An ecological community is complex and the mechanisms behind the assembly of such a community are still poorly understood. Here, we concentrate on the question of what mechanisms affect the proportion of specialists and the proportion of generalists in a community. First, we use an individual-based model to explore the effects of the available resource spectrum on the specialist-generalist balance in well-mixed and spatially structured environments. In the well-mixed model, we uncover a new mechanism which we term `resource spectrum engineering', in which opportunistic specialists occupying small niches in a mostly generalist community can change the resource spectrum that is experienced by other species strongly disfavouring generalists and causing a community-wide shift towards specialist strategies. Extending to a spatially structured model in which the dispersal distance of species may be limited, we find that specialism is linked to intermediate dispersal lengths, whereas generalism is linked to short and long dispersal lengths. We then investigate two real microbial systems, using 16rRNS sequence data. In the first experiment, we identify functional groups of specialists and generalists by perturbing the microbial environment with variable nutrient concentrations and establishing which groups survive across different concentrations and which do not. In the second experiment we use many replicates of samples from the same source to find co-occur find that generalist species may be more likely to be dependant on the presence of each other than on specific environmental conditions.