How does network structure and complexity in river systems affect population abundance and persistence?

Models of habitat connectivity, and how network structure and connectivity affects resident populations, are increasingly being developed for terrestrial habitat networks. Rivers, unlike many terrestrial habitat networks, follow a rigid hierarchical branching structure to form dendritic networks. It has been hypothesised that this unique structure must have implications for population processes. We developed a theoretical model to relate local-scale habitat quality and connectivity to landscape-scale population dynamics of mobile organisms (e.g. fish, aquatic invertebrates). River networks were modelled as directed graphs, with nodes being habitat patches, and edges the connections between them. Using population simulation analyses, we investigated the effects of network structure on population abundance and persistence. Network structural complexity affected landscape-scale population abundance, but the apparent effect depended upon how structure was quantified. There were no noticeable effects of dendritic network structure on population persistence. Previous research on the effects of habitat network structure on population persistence has used metapopulation patch occupancy models, which do not directly consider population dynamics. Our results show that spatially-explicit population modelling is possible, and that it provides information beyond that possible with patch occupancy models (e.g. population abundance). More importantly, it calls into question whether metapopulation models provide an adequate representation of population dynamics in dendritic habitat networks.