Isotopic niche partitioning between two apex predators over time.

Stable isotope analyses have become an important tool in reconstructing diets, analysing resource use patterns, elucidating trophic relations among predators and understanding the structure of food webs. Here, we use stable carbon and nitrogen isotope ratios in bone collagen to reconstruct and compare the isotopic niches of adult South American fur seals (Arctocephalus australis; n = 86) and sea lions (Otaria flavescens; n = 49) - two otariid species with marked morphological differences - in the Río de la Plata estuary (Argentina - Uruguay) and the adjacent Atlantic Ocean during the second half of the 20th century and the beginning of the 21st century. Samples from the middle Holocene (n = 7 fur seals and n = 5 sea lions) are also included in order to provide a reference point for characterizing resource partitioning before major anthropogenic modifications of the environment. We found that the South American fur seals and South American sea lions had distinct isotopic niches during the middle Holocene. Isotopic niche segregation was similar at the beginning of the second half of the 20th century, but has diminished over time. The progressive convergence of the isotopic niches of these two otariids during the second half of the 20th century and the beginning of the 21st century is most likely due to the increased reliance of South American fur seals on demersal prey. This recent dietary change in South American fur seals can be explained by at least two non-mutually exclusive mechanisms: (i) the decrease in the abundance of sympatric South American sea lions as a consequence of small colony size and high pup mortality resulting from commercial sealing; and (ii) the decrease in the average size of demersal fishes due to intense fishing of the larger class sizes, which may have increased their accessibility to those eared seals with a smaller mouth gape, that is, South American fur seals of both sexes and female South American sea lions.

Mercury bioaccumulation and isotopic relation between Trichiurus lepturus (Teleostei) and its preferred prey in coastal waters of southeastern Brazil.

The trophic transfer of total mercury (THg) and its bioaccumulation from prey species to the predator fish Trichiurus lepturus was analysed in coastal waters of southeastern Brazil to evaluate the trace element dynamic in this predator-prey system. The isotopic (δ13C and δ15N) relation between this predator and its prey allowed inferences on prey assimilation and predator feeding habits. The THg increment varied from 4.5 to 19.5 times between prey and predator, with a biomagnification power of 0.59. The prey species could be divided into three groups regarding δ15N values: i) 13.6 to 13.2‰ (juvenile conspecifics, Pellona harroweri, and Peprilus paru); ii) 12.5 to 11.8‰ (Chirocentrodon bleekerianus, Lycengraulis grossidens, and Dorytheuthis plei); and iii) 10.5‰ (Xiphopenaeus kroyeri). Based on δ13C values, the prey groups were: i) -15.3‰ (X. kroyeri); ii) -17.6 to -16.8‰ (C. bleekerianus, D. plei, P. harroweri, P. paru, and juvenile conspecifics); and iii) -18.7‰ (L. grossidens). The values of THg and δ15N highlighted juvenile conspecifics as the main via of this trace element and the most assimilated prey. The isotopic relation between predator and its prey species showed a feeding activity preferably coastal and pelagic.

Stable isotope ratios indicate that body condition in migrating passerines is influenced by winter habitat.

Although predicted some time ago, there has been little success in demonstrating that the overall fitness of migratory birds depends on the combined influences of their experiences over all seasons. We used stable carbon isotope signatures (delta13C) in the claws of migrating black-throated blue warblers Dendroica caerulescens to infer their wintering habitats and investigated whether winter habitat selection can be linked to condition during migration. Resident bird species with low delta13C corresponded to selection of more mesic habitats, and migrating birds with low delta13C were in better condition than conspecifics with higher delta13C signatures. These findings concur with empirical observations on the wintering grounds, where dominants (mostly males) tend to exclude subordinates from mesic areas (considered to be high-quality habitats). We believe that variation in condition during migration may be one of the key factors determining differences in arrival times and condition at the breeding areas, which in turn have a major influence on reproductive success.