Changing diets over time: knock-on effects of marine megafauna overexploitation on their competitors in the South-Western Atlantic Ocean

This study compares the δ15N values and the trophic position of two
seabird species throughout the Late Holocene in three regions in the
South-Western Atlantic Ocean to assess the hypothesis that the decimation
of megafauna lead to changes in the trophic position of mesopredators.
Modern and ancient mollusc shells were also analysed to account for
changes in the isotopic baseline through time. Results revealed that
modern Magellanic penguins have higher δ15N values than their ancient
conspecifics in the three regions, after controlling for changes in the
isotopic baseline. This was also true for modern Imperial shags compared
to ancient unidentified cormorants/shags from the two areas where ancient
specimens were recovered (Southern Patagonia and the Beagle Channel). Such
temporal variability might be caused by three non-mutually exclusive
processes: decreased availability of pelagic squat lobster resulting from
decreasing primary productivity through the Late Holocene, increased
availability of small fishes resulting from the sequential depletion of
other piscivores (South American fur seal and sea lion and Argentine hake)
since the late 18th century and modification of the migratory patterns of
Magellanic penguins. Although disentangling the relative contribution of
all those processes is impossible at this time, the results reported here
demonstrate that the ecology of Magellanic penguins and Imperial shags has
undergone major changes since the Late Holocene.