Mutualism provides the basis for biodiversity in eco-evolutionary community assembly

Unveiling the ecological and evolutionary mechanisms underpinning the assembly of stable and complex ecosystems is a main focus of community ecology. Ecological theory predicts the necessity of structural constraints on the network of species interactions to allow for growth of complexity in assembling multispecies communities. A promising research avenue is the search for an understanding of how the coexistence of diverse species interaction types could influence the development of complexity and how an ideal composition could arise in nature. We propose an ecological model with mixed interaction types incorporating evolutionary assembly by speciation. This framework allows to investigate the eco-evolutionary assembly on complex species interaction networks with multiple interaction types and its consequences for ecosystem stability. Our results show that highly mutualistic communities are conducive of complexity and promote the emergence of consumer-resource interactions. Furthermore, we show that an evolutionary process is required to produce such condition. Moreover, this evolutionary assembly generates a diversity of outcomes and promotes two distinct types of complexity depending on speciation constraints. Assembled communities are thus either larger (more species) or more connected, in agreement with patterns previously observed in microbial communities. Our results produce invaluable theoretical insight into the mechanisms behind the emergence of ecological complexity and into the roles of mutualism and speciation on community formation.