The urban tree of life: synthesizing relationships between body size and urban affinity

This global synthesis of over 30,000 species reveals that while most organisms avoid urban environments, the relationship between body size and urban affinity is highly heterogeneous across taxa, with larger plants generally thriving in cities while animals show variable responses, underscoring the need for tailored strategies to support urban biodiversity.

The Gist

Imagine the city as a giant, chaotic, neon-lit party that nature never invited itself to. For most living things, this party is a disaster. The noise, the concrete, the heat, and the lack of green space make it impossible for them to survive. But for a lucky few, the city is actually a buffet.

This paper is like a massive, global census of who is crashing the party, who is hiding in the bathroom, and who is dancing on the tables. The researchers looked at over 30,000 different species—from tiny beetles to towering trees, from birds to snakes—to figure out the rules of survival in the concrete jungle.

Here is the breakdown of their findings, translated into everyday language:

1. The "Party Guest" Distribution (The SUD)

The researchers discovered a pattern they call the "Species Urbanness Distribution." Think of it like a party guest list:

• The Avoiders (The Majority): Most species are like guests who hate the music and the crowd. They avoid the city entirely. If you look at a city, most of the species that could live there are actually hiding in the nearby forests or parks.
• The Exploiters (The Few): A very small group of species are the "life of the party." They don't just survive; they thrive. Think of pigeons, rats, or dandelions. They have figured out how to use the city's resources better than anyone else.

The shape of this distribution is like a lopsided hill: a huge pile of "avoiders" on one side, and a tiny, steep spike of "exploiters" on the other. This happens everywhere in the world, from London to Tokyo.

2. The Great Size Debate: Does Being Big or Small Help?

The big question the researchers asked was: "Does your size determine if you get to stay at the party?"

In nature, size usually matters a lot. Big animals need big territories; small animals need less food. But in a city, the rules get weird. The researchers found that there is no single rule. It depends entirely on what kind of animal or plant you are.

• For Plants (The Greenery): Generally, bigger is better. Tall trees and large shrubs often do well in cities. Why? Because they can reach above the smog, they are often planted by humans because they look nice, and they can handle the heat better than tiny, delicate wildflowers.
• For Animals (The Wildlife): It's a mixed bag.
  • The "Small is Sweet" Crowd: Some animals, like certain beetles and snakes, tend to be smaller in cities. It's like the city is a hot oven, and being small helps you cool down faster (or maybe the food is just smaller).
  • The "Big is Bold" Crowd: Other animals, like pigeons and doves, tend to be larger. Maybe because they need to fly further to find food in a fragmented city, or because they are tough enough to handle the noise and traffic.
  • The "Too Big to Fit" Crowd: Some big animals, like hawks or large snakes, get kicked out. They need huge hunting grounds or specific quiet places that cities simply don't have.

3. The "One Size Does Not Fit All" Conclusion

The most important takeaway is that you can't guess if an animal will survive in a city just by looking at its size.

Imagine you are a city planner trying to save nature. You might think, "Okay, I'll just protect the big animals," or "I'll just protect the small ones." This paper says: Stop guessing!

• For some groups (like willow trees), being big helps you survive the city.
• For others (like snakes), being big gets you evicted.
• For others (like beetles), being small is the ticket to the VIP section.

The Big Picture: Why This Matters

Cities are expanding fast. If we want to keep biodiversity alive in our concrete jungles, we can't use a "one-size-fits-all" approach.

Think of a city garden as a puzzle. If you only plant tall trees, you might miss out on the small, heat-loving insects that need the ground cover. If you only protect the big birds, you might ignore the tiny bees that need specific flowers.

The Analogy of the Toolkit:
The researchers are handing us a new toolkit. Instead of just asking "Is this animal big or small?", we need to ask, "What specific tricks does this family of animals use to survive?"

• Some need connectivity (bridges between parks) because they can't fly far.
• Some need shade because the city is too hot.
• Some need quiet because they can't handle the noise.

By understanding these specific "body size" rules for different groups, we can design cities that aren't just concrete graveyards, but places where a wider variety of life can actually thrive. We need to stop treating the city as a single environment and start treating it as a complex mosaic where different rules apply to different neighbors.

Technical Summary

1. Problem Statement

Urbanization is a primary driver of global biodiversity loss, yet species responses vary drastically from avoidance to exploitation. While previous studies have examined these responses in specific taxonomic groups (e.g., birds or mammals) or at local scales, there is a lack of global, cross-taxonomic synthesis regarding:

• The distribution of species' affinities to urban environments across the entire tree of life.
• Whether body size, a fundamental ecological trait linked to metabolism, mobility, and space use, consistently predicts a species' ability to tolerate or exploit urban environments.
• How these relationships vary across different taxonomic families and biogeographic regions.

2. Methodology

The study employed a macroecological approach using a massive dataset of occurrence records and trait data, analyzed through Bayesian hierarchical modeling.

