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Are you interested in the relationship between urban biodiversity and ecosystem services?
Our debate today works with the article titled Understanding biodiversity-ecosystem service relationships in urban areas: A comprehensive literature review from 2017, by Nina Schwarz, Marco Moretti, Miguel N. Bugalho, Zoe G. Davies, Dagmar Haase, Jochen Hack, Angela Hof, Yolanda Meleroj, Tristan J. Pett, and Sonja Knapp, published in the Ecosystem Services journal.
This is a great preparation to our next interview with Caroline Howell in episode 456 talking about the need for structural changes for meaningful impact on handling nature in communities.
Since we are investigating the future of cities, I thought it would be interesting to see how we can improve our approach to nature beyond mere green washing. This article calls for more action to help urban designers navigate the trade-offs between promoting nature and ensuring reliable service delivery for residents.
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Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation. This conversation was produced and generated with Notebook LM as two hosts dissecting the whole research.
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Speaker 1: When we build a bridge, we use incredibly precise math. We know the exact tensile strength of the cables, the load-bearing capacity of the concrete.
Speaker 2: We don’t just guess. No engineer would look at a random pile of building materials and assume they’ll, you know, magically form a stable overpass. Just because there’s a wide variety of metals, we demand to know the exact mechanisms.
Speaker 1: Exactly. But when we look at this massive global trend of expanding urban green infrastructure, things like sprawling green roofs or massive urban parks, that engineering precision suddenly gets awfully blurry. It really does. City planners are operating on this huge sweeping assumption that simply increasing the sheer variety of nature in a city automatically improves urban ecosystem services.
Speaker 2: Yeah, the assumption that more plans automatically equals better temperature regulation or cleaner air, which brings us to the central question we are debating today. Do we actually have enough empirical mechanistic evidence to confidently design urban green spaces that reliably deliver these benefits? Or are we operating on untested guesswork that could lead to completely ineffective urban planning?
Speaker 1: I argue that the existing evidence, which is heavily rooted in what we call taxonomic diversity, which basically just means counting the sheer number of different species in a given area, provides a highly practical and sufficient foundation for cities to act right now while we continue to build those complex models in the background.
Speaker 2: We’re essentially flying blind
Speaker 1: I see why you think that, but let me give you a different perspective based on the comprehensive literature review we’re discussing today. The data is still evolving, sure, but it gives us a solid baseline. Out of the two hundred and twenty-eight empirically tested relationships between biodiversity and urban ecosystem services, fifty-two percent, so one hundred and nineteen cases, demonstrated a clear positive relationship.
Speaker 2: But fifty-two percent is basically a coin toss.
Speaker 1: It shows a strong positive trend, and taxonomic metrics were used in forty-one percent of those tests. These are incredibly accessible tools. City planners aren’t all trained ecologists. Counting species gives them a practical, measurable way to enhance human recreation, mental health, and urban pollination right now.
Speaker 2: Uh, I’m sorry, but I just don’t buy that. Let me tell you why. That foundation is remarkably fragile when you actually dig into the numbers in the review. They analysed over three hundred publications and found nine hundred and forty-four mentions of biodiversity improving urban ecosystems. Do you know how many of those claims were actually tested?
Speaker 1: I know it wasn’t all of them.
Speaker 2: Barely a quarter. A staggering forty-seven percent of those benefits were merely asserted by the authors. They just assumed the plants were doing a good job. And here is the fatal flaw. For the twenty-four percent that were actually tested, ninety-seven percent of those rely on simple statistical correlations. Correlation is not causation. Only two point six percent use actual cause and effect models like structural equation modelling to map out the biological domino effect. We are basing multi-billion-dollar urban policies on coincidences.
Speaker 1: But we have to look at how that head count actually translates to physical reality. Taxonomic diversity works as a highly effective proxy in the real world. Let’s look at the empirical data from Lahti, Finland. The soil study. The soil study. Researchers there showed that simply increasing plant species diversity directly increased the soil’s capacity to retain nitrogen, which prevents harmful runoff. And why does that work? Because when you have twenty different plant species, you don’t just have twenty identical green stems. You naturally end up with a vast array of underground architecture.
Speaker 2: Sure. You get different roots.
Speaker 1: Exactly. One plant has a deep tap root, another has fine, fibrous roots gripping the surface. That sheer variety creates a physical net that traps nitrogen. The taxonomic head count gets the job done because variety inherently brings a mix of tools to the table.
Speaker 2: But just counting species without actively measuring what those specific tools are is scientifically shallow. Relying on taxonomic diversity is like hiring a team to build a house based purely on how many people apply, rather than checking what specific skills they bring.
Speaker 1: That’s a bit reductive.
Speaker 2: I don’t think it is. Functional metrics are glaringly absent in the data Take the Chicago bee study from the review. They counted 37 different bee species living in the city, which looks fantastic on a taxonomic spreadsheet. But when they actually measured pollination, the actual service we need them to perform, only five of those species were doing the heavy lifting.
Speaker 1: And what made those five so special in that urban environment?
Speaker 2: That is the question planners should be asking. It comes down to their functional traits. Urban environments are brutal. A bee needs a very specific tongue length for ornamental flowers or a specific body mass to fly through the wind tunnels created by skyscrapers. If a planter just maximizes the sheer number of bee species, they might completely miss the handful of species that actually own the biological tools required to pollinate a city.
