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.
Highlights
Both direct competition and indirect resource competition typically reduce species fitness, and functionally similar species often coexist by niche partitioning
We have shown that native riparian spiders have a higher probability of sharing trophic niche space with invasive trout when compared to native fish
This suggests that invasive trout are more likely to reduce resource availability to, and compete with, riparian spiders than some native fish, potentially explaining the negative effects of trout on riparian consumers elsewhere
Summary
Both direct competition and indirect resource competition typically reduce species fitness, and functionally similar species often coexist by niche partitioning. Due to the considerable flow of energy between adjacent habitats, such as streams and riparian zones (Baxter et al 2005) and marine and coastal systems (Spiller et al 2010), there is potential for resource competition between populations which never interact directly. This is true of invaded systems, where drastic changes in community a 2016 The Authors.
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