Comparisons of isotopic niche widths of some invasive and indigenous fauna in a South African river

Abstract

1. Biological invasions threaten ecosystem integrity and biodiversity, with numerous adverse implications for native flora and fauna. Established populations of two notorious freshwater invaders, the snail Tarebia granifera and the fish Pterygoplichthys disjunctivus, have been reported on three continents and are frequently predicted to be in direct competition with native species for dietary resources. 2. Using comparisons of species' isotopic niche widths and stable isotope community metrics, we investigated whether the diets of the invasive T. granifera and P. disjunctivus overlapped with those of native species in a highly invaded river. We also attempted to resolve diet composition for both species, providing some insight into the original pathway of invasion in the Nseleni River, South Africa. 3. Stable isotope metrics of the invasive species were similar to or consistently mid-range in comparison with their native counterparts, with the exception of markedly more uneven spread in isotopic space relative to indigenous species. Dietary overlap between the invasive P. disjunctivus and native fish was low, with the majority of shared food resources having overlaps of <0.26. The invasive T. granifera showed effectively no overlap with the native planorbid snail. However, there was a high degree of overlap between the two invasive species (~0.86). 4. Bayesian mixing models indicated that detrital mangrove Barringtonia racemosa leaves contributed the largest proportion to P. disjunctivus diet (0.12-0.58), while the diet of T. granifera was more variable with high proportions of detrital Eichhornia crassipes (0.24-0.60) and Azolla filiculoides (0.09-0.33) as well as detrital Barringtonia racemosa leaves (0.00-0.30). 5. Overall, although the invasive T. granifera and P. disjunctivus were not in direct competition for dietary resources with native species in the Nseleni River system, their spread in isotopic space suggests they are likely to restrict energy available to higher consumers in the food web. Establishment of these invasive populations in the Nseleni River is thus probably driven by access to resources unexploited or unavailable to native residents.

Inconsistency for the niche breadth invasion success hypothesis in aquatic invertebrates

“…Dietary niche, as determined by variation in δ 13 C and δ 15 N, was most often broader in less successful invasive species of Cladoceran waterflea and oyster, but more successful species of tunicate. This inconsistency was contrary to our hypothesis of a universally broader isotopic niche in more successful aquatic invasive species, and contrasts with studies that have found broader dietary niches in the most widespread and dominant invasive species of fishes (García‐Berthou ; Pettitt‐Wade et al ), reptiles (Reed et al ), birds, and mammals (Hayes and Barry ), and in aquatic invasive invertebrates compared to native aquatic invertebrates (e.g., Piscart et al ; Hill et al ).…”

“…Insights on the breadth of resources consumed by invasive species in relation to invasion success and phenotypic traits can provide a template for predicting impacts to food web structure following establishment (Rudnick and Resh ). Yet, to our knowledge, this is the first study to compare the isotopic niche of the chosen species pairs of invasive oyster, Cladoceran waterflea and tunicate, and one of the few studies to compare isotopic niches of aquatic invasive invertebrates (but see Rudnick and Resh ; Jackson et al ; Jackson and Britton ; Rosewarne et al , comparisons in the third and fourth studies listed were between invasive fish and invasive crayfish), whereas comparison between native and invasive invertebrates is relatively common (e.g., Olsson et al ; Piscart et al ; Hill et al ; Karlson et al ; Herkül et al ). Ultimately, the findings of this research will provide for a better understanding of the mechanisms for survival across broad geographic ranges and to enhance trait based analysis for species distribution models and risk assessments.…”

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

“…The success of invasive species is often attributed to their plasticity in behavior and diet choice (e.g., Sol et al 2002;Caut et al 2008). Invasive fishes often have higher feeding rates (Alexander et al 2014) and wider niche breadths (Carman et al 2006;Jackson and Britton 2013;Hill et al 2015) than native species which could explain their higher degree of niche overlap with spiders when compared to native fish. However, in streams where invasive trout are functionally redundant due to the presence of a functionally similar native fish, their impact on riparian consumers is likely to be undetectable.…”

“…; Jackson and Britton ; Hill et al. ) than native species which could explain their higher degree of niche overlap with spiders when compared to native fish. However, in streams where invasive trout are functionally redundant due to the presence of a functionally similar native fish, their impact on riparian consumers is likely to be undetectable.…”

Comparing the fish assemblages and food‐web structures of large floodplain rivers

“…The subscript “c” indicates a small sample size correction (Jackson et al., ). All metrics were bootstrapped ( N = 10,000), so comparisons could be made between populations with differing sample sizes (minimum sample size N > 5, Hill, Jones, Hill, & Weyl, ; Jackson & Britton, ; Jackson et al, ). No fishes of the genus Labeo were sampled from the Kwando River, so this group was excluded from the Kwando River analysis.…”