Over the last 7,500 years, the area surrounding the Ross Sea has undergone dramatic environmental change. Once an open body of water; a large, land-fast ice shelves began to form there around 1000 years ago, transforming living conditions for the seals. This has given paleontologists Paul Koch and Emily Brault, from the University of California at Santa Cruz, a unique opportunity to study the long-term impact of changing ice conditions on mammal populations. "Studies of fossils let us see how species do or don't adapt to environmental shifts. Here, we are using that approach to explore the adaptability and vulnerability of different Antarctic seal species to less icy conditions in the near future" Explains Koch.
To understand the effects of ice expansion on the seals, Koch and colleagues from Santa Cruz and the University of Maine amassed a vast collection of seal remains, which were well preserved by the cold, dry conditions in the Dry Valleys of Antarctica. The hoard totaled over five hundred mummified animals, including about 380 crabeater seals, and 170 Weddell seals. Koch estimated the age of the specimens using radiocarbon dating, and using the degree of weathering of the carcasses. Colleagues at the Durham University confirmed species identification with ancient DNA analysis. Finally, the structure of carbon and nitrogen atoms in their bones was used as a window into their ecology. Comparing the ratios of heavy and light forms of these elements, known as isotopes, provided an indication of where seals were hunting and what they were eating.
The isotopes suggest that the response to changing ice conditions varies between seal species. The crabeater seals, which most commonly feed on small plankton such as krill, showed very little isotopic change over the past 2500 years, indicating their diet likely remained unaltered. However, the Weddell seals, that have a mixed diet of fish, squid and crustaceans, underwent a shift in isotopic values around 500 years ago. Likely this isotopic shift reflects a combination of changing diet of the seals and changing ecosystems in the Ross Sea associated with the expansion of the land fast ice shelves.
"It's becoming clear that big changes in the Ross Sea ecosystem occurred at the end of the Holocene." Concludes Brault "We are beginning to understand what that meant for the organisms that lived and died there. For example, primary productivity may have decreased significantly, and predators may have adapted their foraging strategies to a changing environment."
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CORRESPONDING AUTHOR CONTACT INFORMATION
PAUL L. KOCH
University of California, Santa Cruz.
EMILY K. BRAULT
University of California, Santa Cruz.
OTHER EXPERTS NOT DIRECTLY INVOLVED WITH THE STUDY
University of New Mexico
Louisiana State University
ASSESSING THE VULNERABILITY OF ANT ARCTIC SEALS TO ENVIRONMENT AL CHANGE: INSIGHTS FROM STUDIES OF SEAL MUMMIES
KOCH, Paul L., University of California, Santa Cruz, CA, USA
BRAULT, Emily K., University of California, Santa Cruz, CA, USA
WELCH, Andreanna J., Durham University, Durham, United Kingdom
NYE, Jonathan W., University of California, Merced, CA, USA
NIVEN, Laura, University of California, Santa Cruz, CA, USA
HALL, Brenda, University of Maine, Orono, ME, USA
HOELZEL, A. Rus, Durham University, Durham, United Kingdom
Polar systems are shifting rapidly in response to human-induced environmental change. Records of response to past environmental shifts may reveal the vulnerability of species or ecosystems by demonstrating their sensitivity and adaptability to such changes. Our prior work has shown that for much of the Holocene (7000 to 500 years ago), and unlike the situation today, land-fast ice shelves were absent along the southwestern coast of the Ross Sea, Antarctica, allowing the region to be inhabited by a large population of southern elephant seals (Mirounga leonina). We are using data from mummified remains to explore the response of three currently common Antarctic seals (crabeater seals, Lobodon carcinophagus; Weddell seals, Leptonychotes weddellii; leopard seals, Hydrurga leptonyx) to changes in iciness and the abundance of a large potential competitor.
During field seasons in 2012-13 and 2013-14, we collected samples (bone, skin, fur, etc.) from mummified seals that occur in the Dry Valleys region of Antarctica (~76.75 °S to ~78.25 °S and up to 60 km from the coast). While species identification is still underway, we found >300 crabeater seals, nearly 100 Weddell seals, and ~20 leopard seals. We calibrated carcass weathering stage against approximately 130 14C dates. Carcass weathering varies with age to ~1000 years old, then plateaus, and most carcasses are under 1500 years of age. Stable carbon and nitrogen isotope ratio variations in seal bone, which reflect trophic level, foraging zone, and marine biogeochemistry, match expectations among these species. In addition, there are no strong temporal patterns in any species (though data from leopard seals are sparse). Our isotopic data suggest that the ecology of these land-fast ice or pack-ice dependent seals was not affected by the change in iciness or elephant seal abundance that began ~1000 years ago. This contrasts with isotopic data from fossil penguins and elephant seals, which point to drops in 15N-concentration over the last 1000 to 100 years, perhaps due to changes in the productivity of the Ross Sea. The constancy of ecological patterns among crabeater, Weddell and leopard seals and the failure of these seals to respond to shifts in Ross Sea productivity suggest a low adaptability in foraging patterns, which may affect their vulnerability to environmental change. Beyond its ongoing use in species identification, we will use ancient DNA data to explore historical population dynamics and signals for selection in the context of the ecological constancy we observed in the face of environmental shifts.