The small‐island effect (SIE) has become a widespread pattern in island biogeography and biodiversity research. However, in most previous studies only area is used for the detection of the SIE, while other causal factors such as habitat diversity is rarely considered. Therefore, the role of habitat diversity in generating SIEs is poorly known. Here, we compiled 86 global datasets that included the variables of habitat diversity, area and species richness to systematically investigate the prevalence and underlying factors determining the role of habitat diversity in generating SIEs. For each dataset, we used both path analysis and breakpoint regressions to identify the existence of an SIE. We collected a number of system characteristics and employed logistic regression models and an information–theoretic approach to determine which combination of variables was important in determining the role of habitat diversity in generating SIEs. Among the 61 datasets with adequate fits, habitat diversity was found to influence the detection of SIEs in 32 cases (52.5%) when using path analysis. By contrast, SIEs were detected in 26 of 61 cases (42.6%) using breakpoint regressions. Model selection and model‐averaged parameter estimates showed that Number of sites, Habitat range and Species range were three key variables that determined the role of habitat diversity in generating SIEs. However, Area range, Taxon group and Site type received considerably less support. Our study demonstrates that the effect of habitat diversity on generating SIEs is quite prevalent. The inclusion of habitat diversity is important because it provides a causal factor for the detection of SIEs. We conclude that for a better understanding of the causes of SIEs, habitat diversity should be included in future studies.

Background Habitat loss, fragmentation and decrease of habitat quality caused by urbanization have led to a dramatic decline in biodiversity worldwide. For highly urbanized areas, parks have become “islands” or habitat fragments for wildlife. As an important indicator group of urban ecosystem health, the response of birds to urbanization has attracted the global attention of ecologists. Understanding the key factors affecting bird diversity in urbanized environment is crucial to the protection of biodiversity in urban ecosystems. Methods We used the line-transect method to survey birds in 37 urban parks in Nanjing, China. We also measured a number of park characteristics (area, isolation, shape index, environmental noise, distance to city center, and habitat diversity) that are commonly assumed to influence bird diversity. We then used the information-theoretic multi-model inference approach to determine which park characteristics had significant impacts on bird species richness. Results We found that park area, habitat diversity and the distance to city center were the best positive predictors of bird species richness in Nanjing urban parks. By contrast, park isolation, park shape and environmental noise had little or no influence on bird diversity. Conclusions Our study highlights the importance of park area, habitat diversity and the distance to city center in determining bird diversity in Nanjing city parks. Therefore, from a conservation viewpoint, we recommend that large parks with complex and diverse habitats far away from the city center should be retained or constructed to increase bird diversity in urban design and planning.

Nestedness is an important pattern frequently reported for species assemblages on islands or fragmented systems. However, to date, there are few studies that comprehensively investigated faunal nestedness and underlying processes in urbanized landscapes. In this study, we examined the nestedness of bird assemblages and its underlying causal mechanisms in 37 urban parks in Nanjing, China. We used the line-transect method to survey birds from April 2019 to January 2020. We used the program NODF to estimate the nestedness of bird assemblages. We applied spearman partial correlation test to examine the relationships between nestedness ranks of sites and park characteristics (area, isolation, anthropogenic noise, number of habitat types and building index), as well as between nestedness ranks of species and their ecological traits (body size, geographic range size, clutch size, minimum area requirement, dispersal ratio and habitat specificity). We found that bird assemblages in urban parks were significantly nested. Park area, habitat diversity, building index, habitat specificity and minimum area requirement of birds were significantly correlated with nestedness. Therefore, the nestedness of bird assemblages was caused by selective extinction, habitat nestedness and urbanization. However, the nestedness of bird assemblages did not result from passive sampling, selective colonization or human disturbance. Overall, to maximize the number of species preserved in our system, conservation priority should be given to parks with large area, rich habitat diversity and less building index. From a species perspective, we should focus on species with large area requirement and high habitat specificity for their effective conservation.

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