Is biodiversity needed for sustainability? A spotlight on urban landscapes

Abstract

The current rate and magnitude of biodiversity loss is so great that we are in the midst of the Earth's sixth mass extinction (Turvey and Crees, 2019). The unsustainable consumption of resources by humans is directly and indirectly responsible for losses to not only the taxonomic, but also the functional, phylogenetic, and genetic dimensions of biodiversity (MA, 2005). Because of the consistent causal relationships between human activities and biodiversity loss, a prevailing paradigm has been that human societal development is diametrically opposed to biodiversity conservation. However, recent work, summarized in the latest Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) report, indicates that the United Nations’ (UN) global sustainable development goals (UN, 2015) can be addressed while simultaneously conserving biodiversity (IPBES, 2019). To realize these win–win scenarios, business-as-usual practices need a transformational overhaul to more sustainable activities that are biodiversity friendly, yet this IPBES report implies that biodiversity and human development need not always be at odds. Furthermore, the report indicates that reaching sustainable development goals may be undermined by the erosion of biodiversity (IPBES, 2019). Meaning, biodiversity—explicitly a diversity of life forms—is essential for the efficiency in resource production and waste removal that is the foundation of sustainable development. Under this paradigm, biodiversity shifts from a casualty of human development to an essential ingredient in the sustainable development of human societies. From a conservation standpoint especially, it is appealing to view biodiversity as a savior for humankind; however, major research questions remain regarding how and under what circumstances biodiversity contributes to sustainability. In this essay, I assert that the mechanistic links between biodiversity and sustainability need to be explored, especially in the context of urban landscapes. Due to high human population densities and resource demands, urban landscapes are a critical location for researching sustainable development solutions that fulfill societal needs without overexploiting resources (Elmqvist et al., 2019). Yet, studies investigating the contribution of biodiversity to sustainability in urban landscapes are scant, possibly due to a perception of cities as being species-poor (Lepczyk et al., 2017). By exploring biodiversity–sustainability relationships in the unique conditions presented by urban landscapes, we can get closer to reaching the UN's global sustainable development goals. Broadly, the expected link between biodiversity and sustainability is as follows: biodiversity influences the ecosystem functioning and ecosystem services that generate landscape sustainability (Fig. 1). Ecosystem functions are the processes that transfer energy and/or information within and between ecosystems (e.g., biomass production, gene flow), and ecosystem services are the outputs from ecosystems, usually resulting from one or many ecosystem functions, that benefit humans (e.g., pollination) (Hooper et al., 2005). A landscape's sustainability is based on its ability to provide consistent ecosystem services into the future and is influenced by the biological, physical, and social components that compose the landscape (Wu, 2013). From a mechanistic perspective, commonly observed positive correlations between biodiversity (usually measured as species richness) and ecosystem functioning are explained by several mechanisms including niche complementarity, the selection effect, and functional redundancy (Hooper et al., 2005). Under niche complementarity, single species’ monocultures have lower functioning than diverse assemblages of species due to synergistic effects among the species when functioning together. In comparison, under the selection effect, diverse assemblages and monocultures of high functioning species have similar levels of functioning, and the positive biodiversity–ecosystem functioning correlation is due to one or a few high performing species that are more likely to be present in species-rich assemblages. Both niche complementarity and selection effect mechanisms assume variation in species’ contributions to functioning, but this may not always be the case. Species-rich assemblages can contain functionally redundant species that act to stabilize ecosystem functioning in the face of disturbances that cause species extirpations. Most experimental and observational studies exploring these mechanistic links between biodiversity and ecosystem functioning have been conducted in rural (natural, agricultural) landscapes (Fig. 1) (Plas, 2019). While important, the direct implications and patterns discovered in these studies may have limited transferability to urban landscapes (Schwarz et al., 2017). Although most ecosystem services are regarded as single or composite ecosystem functions, the full nature of the relationship between ecosystem functioning and ecosystem services has yet to be established (Fig. 1). However, given that most ecosystem services are thought to comprise one or many ecosystem functions, there is an expectation that niche complementarity, the selection effect, and functional redundancy also underlie the relationship between