Urban densification reduces food-web robustness despite partial gains through local habitat improvements

Merin Reji Chacko , Joan Casanelles Abella, Giulia Casciano, Bertrand Fournier, David Johannes Frey, Kilian Perrelet, Marco Moretti

Urban densification is widely promoted to curb sprawl, yet the replacement of existing green infrastructure threatens species across multiple trophic levels. Species are embedded in complex food webs, and understanding how densification restructures urban food webs is essential for predicting functional change and evaluating whether local habitat improvements (e.g. greater vegetation diversity) can yield cascading effects across trophic levels. Using a metaweb approach and empirical information on species co-occurrences, we inferred multi-trophic food webs for 84 gardens in Zurich, Switzerland, selected along two largely independent gradients: landscape-scale urban densification, and local habitat quality. Densification decreased primary consumer and predator richness, decreasing food web connectance, generality and modularity. It also reduced structural robustness to secondary extinctions (Rcoef) but not the network collapse threshold R50. Improved local habitat quality increased primary consumer and predator richness as well as null-standardised degree skewness, and indirectly increased R50, but not Rcoef. Our findings indicate that local habitat improvements within individually managed gardens alone are unlikely to maintain the structural robustness of food webs to densification. Maintaining connected, high-quality green infrastructure across the urban landscape will also be necessary to sustain robust food webs and their contributions to city dwellers.