Speaker 1: That is a fair point, and the mass ratio hypothesis in ecology absolutely acknowledges this, the idea that the physical traits of the dominant species control how the whole ecosystem functions. We see this with keystone species, right? In fragmented urban forests, it’s the specific identity of burying beetles that drives the decomposition of animal carcasses. Because of their biology, they out-compete typical vertebrate scavengers. So yes, specific identities drive these services. But here is my counterpoint: maintaining a high taxonomic diversity is the exact insurance policy that guarantees those keystone heavy lifters are present in the first place.
Speaker 2: But you’re assuming native keystone players will just seamlessly thrive if we invite them. Cities are highly altered, novel environments. The traditional rules of ecology do not seamlessly apply here.
Speaker 1: Because of the abiotic conditions?
Speaker 2: We have extreme heat islands. We have artificial light that tricks trees into keeping their leaves too long, throwing off their circadian rhythms. Because of this intense stress, urban spaces are heavily populated by non-native species. Yet the review shows that while 41% of publications looked at native and non-native species, the vast majority completely failed to tease apart their specific effects.
Speaker 1: Actually, if we look at the mechanisms, non-natives can be highly efficient in these environments, sometimes more efficient than natives. Let’s look at the Asian tree, Ailanthus altissima, the tree of heaven.
Speaker 2: Oh, the invasive one.
Speaker 1: Invasive, yes, but in highly degraded urban streams, studies show its leaf litter decays much faster than native species. Why? Because the leaves lack the tough defensive chemicals found in native oaks. It provides rapid nutrient cycling in a polluted environment where native trees struggle to even survive. Sometimes a novel species brings exactly the functional trait you need.
Speaker 2: That’s a compelling argument, but have you considered the concept of ecosystem disservices? A non-native tree may drop leaves that decay quickly, giving you a short-term bump in nutrient cycling, but what happens over three decades?
Speaker 1: The environment adapts.
Speaker 2: Or it degrades further. That same tree might aggressively alter the soil chemistry, actively suppressing native flora. There was a brilliant study on plant pollinator interactions along a gradient moving into the dense city. It showed that a higher diversity of non-native plants actually decreased pollinator efficiency. It reduced the seed set of native plants, so you get fast leaf decay, but you break the entire reproductive network of the native ecosystem.
Speaker 1: Which perfectly illustrates the immense danger of trying to optimize our cities for just a single service. I am entirely against finding one hyper-efficient species and plastering it everywhere. There was a fascinating modelling study from an English city looking at carbon sequestration. But
Speaker 2: the eucalyptus.
Speaker 1: Yes. They found that if they planted just two specific tree species, eucalyptus gunnii and Populus tremula, they would outperform the current diverse urban tree stock in carbon capture by a factor of 12.
Speaker 2: Because their biology is hyper-optimized for sucking CO2 out of the air?
Speaker 1: Exactly. But the researchers explicitly warned against doing this because the aesthetic and biological cost would be terrible. Those eucalyptus trees have a sparse canopy architecture. They don’t cast the dense shade needed to cool the asphalt. Their leaves don’t support the native caterpillars that urban birds eat. Plant diversity is strictly required to maintain multifunctionality. We see this in green roof studies everywhere.
Speaker 2: I agree completely on the dangers of monocultures, but this is exactly why we urgently need to adopt the response effect traits framework.
Speaker 1: Without deeply studying a wider spectrum of functional traits, we risk accidentally stumbling into that monocrop dystopia.
Speaker 2: It means we measure two things: how a plant responds to the environment and what effect it has on it. A response trait is how it handles a shock. Can it survive a sudden freeze? The effect trait is what it does for us. How much storm water can its canopy intercept? Currently, if scientists measure anything, they just measure simple things like specific leaf area. We need to look at complex mechanisms, like canopy architecture. Otherwise, we lose response diversity, the ability to adapt to climate change.
Speaker 1: That’s an interesting point, though I would frame it differently. While mapping out every response and effect trait is the holy grail of urban ecology, it’s our current broad-based taxonomic efforts that are actively preventing that monocrop dystopia right now. By demanding a new park contains 30 different species, planners are accidentally but effectively ensuring a wide spread of traits.
Speaker 2: But a safety net woven from statistical coincidences will eventually tear when put under the unprecedented stress of a changing climate.
Speaker 1: To summarize my position, I maintain that while functional trait data is lacking, the strong presence of positive correlations in existing taxonomic studies provides a highly useful pragmatic guide. We have enough evidence to keep our cities green and multifunctional while the rigorous science evolves.
Speaker 2: And I maintain that until we move from correlative taxonomic data to cause-and-effect functional trait models- Urban planners are essentially guessing. We risk building fragile green spaces that look beautiful on opening day, but completely lack long-term resilience.
Speaker 1: We certainly agree that urban areas are highly novel ecosystems and that the role of non-native species requires urgent study.
Speaker 2: Absolutely. And we both recognize that maximizing a single ecosystem service can dangerously homogenize urban nature.
Speaker 1: Designing nature into human spaces is incredibly complex. Human socioeconomic decisions really act as secondary filters for biodiversity here. There is so much more to explore in the source material regarding how we basically curate the biological machinery of our cities.
Speaker 2: It comes back to the physics of the bridge. If we want our green infrastructure to hold up under climate change, we have to deeply understand the structural integrity of every root and leaf.
Speaker 1: So the next time you walk through a city park, take a closer look at the living infrastructure around you. Observe the traits of those plants and form your own conclusions on whether they are truly thriving or just hanging on.
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