Human population density exhibits varying relationship with bird assemblage species diversity, phylogenetic structure and co-occurrence network in high-intensity built up environment

High-intensity built-up environment are also frequent human-nature interaction areas. However, how human influence natural relationship in such region requires further attentions. We took the human-bird relationship in the Guangdong-Hong Kong-Macao Greater Bay Area as the research system, exploring the relationship between population density gradients and the diversity and structure of bird assemblage in high-intensity built-up environment. We found that high-intensity built-up environment still conserved high bird species diversity and richness; the Shannon-Wiener index of the bird assemblage was highest at the gradient of 1,000 people/km 2 and lowest at gradients exceeding 3,000 people/km 2 , showing an overall declining trend as population increased; the Simpson index showed no significant difference among human population gradients; the Pielou index trend was opposite to the Shannon-Wiener index. Phylogenetic structures showed no significant difference across human population density gradients. However, in terms of co-occurrence network structure, the global node strength first increased and then decreased, peaking at 2,000 people/km 2 , while the local node strength first decreased and then increased, reaching its lowest at 3,000 people/km 2 . Our study revealed that bird assemblage took a complex organization in response to increasing population density, which cannot be fully captured by traditional diversity metrics alone. Our finding highlighted a critical shift in bird assemblage network organization. This implied that urban ecological management should not only focus on preserving species diversity but, more importantly, on maintaining habitat connectivity and complexity at urban environment to safeguard the species interaction networks—a key component of ecosystem sustainability.