Invading rainbow trout usurp a terrestrial prey subsidy from native charr and reduce their growth and abundance

Abstract

Movements of prey organisms across ecosystem boundaries often subsidize consumer populations in adjacent habitats. Human disturbances such as habitat degradation or non-native species invasions may alter the characteristics or fate of these prey subsidies, but few studies have measured the direct effects of this disruption on the growth and local abundance of predators in recipient habitats. Here we present evidence, obtained from a combined experimental and comparative study in northern Japan, that an invading stream fish usurped the flux of allochthonous prey to a native fish, consequently altering the diet and reducing the growth and abundance of the native species. A large-scale field experiment showed that excluding terrestrial invertebrates that fell into the stream with a mesh greenhouse reduced terrestrial prey in diets of native Dolly Varden charr (Salvelinus malma) by 46-70%, and reduced their growth by 25% over six weeks. However, when nonnative rainbow trout (Oncorhynchus mykiss) were introduced, they monopolized these prey and caused an even greater reduction of terrestrial prey in charr diets of 82-93%, and reduced charr growth by 31% over the same period. Adding both greenhouse and rainbow trout treatments together produced similar results to adding either alone. Results from a comparative field study of six other stream sites in the region corroborated the experimental findings, showing that at invaded sites rainbow trout usurped the terrestrial prey subsidy, causing a more than 75% decrease in the biomass of terrestrial invertebrates in Dolly Varden diets and forcing them to shift their foraging to insects on the stream bottom. Moreover, at sites with even low densities of rainbow trout, biomass of Dolly Varden was more than 75% lower than at sites without rainbow trout. Disruption of resource fluxes between habitats may be a common, but unidentified, consequence of invasions, and an additional mechanism contributing to the loss of native species.

Competitive interactions for food resources between invasive racer goby Babka gymnotrachelus and native European bullhead Cottus gobio

“…), it is probable that racer goby are forcing bullheads to abandon their most profitable habitats and to occupy energetically inferior locations, and may even be eliminating the bullhead from parts of its native European range. There is strong evidence that the interference competition of introduced fishes, which use aggression in their interactions with native species (Connell 1983;Schoener 1983), could be adversely affecting the foraging conditions and growth of the latter (Marchetti 1999;Lawler et al 1999;Baxter et al 2007;Blanchet et al 2007), though this is not always the case (Fobert et al 2011). European bullheads can survive in small streams with strong water current, unsuitable to racer gobies, which are normally associated mainly with lentic, slowly flowing waters (Smirnov 1986;Pinchuk et al 2003a).…”

Crossing boundaries: Shigeru Nakano's enduring legacy for ecology

“…5), and mesh covers over riparian shrubs, eventu-ally completing a total of five major experiments and eight studies, before and after Nakano's death (Nakano et al 1999c;Murakami and Nakano 2002;Kato et al 2003;Kawaguchi et al 2003;Baxter et al 2004Baxter et al , 2007Fukui et al 2006;Nakano et al unpublished). These showed, for example, that emerging aquatic insects had positive indirect effects on riparian shrubs during spring by drawing birds into the riparian zone.…”

“…These birds not only fed on the emerging insects but also gleaned terrestrial invertebrates from shrubs that normally damage their leaves (Murakami and Nakano 2002). In turn, cutting off the input of terrestrial invertebrates into streams using greenhouses, which provides about half of the annual prey biomass and annual energy budget for stream fish (Kawaguchi and Nakano 2001;Nakano and Murakami 2001), either caused half the fish biomass to emigrate (Kawaguchi et al 2003) or, if fish were enclosed, it reduced their growth markedly (Baxter et al 2007) and caused them to crop the benthos drastically (Baxter et al 2004;see Fausch et al 2010 for a review). In turn, the reduced benthos resulted in markedly reduced insect emergence, a predictable decline in tetragnathid spider abundance, and a trophic cascade that increased biomass of streambed algae (Baxter et al 2004).…”

“…These investigators have assessed how the effects of human land uses such as cattle grazing Fausch 2007, 2012), logging (Inoue et al 2013), and mining (Kraus et al 2016), as well as the introduction and invasion of species like nonnative trout (Baxter et al 2004(Baxter et al , 2007Benjamin et al 2011Benjamin et al , 2013Lepori et al 2012) and global climate change (Larsen et al 2016), can all have strong effects on the processes that produce or shape the flux of invertebrate prey across the stream-riparian boundary. For example, Benjamin et al (2013) reported that nonnative brook trout reduced the emergence of adult aquatic insects by 55% compared to native cutthroat trout (O. clarkii), and projected that this was sufficient to eliminate emerging insects from the diets of two-thirds of riparian birds that depend, in part, on this food source.…”

Ecosystem Services Provided by the Little Things That Run the World

“…Numerous insect orders have been used as complementary food sources for poultry [29]. Terrestrial invertebrates can comprise more than half of energy ingested by stream fishes [30,31] and are often the preferred prey of juvenile salmonids [32] with significant consequences for fish growth and abundance [33,34]. Insects falling into streams are estimate to comprise about 30-80% of the diets of young salmon [31], 56.7% of masu salmon, 50.5% of white-spotted char, 46.3% of rainbow trout, 22.6% of Dolly Varden, and 12.0%…”