Habitat partitioning by native and alien fishes and decapods in novel habitats of the upper San Francisco Estuary

The rate of alien species introductions is increasing in aquatic ecosystems, but many invaders appear to co-exist with previous residents, resulting in persistent mixed assemblages of non-coevolved organisms. This study focuses on species’ habitat partitioning of edge habitats in the San Francisco Estuary, a highly altered novel ecosystem that supports mixed assemblages of native and alien fishes and macroinvertebrates. We used minnow traps to address two questions about the demersal assemblage along levees in large tidal sloughs: (1) What is the relative abundance of demersal fish and macroinvertebrate species in shallow water adjacent to levee edges? and (2) Do native and alien species occupy different habitat types? We used our findings to address the broader question: do native and non-native species show habitat segregation, suggesting development of assemblage structure similar to that of co-evolved assemblages? Results indicate that habitat partitioning occurs among some clusters of species, but not all. Native Prickly Sculpin was found most often near riprapped levee edges, while alien Yellowfin Goby occupied adjacent muddy habitat, and the alien Shimofuri Goby showed no preference. Where two non-native species of caridean shrimp co-occurred, Siberian Prawn occupied unvegetated mud, while Oriental Grass Shrimp used primarily riprap. However, when only Siberian Prawn was present, it showed no preference for habitat type. Habitat associations changed slightly in response to seasonal shifts in habitat and spawning requirements. This study demonstrates that non-coevolved assemblages of organisms can develop resource partitioning to assist co-existence in novel habitats