Integrating functional and phylogenetic diversity to assess bird community assembly along the major rivers of Hainan Island, south China

Hainan Island, located in the south of China (18°10′-20°10′ N, 108°37′-111°03′ E), is the second largest island in China, with a tropical oceanic climate which is warm and hot throughout the year, with an average annual temperature of between 23 and 26°C (Li et al. 2013). The entire island receives abundant rainfall, with an annual average of >1,600 mm, larger amounts of which fall in the east than in the west. Accordingly, the central and eastern parts of the island tend to be relatively wet, whereas the southwestern coastal areas are relatively dry. The distribution of rainfall is uneven across the different seasons, with less rain in winter and spring (dry season), and larger amounts in the summer and autumn (rain season) (Rao et al. 2017; Liu and Liang 2021). The terrain of the peripheral areas of Hainan Island is generally low and flat, whereas that of the center, in which Wuzhi and Yinggeling Mountains form the core of the uplift is contrastingly elevated. The islands rivers flow into the sea from the central mountainous or hilly areas through the surrounding lowland areas according to the terrain, forming a radial island river system (Rao et al. 2017). The three main watersheds on Hainan Island, all originating in the central mountainous areas, are those of the Nandu River (333.8 km, flowing northward), Changhua River (232 km, flowing westward) and Wanquan River (157 km, flowing southward). We employed digital elevation maps to discern ridges and valleys, delineate river channels and watershed divides, and subsequently divided the river into approximate thirds: upper, middle, and lower reaches.In this study, we used the line transect method to survey breeding birds along the three major rivers of the Nandu River, Changhua River and Wanquan River on Hainan Island, south China. The patterns of community assembly were subsequently assessed by integrating functional and phylogenetic diversity, whereas environmental factors and interspecific competition intensity were incorporated to determine whether community assembly in these rivers is driven by environmental filtering or interspecific competition. Our findings revealed that bird communities within rivers were characterized by an overall slight clustering (i.e., more similar species), with the upper reaches of the Changhua River and the lower reaches of the Nandu River showing over-dispersion, whereas the lower, middle, and upper reaches of the Wanquan River all showed clustering. Altitude and the human influence index were identified as the main factors driving bird community assembly within the three major rivers. Notably, for bird communities along different river reaches, the integration of functional and phylogenetic diversity prevented the mis-classification of over-dispersion or clustering in community structure caused by traits with weak phylogenetic signals, or the observation of traits unrelated to community assembly patterns. This empirical study demonstrates the importance of integrating functional and phylogenetic diversity, which not only contributes to gaining an understanding of the mechanisms underlying the development of community assemblies but also facilitates a determination of the extents to which function and phylogeny contribute to shaping the patterns of communities.