Species replacement by a nonnative salmonid alters ecosystem function by reducing prey subsidies that support riparian spiders

Abstract

Replacement of a native species by a nonnative can have strong effects on ecosystem function, such as altering nutrient cycling or disturbance frequency. Replacements may cause shifts in ecosystem function because nonnatives establish at different biomass, or because they differ from native species in traits like foraging behavior. However, no studies have compared effects of wholesale replacement of a native by a nonnative species on subsidies that support consumers in adjacent habitats, nor quantified the magnitude of these effects. We examined whether streams invaded by nonnative brook trout (Salvelinus fontinalis) in two regions of the Rocky Mountains, USA, produced fewer emerging adult aquatic insects compared to paired streams with native cutthroat trout (Oncorhynchus clarkii), and whether riparian spiders that depend on these prey were less abundant along streams with lower total insect emergence. As predicted, emergence density was 36% lower from streams with the nonnative fish. Biomass of brook trout was higher than the cutthroat trout they replaced, but even after accounting for this difference, emergence was 24% lower from brook trout streams. More riparian spiders were counted along streams with greater total emergence across the water surface. Based on these results, we predicted that brook trout replacement would result in 6-20% fewer spiders in the two regions. When brook trout replace cutthroat trout, they reduce cross-habitat resource subsidies and alter ecosystem function in stream-riparian food webs, not only owing to increased biomass but also because traits apparently differ from native cutthroat trout.

Non‐native rainbow trout (Oncorhynchus mykiss) occupy a different trophic niche to native Breede River redfin (Pseudobarbus burchelli) which they replace in South African headwater streams

“…These findings have important management implications because changes in the functioning of fish assemblages can potentially alter the structure and function of entire aquatic communities (Benjamin et al, 2011). Our GCA analyses show terrestrial invertebrates to be a relatively important prey source for trout, potentially offsetting predation pressure on aquatic invertebrates in the streams where they have replaced redfin and the other small-bodied native species.…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Spiders -Spiders living in riparian zones receive a large proportion of food resources from freshwaters in the form of emerging aquatic insect subsidies [58][59][60]. Briers et al (2005) found that over 40% of the diet of riparian spiders comprised of adult aquatic insects, but this decreased to less than 1% at 20 m from the stream [61].…”

“…If there is a disruption in the flow of allochthonous resources due to landscape degradation, not only will the consumer be impacted, but the entire trophic system has the potential to be altered. The impacts of invasive species can propagate to adjacent ecosystems, for instance, the invasive brook trout reduced emergence rates of aquatic insects by 24%, which caused 6-20 % fewer spiders in the riparian zone, and therefore, altered ecosystem function in stream-riparian food webs [59].…”

Non-native rainbow trout change the structure of benthic communities in headwater streams of the Cape Floristic Region, South Africa

“…If they consume similar types and amounts of prey to the native predators, then impacts on lower trophic levels may be minimal. On the other hand, if prey selectivity or consumption rates differ from the native predator(s), then it is likely that prey assemblages will be altered, with potential indirect effects on lower trophic levels (Benjamin et al, 2011;Lepori et al, 2012).…”