title: 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.
authors.affiliation: Department of Zoology and Entomology Centre for Invasion Biology University of Pretoria Private Bag X 20 Hatfield 0028 South Africa.
authors.forename: Michelle C
authors.initials: MC
authors.lastname: Jackson
authors.affiliation: School of Animal, Plant and Environmental Sciences University of the Witwatersrand Johannesburg South Africa; South African Institute for Aquatic Biodiversity (SAIAB) Grahamstown 6140 South Africa; Centre for Invasion Biology SAIAB Grahamstown 6140 South Africa.
authors.forename: Darragh J
authors.initials: DJ
authors.lastname: Woodford
authors.affiliation: South African Institute for Aquatic Biodiversity (SAIAB) Grahamstown 6140 South Africa; Centre for Invasion Biology SAIAB Grahamstown 6140 South Africa; Department of Zoology and Entomology Rhodes University Grahamstown 6140 South Africa.
authors.forename: Terence A
authors.initials: TA
authors.lastname: Bellingan
authors.affiliation: South African Institute for Aquatic Biodiversity (SAIAB) Grahamstown 6140 South Africa; Centre for Invasion Biology SAIAB Grahamstown 6140 South Africa.
authors.forename: Olaf L F
authors.initials: OL
authors.lastname: Weyl
authors.affiliation: Department of Zoology and Entomology Centre for Invasion Biology University of Pretoria Private Bag X 20 Hatfield 0028 South Africa.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Potgieter
authors.affiliation: Centre for Water Resources Research University of KwaZulu-Natal Private Bag X01 Scottsville 3209 South Africa.
authors.forename: Nick A
authors.initials: NA
authors.lastname: Rivers-Moore
authors.affiliation: South African Institute for Aquatic Biodiversity (SAIAB) Grahamstown 6140 South Africa; Centre for Invasion Biology SAIAB Grahamstown 6140 South Africa.
authors.forename: Bruce R
authors.initials: BR
authors.lastname: Ellender
authors.affiliation: Department of Zoology and Entomology Centre for Invasion Biology University of Pretoria Private Bag X 20 Hatfield 0028 South Africa.
authors.forename: Hermina E
authors.initials: HE
authors.lastname: Fourie
authors.affiliation: Department of Zoology and Entomology Centre for Invasion Biology University of Pretoria Private Bag X 20 Hatfield 0028 South Africa.
authors.forename: Christian T
authors.initials: CT
authors.lastname: Chimimba
keywords: Competition; invasive species; niche overlap; nicheROVER; stable isotope; trophic subsidies