In sustainability science, interdisciplinarity, i.e., the integration of perspectives from different disciplines, is built collectively from interactions among researchers of various disciplines (“collective interdisciplinarity”) but also results from the fact that researchers have backgrounds in multiple disciplines (“individual interdisciplinarity”). We applied social network analysis tools to analyze how individual interdisciplinarity influences collective interdisciplinarity, using the case of a forest sustainability science group. We hypothesized that researchers with higher individual interdisciplinarity had more interdisciplinary interactions and were interdisciplinary brokers within the group. We first analyzed individual interdisciplinarity using a bipartite network of researchers and disciplines. We then analyzed networks of management, research, and publication interactions among researchers in the research group. This showed how disciplines influenced interactions and how researchers contributed to interdisciplinary interactions and brokerage. Results of the first analysis identified large disciplinary communities in the center of the bipartite network, whereas smaller ones were more distant. The second analysis highlighted disciplinary homophily in interaction networks, as two researchers interacted more if they were from the same disciplinary community. Results also showed that the interactions among researchers were structured not only by disciplinary homophily, but also by other forms of homophily related to location or region of work. The key brokers of interactions across disciplinary communities were distributed across several communities, showing that brokerage was not controlled by the large, dominant communities. Analysis of correlations between individual interdisciplinarity and contributions to collective interdisciplinarity did not support our hypothesis but rather hinted at the alternative hypothesis that researchers with high individual interdisciplinarity interacted less with other disciplinary communities.