Overcoming urban stream syndrome: Trophic flexibility confers resilience in a Hawaiian stream fish

Abstract

Urbanisation is widely associated with a suite of physical, chemical and biological degradation of stream ecosystems, known as “urban stream syndrome.” It is unclear whether urban stream syndrome is applicable to oceanic islands, where marine dispersal of larvae enables diadromous species to continuously recolonise even highly degraded urban streams. The depauperate native fauna of oceanic island streams can be entirely composed of diadromous species, but urban streams food webs are often dominated by introduced predators, competitors and functional groups derived from continental systems. Despite these challenges, some native species appear to thrive in urbanised catchments. Here, we test for urban stream syndrome on oceanic islands by quantifying catchment land use, nutrient concentrations and fish community composition for 37 streams across the Hawaiian archipelago. To assess how native species adapt to food webs altered by species introductions, we quantified trophic responses by examining stomach contents, nitrogen stable isotopes and body condition of Awaous stamineus (an omnivorous goby) in each stream. Urbanisation was consistently associated with nitrogen pollution and replacement of native species with more tolerant exotics. Population densities of three of five native goby species declined sharply with urbanisation, whereas the two other native gobies species were resilient. The trophic position of the omnivore A. stamineus was elevated in urban streams compared to forested catchments, reflecting a shift in stomach contents from algae to greater reliance on exotic aquatic and terrestrial invertebrates. Comparable body condition and resilient population density of A. stamineus across the urbanisation gradient suggest that dietary flexibility buffers this species against environmental degradation. Our findings indicate that the concept of urban stream syndrome is applicable to oceanic islands, yet A. stamineus shows striking resilience. Flexibility in diet, life history and habitat use of this native goby appear to buffer it against the effects of urbanisation compared to most other amphidromous fishes in Hawaiian streams.

Migratory gauntlets on oceanic islands: Watershed disturbance increases the cost of amphidromy

“…This finding is also inconsistent with prior work (Lisi et al, 2018) showing that L. concolor is sensitive to land use modification. This finding is also inconsistent with prior work (Lisi et al, 2018) showing that L. concolor is sensitive to land use modification.…”

“…It also parallels as- (Table 1). This increases the potential for outcomes arising from upstream conditions, especially for E. sandwicensis, which exhibits greater sensitivity than S. hawaiiensis to land use intensification (Lisi et al, 2018). Without the ability to climb, E. sandwicensis and S. hawaiiensis are limited to low elevation reaches and estuarine habitat.…”

“…For example, even in watersheds with low land use development, Sicydiine densities were low (or the species were absent) when Poeciliids were present (Figure 4). Lisi et al (2018) Figure 3). This stands in contrast to A. stamineus and S. hawaiiensis, which appear to be more resilient to the presence of Poeciliid species.…”

“…Further work is thus warranted to more carefully attribute and partition out pressures on native species arising from the often diverse assemblage of invasive species present in Hawaiian streams Brasher, 2003;Lisi et al, 2018;Moody et al, 2017). This raises the possibility that the presence of additional invasive species alongside Poeciliids might lead to additive or multiplicative pressures on native species.…”

“…Management of watersheds harbouring amphidromous fauna has largely been defined by concerns about primary habitat and upstream conditions (reviewed in Blum et al, 2014), but a growing literature indicates that greater consideration should be given to conditions encountered along migratory pathways. Similarly, biotic surveys suggest that coastal urbanisation reduces migratory passage of amphidromous fishes in the Hawaiian Islands Brasher, 2003;Lisi et al, 2018;Moody et al, 2017), and conservation priority areas have been identified for amphidromous fishes in Fiji based on loss of downstream migratory pathways (Jenkins, Jupiter, Qauqau, & Atherton, 2010). Similarly, biotic surveys suggest that coastal urbanisation reduces migratory passage of amphidromous fishes in the Hawaiian Islands Brasher, 2003;Lisi et al, 2018;Moody et al, 2017), and conservation priority areas have been identified for amphidromous fishes in Fiji based on loss of downstream migratory pathways (Jenkins, Jupiter, Qauqau, & Atherton, 2010).…”