Interplay of immigration and coevolution constrains the structure of specialist-generalist communities

Species and populations in nature fall along a specialist-generalist continuum in habitat preferences and resource use. However, most studies focusing on assembly, niche packing and diversification have considered solely niche positions within a community. We study how the co-evolution of niche position and degree of specialization affects packing, coexistence, and properties of the resulting communities under different modes of community assembly. We observe that evolution can lead to diversification and spontaneous emergence of asymmetric configurations of generalist and specialist phenotypes. Moreover, there exist multiple alternative evolutionarily stable states, with either many specialists or fewer generalists, some of which are only locally uninvadable. Contingency on assembly history shows that both local evolutionary adaptation and dispersal processes influence community-level eco-evolutionary dynamics and the structure of communities. By comparing communities along a continuum of assembly modes going from pure coevolution to pure dispersal, we find that high rates of evolution constrain richness while promoting generalism, partly because pure evolutionary communities become trapped at local evolutionary optima. In contrast, communities shaped by strong immigration tend to be rich in specialist species. Finally, high rates of evolution decrease invasion success in mixed assembly scenarios, a pattern consistent with the phenomenon of community monopolization.