The introduction of exotic species to island environments often has debilitating effects on native ecosystems. Remote island ecosystems are particularly vulnerable to species invasions because endemic species evolved with minimal exposure to the competition and predation stresses that are encountered in environments with greater species diversity. In freshwater ecosystems in Hawai‘i, native amphidromous fish and crustaceans migrate upstream to higher elevation reaches to avoid predation by estuarine species and competition with biota that lack climbing abilities. The introduction of the amphidromous Macrobrachium lar (Tahitian Prawn) to Hawaiian streams in the 1950s quickly resulted in the ubiquitous colonization of streams across the island chain, but the degree to which this species is co-located with endemic species, and possibly competing with or predating on these species, is not well quantified. To quantify patterns of habitat use and associations with native species, we conducted visual surveys (n = 948) in 48 stream reaches under low-flow conditions across a range of stream sizes and terminal reach conditions (e.g., waterfall, estuary) from 2020 to 2022 on Maui island: 13 low elevation (<200 ft asl) reaches; 17 middle elevation (200-600 ft asl) reaches; 18 high elevation (>600 ft asl) reaches.
Macrobrachium lar abundance was greatest in streams with estuaries, with abundances 1.9x, 1.6x, and 2.7x greater compared to streams with terminal waterfalls for high, middle, and low elevation reaches, respectively. In streams with estuaries, abundances were 3.7x and 4.6x greater at the mouth than at middle elevation and high elevation reaches, respectively. Abundance did not vary across habitat (e.g., pool, riffle, run) in streams with terminal waterfalls. However, in low elevation reaches above estuaries, abundances were 2.1x and 2.4x greater in pool habitats compared to run and riffle habitats, respectively. We document co-occurrence with the endemic fish and invertebrates in low and middle elevation reaches.