|Southern Hairy Nosed Wombat, 2007, survived the extinction|
The 1994 trip was my first international field expedition. I did not have proper plant import permits from the United States Department of Agriculture, so when we returned to the United States, my plants were confiscated at the border. I learned my lesson. Through contacts at the Smithsonian, I obtained a permit and after a nervous month, all of my samples entered the United States and were delivered safely to my lab. I brought back 250 plant specimens and over the next year we analyzed their carbon and nitrogen isotopic compositions. Grasses in Australia shift from C4 metabolism in the North to mixtures of C4 and C3 grasses in the very south. Acacia species, both shrubs and trees, and eucalyptus trees are C3 plants. In general, the vegetation in Australia has mixtures of grasses, Acacia, and Eucalypts with variable proportions of herbaceous C3 plants and chenopods. Our goal was to determine the extent of C4 grasses in an Australian animal’s diet. Chenopods can be the predominant plants in many locations. Individual species of chenopods can use C3, C4, or a combination of C3-C4 types of photosynthesis, therefore it was important to keep plant composition in mind when interpreting the δ13C in eggshells.
Because we were ultimately studying the diets of emu and Genyornis, my field collections centered around two things: the general vegetative landscape and potential emu diet. Emus consume the seeds and flowers of all plants except eucalypts; they sometimes eat whole leaves and consume fruits when available. Without question, emus will also eat any insect or lizard that they can catch, which augments their protein intake substantially during the nesting season, particular in arid areas. With subsequent trips in 1998, 1999, 2000, 2001, and 2008, I amassed nearly 1000 plant specimens from all over the continent. Based on this work, we were able to assemble a graph of the nitrogen isotopes of plant tissue as a function of precipitation. Also, we determined a gradient in carbon isotope composition for C3 plants, as a function of precipitation with more positive as compositions in the most arid regions to the most negative in the wet tropical north.
|Seth Newsome and Marilyn with modern eggshell|
Our work in Australia intersected with one of the major controversies in paleontology today: whether the extinctions of megafauna occurred because of human interactions or because of climate change. On every major continent, large mammals, reptiles, and birds have gone extinct in the past 10,000 to 50,000 years (Barnosky et al., 2004). In North America, mammoths and mastodons were extant when the earliest humans arrived a few thousand years after the Last Glacial Maximum (LGM). These huge animals went extinct within 4,000 years of human arrival. In Australia, the timing of the extinction event as well as the arrival of humans was unknown when we began our work. Because each eggshell sample was dated by amino acid racemization, we could determine that Genyornis went extinct at 45,000±2,000 years ago throughout Australia. An early criticism of our extinction dates was that our samples were collected only in the Lake Eyre basin. Subsequent field work across western Australia, as well as in the Lake Frome and the Murray-Darling River Basin in the south, proved that we were recording a continent-wide extinction event.
Carbon isotopes in the eggshells opened a new window and revealed much about how the Australia Outback ecosystem changed over time. Prior to the arrival of humans, the carbon isotopes of emu eggshell reflected the full range in potential diet from 100% C3 to 100% C4 vegetation. Genyornis’ diet was less varied than that of co-exisiting emu and always included a significant component of C4 vegetation (presumably grasses). Earlier estimates by paleontologists had presumed that Genyornis was a browser, eating only leaves from trees. We found this not to be the case. The carbon isotope data throughout the continent portrayed a rich mosaic of vegetation composed of both C3 and C4 plants prior to the arrival of humans and the megafaunal extinction.
After Genyornis disappeared from the fossil record, the carbon isotopes of emu eggshell shifted dramatically to more C3 vegetation indicating that there was considerably less plant diversity in their habitat. We termed this an “ecosystem collapse”, where large animals like Genyornis and Diprotodon that relied on grass could no longer be sustained by diminished grasslands. Diprotodon, the first fossil mammal described from Australia (Owen, 1838) was a large wombat-like marsupial that was widespread across the continent when humans arrived. The widespread ecosystem collapse--meaning vegetational change--requires large-scale phenomena to drive the continental shift in vegetation composition. We proposed that human use of fire might well have caused the collapse.
|Looking for eggshells: Marilyn, Giff, Bev Johnson (background), Evan foreground|
The Australian Outback is known for its harsh, dry conditions, but it hasn’t always been that way! Geologists look at sediments around lake beds that are now dried out and can tell that in the past, these lakes were filled with water and surrounded by animals and humans. One of the goals of our study has been to figure out the wet-dry cycles that have occurred over the past 120,000 years. Also, we asked the question: did early humans have an influence on the amount of rainfall that fell on Australia? Are the current dry conditions in the Outback related to human influences?
Eggshells from emus and Genyornis have given us some independent answers from what geologists have learned from lake sediments. Emus are known omnivores—they eat mostly plants but when they are laying their eggs, they need extra protein, which they get from insects and lizards. Watch an emu foraging along the roadside and you’ll see them pecking at bushes and darting their long necks at passing grasshoppers. The protein in their diet contains the element nitrogen. It turns out that nitrogen and its isotopic forms (14N and 15N) in plants, insects, and lizards are related to the amount of rainfall over the last growing season. Our team has collected plants and insects from all over the Australian Outback starting in 1994 to 2010. We have learned that plants, especially, growing in the drier places, like north of Port Augusta, have more 15N in them than plants growing in wetter areas, like near the Top End of Northern Australia. So, we can make a graph of the amount of rainfall in modern times against the nitrogen isotopes in plants and we find a nice relationship.
But we don’t find plants in the fossil record, so we need to rely on eggshells! As our team worked around the country over the years, we collected modern emu eggshells, often from the same places where we collected the plants. It turns out that the nitrogen isotopes in the eggshells showed nearly the same relationship to rainfall as did the plants, but with a small difference. Basically, you are what you eat! Because we know how rainfall affects plants and modern eggshells, we used this information to figure out wet and dry periods from nitrogen isotopes in the fossil eggshells. We found that we could identify certain places that were wetter on the continent than others. We also saw that the Outback became drier over time, although there were swings in Wet-Dry throughout our study period. Today, the Outback is significantly drier than it was in the years prior to the Megafauna Extinction event, 45,000 years ago. Changes from European settlements and ranching probably have served to create an even drier Outback than there might have been otherwise.