The staggering biodiversity observed around the globe has inspired ecologists to investigate how species-rich communities are formed and maintained. A particularly interesting process is that of community assembly: take barren ground, and introduce new species at a certain rate---how many species will be able to establish? Are there assembly rules that govern this process?
I present two connected research projects on this theme. First, I ask how many species would coexist if we were to start from a random pool of species, and show how we can calculate this quantity when sampling species at random. Second, I simulate ecological assembly and show how choosing the next species to add to the community in different ways results in completely different community structures. Interestingly, many of the network patterns that are believed to be resulting from a "selection for stability" process could be by-products of community construction rules.