Carbon isotope evidence for an abrupt reduction in grasses coincident with European settlement of Lake Eyre,South Australia

Abstract

Stable carbon isotopes in emu eggshell (EES) reflect emu diets and consequently the vegetation available for food sources. At Lake Eyre, South Australia, isotopic data suggest that there has been a rapid and dramatic change in vegetation at some point over the last 200 years when compared with the rest of the Holocene. The proportion of C₄ plants in emu diets has been reduced by approximately 20% in response to an overall shift in C₄/C3 biomass at Lake Eyre. Isotopic measurement of the dominant plants at Lake Eyre indicate that the C₄ plants are almost entirely comprised of grasses and some chenopods and the C3 plants are comprised of the dominant chenopods, shrubs, trees and forbs. We surmise that the -20% reduction in C₄ plant biomass reflects landscape degradation and loss of C₄ grasses resulting from a combination of effects, including overgrazing by both introduced (e.g., sheep, cattle and rabbits) and native (e.g., kangaroos) animals, increasing drought and a change in fire regime beginning in the late 1890s. The magnitude of vegetation change that occurred in the last 200 years is as great as that which occurred during the last glacial maximum (-21 000 years ago), and provides the first evidence for major environmental change at Lake Eyre soon after Europeans settled the arid zone.