Physicochemical tolerance, habitat use and predation are drivers of patterns of coexistence and exclusion among invasive and resident amphipods

Summary Patterns of coexistence and exclusion among resident and invading species in freshwaters may be generated by direct biotic interactions well as by indirect interactions with the broader abiotic and biotic environments. The N orth A merican ‘shrimp’ C rangonyx pseudogracilis ( C rustacea: A mphipoda) is invasive in Europe where it forms complex patterns of apparent exclusion and coexistence with resident G ammarus spp. amphipods. Using a comprehensive integrated approach, we investigated the potential biotic and interacting abiotic factors driving these distribution patterns. A 2009 survey of 69 sites on a small British Island, The Isle of Man, revealed that of 56 river sites containing amphipods, only six contained C . pseudogracilis and these always co‐occurred with G ammarus spp. In contrast, C . pseudogracilis was the only species present in the 12 ponds/reservoirs containing amphipods. Field transplant experiments in ponds and laboratory oxygen tolerance experiments revealed that C . pseudogracilis tolerates physicochemical regimes which G ammarus spp. are incapable of surviving. River microhabitat sampling showed C . pseudogracilis dominating in slower, more pooled and macrophyte‐dense patches, while G ammarus spp. were dominant in faster, more riffled areas. Field bioassays indicated that predation of C . pseudogracilis by G ammarus spp. may be frequent in patches of rivers if/when the species meet. River drift sampling revealed that C . pseudogracilis was greatly underrepresented in night/day drift relative to the G ammarus spp. Laboratory studies showed C . pseudogracilis to be more photophobic and less active than G ammarus spp., both behaviours potentially contributing to low drift prevalence and consequent reduced exposure to shared drift predators. These interacting factors may ultimately contribute to the coexistence, exclusion and relative distributions of C . pseudogracilis and G ammarus spp. The former is potentially subject to intense predation from the latter if they encounter one another in the same microhabitat. However, with C . pseudogracilis being more physicochemically tolerant and displaying different habitat utilisation patterns than the G ammarus spp. in respect of the benthos and drift, such encounters are probably minimised. Hence C . pseudogracilis can persist in the same sites with the G ammarus spp., albeit in different microhabitats.