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.
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