Year and seasonal plant-frugivore network
During the observations throughout the year, we recorded 2235 interactions events between 14 bird species (2 Order 7 families) and 13 plant species (9 Order 10 families) connected by 110 links (Fig.2). Each plant species interacted on an average with 7.08±1.01 (Mean±SD) birds, and each bird species interacted on an average with 6.13±1.15 plant species (Fig.2). We registered 9,106 fruit-eating individual birds, and 92.89% of them foraged fruits of more than five individual plants (Fig.2). Passerine frugivores were relatively important in our study system, involving 13 species that consumed fruits, representing 98.26% of the total interaction number (Fig.2). The plant species most commonly consumed by birds was Cayratia japonica with 724 interactions (32.39%), while the bird species most frequently recorded wasPycnonotus xanthorrhous with 644 interactions (28.81%) (Fig.2).
Compared with the predicted networks generated by the null model (N=1000), the observed plant-frugivore network exhibited relatively a lower connectance (C = 0.604), lower interaction diversity(H2 =3.91), lower interaction evenness (E2 =0.751), higher nestedness (nestedness=0.715), and higher specialization ( =0.091) (Fig.3). These results indicated that the observed network had fewer realized connections and a more stable subset of interactions than that of the predicted network, species exhibiting higher feeding dependence, and lower interaction frequencies.
The contributions of the interaction connections, to the total connections of the network, in different seasons were different (Fig.4). Interaction connections contributed 13.38% and 14.68% to the annual network in spring and summer, and 38.79% and 33.15% to the annual network in autumn and winter, respectively. The specialization (H2´ ), and interaction evenness (E2 ) of the network were higher in spring and summer than that in autumn and winter. However, connectance (C), nestedness, and interaction diversity (H2 ) were contrary to the specialization and interaction evenness (Table1). The number of frugivore bird and fruit plant species was positively correlated with the contributions of the interaction connections between birds and fruit plants in different seasons (Rb2=0.954, P=0.023; Rp2=0.297, P=0.455).
Fig.4 The connections of different seasons contribution to the total plant-frugivore network, the color bars with the gradient of pink to yellow below the main figure indicate the proportion of interaction connections to the total network in different seasons.
Table 1 Main quantitative description of seasonal variation of interaction network