biodiversity and ecosystem services (Fig. 1), yet consistent evidence for these and/or other mechanistic links is still being sought (Duncan et al., 2015). There have been comparatively more explorations of the correlation between biodiversity and ecosystem services in urban landscapes (Ziter, 2016) as well as the spatial configuration of ecosystem services generated across urban landscapes (Lovell and Taylor, 2013), but mechanistic studies are rare (Fig. 1). While the entire mechanistic biodiversity–sustainability relationship (Fig. 1) requires research attention in all landscape types, urban landscapes present unique conditions, such as unique species compositions, scales of species turnover, and ecosystem service demands, that require special attention and may generate distinct patterns. The composition of species in urban landscapes can deviate substantially from rural landscapes; however, the effect of these deviations on ecosystem functioning and ecosystem services has been little explored. Urban environments filter for species such as non-native, ornamental, and generalist species, that can tolerate and exploit urban conditions. Non-native and ornamental species with novel levels of ecosystem functioning not otherwise found in the landscape could possibly skew “natural” patterns of functional redundancy and influence ecosystem resilience. Comparatively, a predominance of generalist species could cause higher functional redundancy relative to rural landscapes. Non-native and ornamental species may also influence the capacity for niche complementarity. Because the community assembly processes in urban landscapes are strongly influenced by humans and do not always result in assemblages of species with a shared co-evolutionary history, niche complementarity may be less likely as a mechanism that maintains ecosystem functioning (Flombaum et al., 2017), but these ideas need further exploration. Rates and patterns of species turnover in both temporal and spatial dimensions may also vary considerably in urban landscapes, affecting how biodiversity and species compositions are maintained. For example, dispersal in urban landscapes may be altered; animal seed dispersers may be absent or different, and wind dispersal can be dampened or augmented by the urban environment. As such, the temporal scale of species turnover within sites may happen at vastly different rates than in rural landscapes (Thuring and Dunnett, 2019). Spatially, landscape sustainability can be maintained by spatial heterogeneity in the habitat types and species composition that generate ecosystem services (Plas et al., 2019), and this landscape sustainability is partly dependent on the scale that spatial heterogeneity is measured. Urban landscapes have the potential for high spatial heterogeneity in species composition at smaller spatial scales relative to rural landscapes, especially when human-managed green infrastructure is considered (Fig. 2). Spatial and temporal variation in environmental conditions can also promote niche complementarity and functional redundancy (Loreau et al., 2003), yet species must be able to persist and disperse in the landscape for these effects to be realized, which may be challenging in urban landscapes for some species. Research on how the rescaling of these temporal and spatial processes influences the maintenance of biodiversity and subsequent ecosystem functioning, ecosystem services, and sustainability is sorely needed. Finally, the constellation of ecosystem services used and valued by residents differs in urban and rural landscapes. Across regions, demand for ecosystem services is highest in urban versus rural landscapes due to high population densities. Urban populations also place a higher importance on regulating services like air and water purification that counteract the waste and pollution urban residents regularly experience that is produced from resource consumption (Martín-López et al., 2012). In addition, the cultural ecosystem services generated by urban green infrastructure can provide benefits such as fewer gun assaults and improved mental health, and these benefits may be enhanced in lower-income neighborhoods (Kondo et al., 2017; South et al., 2018). Because these urban-valued ecosystem services may not be investigated in rural areas, how biodiversity supports these and other services is not well understood. Going forward, to advance this research agenda, mechanistic studies of how biodiversity contributes to ecosystem functioning and ecosystem services in urban landscapes are a critical start. Such studies should directly quantify the strength of ecosystem functioning and services provided by different species assemblages and assess the factors that contribute to community stability and species turnover in urban landscapes. In addition, identifying how biodiversity influences the spatial variation in ecosystem services generated within an urban landscape will provide useful estimates of landscape sustainability. Using this knowledge, then, creative win–win solutions for biodiversity and sustainability can be devised and get us closer to reaching the UN's sustainable development goals. I thank two anonymous reviewers, Pamela Diggle, Matthew Helmus, and Timothy Swartz for helpful comments that improved the content and focus of this essay.

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