Trophic overlap between fish and riparian spiders: potential impacts of an invasive fish on terrestrial consumers

Abstract

Studies on resource sharing and partitioning generally consider species that occur in the same habitat. However, subsidies between linked habitats, such as streams and riparian zones, create potential for competition between populations which never directly interact. Evidence suggests that the abundance of riparian consumers declines after fish invasion and a subsequent increase in resource sharing of emerging insects. However, diet overlap has not been investigated. Here, we examine the trophic niche of native fish, invasive fish, and native spiders in South Africa using stable isotope analysis. We compared spider abundance and diet at upstream fishless and downstream fish sites and quantified niche overlap with invasive and native fish. Spider abundance was consistently higher at upstream fishless sites compared with paired downstream fish sites, suggesting that the fish reduced aquatic resource availability to riparian consumers. Spiders incorporated more aquatic than terrestrial insects in their diet, with aquatic insects accounting for 45–90% of spider mass. In three of four invaded trout rivers, we found that the average proportion of aquatic resources in web‐building spider diet was higher at fishless sites compared to fish sites. The probability of web‐building and ground spiders overlapping into the trophic niche of invasive brown and rainbow trout was as high as 26 and 51%, respectively. In contrast, the probability of spiders overlapping into the trophic niche of native fish was always less than 5%. Our results suggest that spiders share resources with invasive fish. In contrast, spiders had a low probability of trophic overlap with native fish indicating that the traits of invaders may be important in determining their influence on ecosystem subsidies. We have added to the growing body of evidence that invaders can have cross‐ecosystem impacts and demonstrated that this can be due to niche overlap.

An Evaluation of the Impacts of Alien Species on Biodiversity in South Africa Using Different Assessment Methods

“…They are generalist predators that feed primarily on aquatic invertebrates, but will also opportunistically prey on terrestrial invertebrates, fish and amphibians. Their impacts therefore span numerous trophic levels, and in South Africa include the decline, and in some cases local extirpation, of native invertebrates, frogs and fish (Karssing et al 2012;Rivers-Moore et al 2013;Jackson et al 2016). Oncorhynchus mykiss and S. trutta have also been implicated in the decline of populations of Hadromophryne natalensis (Natal Cascade Frog) in a streams in the uKhahlamba Drakensberg Park (Karssing et al 2012).…”

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…Researchers are increasingly using stable isotope analysis as a tool for inferring the trophic ecology of animals, providing insight into trophic niche separation (Cherel et al 2007, Plass-Johnson et al 2013, trophic overlap (Foley et al 2014, Jackson et al 2016, diet composition (Semmens et al 2009), and diet shifts (Ben-David et al 1997, MacNeil et al 2005. Stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) are 2 of the most commonly used isotopes in ecological studies, as they can provide information on feeding locations and prey types due to predictable changes in δ 13 C across habitats (France 1995) and increases in δ 15 N at higher trophic levels (Owens 1987).…”

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

“…In many instances, introduced salmonids perform a functional role that is different from that performed by native predators, and community-wide effects have ensued. In South Africa (but outside of the Cape Floristic region (CFR)), rainbow (Oncorhynchus mykiss) and brown trout have been shown to differ from the native fish they replace in their consumption of emerging aquatic insects, thus affecting both aquatic and adjacent terrestrial food webs through this replacement (Jackson et al, 2016). in many streams (Townsend & Crowl, 1991), and because they have a foraging behaviour that differs from that of the galaxiids (McIntosh & Townsend, 1995), their impact has extended beyond the replacement of native fish, causing shifts in prey assemblage structure and ultimately modifying the entire stream food web (Flecker & Townsend, 1994;McIntosh & Townsend, 1996;Townsend, 2003).…”

“…in many streams (Townsend & Crowl, 1991), and because they have a foraging behaviour that differs from that of the galaxiids (McIntosh & Townsend, 1995), their impact has extended beyond the replacement of native fish, causing shifts in prey assemblage structure and ultimately modifying the entire stream food web (Flecker & Townsend, 1994;McIntosh & Townsend, 1996;Townsend, 2003). In South Africa (but outside of the Cape Floristic region (CFR)), rainbow (Oncorhynchus mykiss) and brown trout have been shown to differ from the native fish they replace in their consumption of emerging aquatic insects, thus affecting both aquatic and adjacent terrestrial food webs through this replacement (Jackson et al, 2016).…”