rich families include Asteraceae (8 species), Moraceae (4 species), Poaceae (4 species), and Vitaceae (4 species). Broussonetia papyifera (Paper Mulberry) presented the highest frequency (observed in 81 out of the 83 study sites). Most of the species present relatively low frequencies

Discussion

Our results demonstrate that the local-habitat variables and landscape variables can explain substantial fractions of plant diversity variation on the ancient city wall of Nanjing (ACWN), suggesting that the local-scale and landscape-scale filters could underlie the spontaneous plant diversity in this special class of informal urban green spaces. Our work thus contributes to better understand urban biodiversity with ecological theories and to disentangle the intricate processes shaping urban

CRediT authorship contribution statement

Shuqing N. Teng: Conceptualization, Methodology, Visualization, Funding acquisition, Project administration, Supervision, Writing – original draft, Writing – review & editing. Chi Xu: Conceptualization, Methodology, Investigation, Visualization, Funding acquisition, Project administration, Supervision, Writing – original draft, Writing – review & editing. Xinyu Miao: Methodology, Investigation, Visualization, Writing – original draft, Writing – review & editing. Yuhan Pan: Methodology,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Lin Ma and Yuan Zhou at Nanjing City Wall Protection and Management Center for their assistance in the field investigation. We thank Renping Jiang, Hanchao Zhang, Wenli Xia, Jingyi Zhang, Zhen Guo, Hanxuan Diao and Li Zhang for assistance in data collection. We also thank three anonymous reviewers for their valuable suggestions on an earlier version of this manuscript. This study is supported by the National Natural Science Foundation of China (32061143014, 31870705 and 42001044), the

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