Multiple small patches have higher taxonomic, phylogenetic, and functional diversity as well as more clustered assemblages than fewer large patches of comparable area

The long-standing SLOSS debate concerns whether several small patches (SS) or a single or fewer large patches (SL) better support biodiversity. Recent literature reviews and meta-analyses highlight the importance of small patches, yet much of this focus has centered on taxonomic diversity, overlooking phylogenetic and functional dimensions that offer deeper insights into community structure. We investigated the SLOSS question by comparing the taxonomic, phylogenetic, functional diversity, as well as phylogenetic and functional structure, of breeding birds across 106 equal-area combinations of SS and SL configurations derived from 30 remnant woodlots in an urban landscape, southwest China. We used line-transect techniques to survey birds during the breeding season (April to August) from 2017 to 2023. We pruned the global phylogenetic tree of birds to include bird species recorded in the study. We selected fifteen bird functional traits to construct a functional tree. We calculated Faith’s phylogenetic diversity (PD) and functional diversity (FD) for comparison with species richness. We further calculated the standardized effect sizes of the mean pairwise phylogenetic distance (SES.MPD) and mean pairwise functional distance (SES.MFD) as measures of phylogenetic and functional structure, respectively. We recorded 80 bird species across 30 patches, with species richness per patch varying from 14 to 58. Species richness, PD, and FD increased significantly with patch area, with weaker effects for PD and FD compared to species richness. The values of all three metrics were significantly higher in SS than in SL. SES.MPD and SES.MFD were predominantly below 0 across patches, suggesting a trend toward phylogenetic and functional clustering. Both metrics, particularly SES.MFD, increased considerably with patch area. However, SES.MPD was significantly lower in SS than in SL, while SES.MFD was significantly lower in SS only when patches less than 50 m apart were excluded. These findings suggest that while multiple small patches support higher species diversity, including taxonomic, phylogenetic and functional diversity, their assemblages may be more phylogenetically or functionally clustered than those found in single or fewer large patches of comparable area. This highlights the complementary roles of small and large patches in biodiversity conservation, with multiple small patches contributing to landscape-scale species diversity and large patches maintaining diverse functional or evolutionary lineages.