Designing urban green-space heterogeneity across urbanisation gradients to maintain avian richness

Urban expansion drives land cover change and habitat simplification, contributing to biodiversity loss. Urban green spaces can mitigate these impacts, but their effectiveness depends on its configuration and implementation. Here, we examine how three complementary dimensions of environmental heterogeneity--plant species richness, habitat heterogeneity, and foliage-layer richness--shape bird richness along an urbanisation gradient in the Netherlands. Using bird and plant occurrence data, LiDAR-derived vegetation structure, and land-use data, we fitted generalized additive models at three spatial scales (100, 200, and 300 m) to assess how these relationships vary across the urbanisation gradient. Plant species richness showed the strongest and consistent positive effect on bird richness, disregarding urbanization intensities. Habitat heterogeneity showed most pronounced positive effects at intermediate levels of urbanisation. In contrast, foliage-layer richness had weak associations with bird richness across urbanization intensities. Together, these results demonstrate that sustaining urban bird diversity requires urbanisation-intensity-dependent design of green-space heterogeneity. Increasing plant richness is generally recommended across urbanization intensities. Increasing habitat heterogeneity is more effective at intermediate levels of urbanisation and appears less suitable in highly urbanised contexts. Beyond simply expanding green space area or their spatial complexity alone, urban planning should focus on the thoughtful design of different types of environmental heterogeneity. This includes city-wide species-rich planting and structurally diverse habitat mosaics in mid-density areas to sustain urban bird diversity.