doi.org
Isotopic niches of native and invasive large decapods along a steep estuarine gradient - Biological Invasions
Biological invasions are reshaping aquatic food webs through novel interactions, yet how invaders integrate cross-ecosystem resources along estuarine gradients remains poorly understood. We used stable isotopes of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S) to characterise isotopic niches of a large decapod community along a 3-km gradient in a small Atlantic estuary in southern Iberia. The lower sector (i.e. at the river mouth) supported a native crab assemblage (Carcinus maenas, Afruca tangeri and Pachygrapsus marmoratus) co-occurring with the invasive Atlantic blue crab (Callinectes sapidus), whereas the upstream freshwater sector was dominated by the invasive red swamp crayfish (Procambarus clarkii); both invaders overlapped spatially in the brackish ecotone. A shift in the origin of assimilated resources over ≤ 1 km was noted, from marine- to inland-derived, and δ34S integrates patterns consistent with ontogenetic blue crab migration. Moreover, isotopic relationships (δ15N and δ34S) suggest an important contribution of prey migrating from downstream in the diet of the decapods inhabiting upper reaches. Bayesian ellipsoid volumes in δ13C–δ15N–δ34S space indicated that the blue crab exhibited the largest isotopic niche, particularly in the brackish sector and substantially larger than that of the invasive crayfish. In the lower sector, crab species showed niche segregation, except for Afruca tangeri and Carcinus maenas, which exhibited substantial overlap. In the brackish sector, blue crabs displayed higher δ15N and δ34S values than the crayfish, and the crayfish niche was largely encompassed by that of the blue crab but not vice versa, suggesting asymmetric interactions. Both invaders showed clear ontogenetic shifts. Blue crab niche peaked in juveniles, and suggests an important change in habitat and resource use through ontogeny. Crayfish niche indicated a dietary change towards greater plant/detritus intake in adults, but exhibited less variable diet than the blue crab across life stages. Overall, these results describe consistent taxonomic, spatial and ontogeny-dependent isotopic patterns in estuarine food webs invaded by large decapods.
