Previous models for assembling ecological networks did not include stochasticity at the level of population dynamics (e.g., demographic noise, environmental noise) and focused mainly on food webs. Here, we present a model for the assembly of mutualistic bipartite networks, such as plant-pollinator networks, and examine the influence of demographic noise on the trajectory of species and strategy diversity, i.e., the range of present strategies from specialism to generalism. We find that assembled communities show at intermediate assembly stages a maximum of species diversity and of average generalization. Our model thus provides a mechanism for non-linear, hump-shaped diversity trajectories at intermediate succession, consistent with the intermediate disturbance hypothesis. Long-term coexistence of specialists and generalists emerges only in the presence of demographic noise and is due to a persistent species turnover. These findings highlight the importance of stochasticity for maintaining long-term diversity.

Data were generated by computer simulations, the setup of which is described in our manuscript. The nestedness of assembled networks was evaluated using ANINHADO (Guimarães Jr. and Guimarães, 2006). The corresponding data can be found in the subfolders named "Binary Matrices". The data produced directly by assembly simulations as well as those produced using ANINHADO were analyzed using Mathematica. The Mathematica notebooks used are included in the software package.