Seasonal dynamics and functional diversity of frugivorous bird-plant interaction network in the karst habitat

Introduction The karst region of southwest China is characterized by fragmented habitats with high plant diversity but low resource availability, and its strong topographic and edaphic heterogeneity drives spatial variation in resource distribution, ultimately structuring frugivorous bird-plant interaction networks. Methods We established six 2-3 km transects in a nature reserve in southern China. We conducted year-round observations of interactions between plants and frugivorous birds, subsequently using metrics to examine network structure and the functional trait of plants and birds shaping their network roles. Results We recorded 196 unique links and 3,327 interactions events involving 19 frugivorous bird and 23 fruiting plant species. Compared with the null-model network (N=1000), the observed network exhibited lower connectance (C z-score =-23.845) and weighted nestedness ( w NODF z-score =-9.215), as well as higher specialization (H2´ z-score =55.042) and higher modularity (Q z-score =20.635). Connectance and weighted nestedness were higher during the dry season, whereas modularity and specialization peaked in the rainy season. Plants had higher functional richness and evenness, and birds had higher functional redundancy across periods. Plants experienced a notable increase in functional richness and dispersion from the dry to the rainy seasons, whereas birds had more stable functional diversity. Pycnonotus jocosus and P. sinensis birds showed the highest species degree (both 23), closeness centrality (12.145 and 10.070, respectively), and betweenness centrality (0.434 and 0.176, respectively). Ficus concinna and F. altissima exhibited the highest species degree (both 16), and F. concinna and F. religiosa shared the highest closeness centrality (both 11.757). Bird body mass was the only trait with a significant negative impact on species degree (β=-1.280, P=0.014). Discussion These findings highlight the seasonal variation in frugivorous bird-plant networks in the karst habitats and species traits affecting network functional roles, providing new insights into the mechanisms sustaining interaction networks in heterogeneous, resource-limited ecosystems.