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 crossecosystem
impacts and demonstrated that this can be due to niche overlap.
