Shifting niches of marine predators due to human exploitation: the diet of the South American sea lion (Otaria flavescens) since the late Holocene as a case study

Abstract

Abstract—Stable isotope ratios of carbon and nitrogen in archaeological and modern bone samples have been used to reconstruct the dietary changes of the South American sea lion Otaria flavescens from the late Holocene to the present in the southwestern Atlantic. We sampled bones from archaeological sites in northern-central and southern Patagonia, Argentina, and bones housed in modern scientific collections. Additionally, we analyzed the stable isotope ratios in ancient and modern shells of intertidal molluscs to explore changes in the isotope baseline and allow comparison between bone samples from different periods after correction for baseline shifts. Results confirmed the trophic plasticity of the South American sea lion, demonstrated the much larger impact of modern exploitation of marine resources as compared with that of hunter-gatherers, and underscored the dissimilarity between the past and modern niches of exploited species. These conclusions are supported by the rather stable diet of South American sea lions during several millennia of aboriginal exploitation, in both northern-central and southern Patagonia, and the dramatic increase in trophic level observed during the twentieth century. The recent increase in trophic level might be related to the smaller population size resulting from modern sealing and the resulting reduced intraspecific competition. These results demonstrate how much can be learned about the ecology of modern species thanks to retrospective studies beyond the current, anthropogenically modified setting where ecosystem structure is totally different from that in the pristine environments where current species evolved.

Isotopic niche partitioning between two apex predators over time

“…Therefore, the organic matter trapped in the mineral matrix of mollusc shells was used to identify and correct for shifts in the isotope baseline (Casey & Post ; Zenteno et al . ).…”

Back to the future? Late Holocene marine food web structure in a warm climatic phase as a predictor of trophodynamics in a warmer South‐Western Atlantic Ocean

“…Previous research on fur seals and sea lions has suggested an increase in the trophic position of these species after European settlement and intense exploitation, as a result of reduced intraspecific competition (Saporiti, Bala, et al, ; Vales et al, ; Zenteno, Borella, et al, ). The results reported here support that conclusion using a different dataset of zooarchaeological samples.…”

“…It should be noted that the topology of the community within the δ 13 C–δ 15 N space and the Layman metrics are not affected by shifts in the isotopic baseline and hence the latter allow the comparison between communities that differ in their isotopic baselines. However, changes throughout time in the isotopic baseline hinder any direct comparison of ancient and contemporary stable isotope ratios, unless shifts are incorporated into the analysis (Bailey, Barrett, Craig, & Milner, ; Casey & Post, ; Drago et al, ; Saporiti, Bala, et al, ; Vales et al, ; Zenteno, Borella, et al, ).…”

“…The stable isotope ratios of modern and ancient organisms cannot be compared directly, because temporal variations in the isotopic baseline may exist (Casey & Post, ). Nonetheless, the proteins that make up the organic matrix of mollusc shells can become encased within mineral crystals and preserved, hence offering a material suitable to reconstruct the changes in the isotopic baseline (Casey & Post, ; Drago et al, ; Misarti, Gier, Finney, Barnes, & McCarthy, ; Vales et al, ; Zenteno, Borella, et al, ). Accordingly, values of δ 13 C and δ 15 N of limpets and mussels were compared independently using General Linear Models (GLM) as run in IBM SPSS Statistics (Version 23.0.0.2 for Mac) with two fixed factors (species and period) to assess the temporal variation of the δ 13 C and δ 15 N values in shells and, when necessary, correct for any isotopic baseline shift between the two periods considered.…”

Compound‐specific amino acid δ15N values in archaeological shell: Assessing diagenetic integrity and potential for isotopic baseline reconstruction

“…For example, carbonate shell matrices decalcify over long periods of time, meaning that either bacterial degradation (i.e., the selective degradation of native matrix organic matter) or the introduction of exogenous organics might alter organic bulk matter isotope ratios. A number of papers have reported extractions of bulk organic matter or protein from shell for bulk SI analysis using bivalve species, as well as mussels, urchins, or limpets …”