A. Data Collection & Processing

• Occurrence Data: Downloaded ~1.4 billion biodiversity records from the Global Biodiversity Information Facility (GBIF) (filtered for coordinate uncertainty <1 km).
• Urban Proxy: Used VIIRS night-time light intensity (remotely sensed data from 2015–2021) as a continuous proxy for urbanization.
• Body Size Data: Aggregated body size metrics (e.g., mass, length, height, wingspan) from literature, databases (e.g., TRY for plants), and direct correspondence.
  • Scope: 30,373 species (17,722 with both urban affinity and body size data) spanning 47 taxonomic classes and 371 families.
• Stratification: Analyses were stratified across 52 biogeographic subrealms (based on the One Earth framework) to account for regional species pools and baseline urbanization levels.

B. Metric Definition: Urban Affinity
The authors developed a continuous metric of Urban Affinity ($A_{s,r}$) for each species ($s$) within a subrealm ($r$):

1. Urban Score ($U_{s,r}$): The mean VIIRS radiance across all occurrence locations of a species in a subrealm.
2. Affinity Calculation: $A_{s,r} = U_{s,r} - \text{mean}(U_r)$, where $\text{mean}(U_r)$ is the average radiance of all species in that subrealm.
   • Interpretation: Negative values indicate "urban avoiders" (occur in less urbanized areas than the regional average); positive values indicate "urban exploiters."

C. Statistical Modeling

• Approach: Bayesian Hierarchical Modeling using the brms package in R.
• Scale of Analysis:
  • Global/Kingdom Level: Tested interactions between body size and kingdom (Plant vs. Animal) with random effects for Class, Order, and Family.
  • Family Level (Primary Analysis): Fitted individual models for 371 families (requiring $\ge$10 species per family). This "many models" approach allowed for the detection of heterogeneous relationships without being dominated by data-rich groups.
• Variables:
  • Response: Urban Affinity.
  • Predictor: Log-transformed, scaled, and centered body size.
  • Random Effects: Subrealm and metadata sources (to account for variation in body size measurement types).

3. Key Contributions

• Species Urbanness Distribution (SUD): The authors introduce the concept of SUD, a novel framework analogous to Species Abundance Distributions (SADs). It describes the full distribution of urban affinity values within a community, revealing a consistent skew across the globe.
• Global Synthesis: This is the most extensive assessment to date, covering >30,000 species across plants and animals, moving beyond single-taxon studies to a "tree of life" perspective.
• Context-Dependent Filtering: The study demonstrates that body size is not a universal predictor of urban success; its effect is highly heterogeneous and dependent on taxonomic group and ecological context.

4. Key Results

A. The Shape of Urban Affinity (SUDs)

• Skewed Distribution: Across all subrealms and taxa, SUDs exhibit a pronounced peak at negative values with a long right-skewed tail.
• Implication: The vast majority of species are urban avoiders, while only a small subset are urban exploiters. This suggests that traits allowing for urban persistence are unevenly distributed (potentially "exaptive" traits) rather than a result of widespread adaptive evolution.

B. Body Size vs. Urban Affinity

• No Universal Pattern: There is no single relationship between body size and urban affinity across the tree of life.
• Plants vs. Animals:
  • Plants: Larger body size (height) generally correlates with higher urban affinity (positive association), though with high variability.
  • Animals: The relationship is mixed. While some groups show positive correlations, others show negative ones.
• Family-Level Heterogeneity:
  • Only 22% of families (83 out of 371) showed significant body size filtering.
  • Positive Associations: Notable in families like Salicaceae (willows) and Columbidae (pigeons/doves), where larger species are more urban-tolerant.
  • Negative Associations: Strongest in Dipsadidae (snakes) and Accipitridae (raptors), where larger species are less urban-tolerant.
  • Ectotherms vs. Endotherms: Ectotherms (e.g., beetles, snakes) often show a shift toward smaller sizes in cities (potentially due to urban heat island effects), whereas some endotherms (e.g., birds) show shifts toward larger sizes (potentially due to mobility needs in fragmented landscapes).

C. Drivers of Variation
The variability in size-affinity relationships is driven by:

• Thermal Stress: Urban Heat Islands may select for smaller ectotherms (Atkinson's rule).
• Mobility & Fragmentation: Larger animals may be favored if size correlates with mobility to traverse fragmented habitats, or disfavored if large home ranges are required (e.g., raptors).
• Human Preferences: In plants, the prevalence of large ornamental species in cities may artificially inflate the positive size-affinity link.

5. Significance and Implications

• Theoretical Advancement: The study challenges the notion of a single "urban filter" based on body size. Instead, it highlights that urban affinity emerges from complex, multivariate trait combinations that vary by lineage.
• Conservation Planning: The findings suggest that conservation strategies must be taxon-specific and context-dependent.
  • Example: In assemblages where large species are filtered out (negative size-affinity), conservation should focus on habitat connectivity and reducing heat islands.
  • Example: In assemblages where small species are vulnerable, improving functional connectivity is prioritized.
• Methodological Utility: The SUD framework and the calculated Urban Affinity scores provide a quantitative tool for monitoring urban biodiversity health and evaluating the success of green infrastructure restoration.
• Future Directions: The authors emphasize the need to integrate multiple functional traits (beyond just body size) and to incorporate intraspecific variation and socio-ecological factors (e.g., human management) into future urban ecology models.