Climate change and humans together pushed Australia’s biggest beasts to extinction

25 11 2019

people-megafaunaOver the last 60,000 years, many of the world’s largest species disappeared forever. Some of the largest that we generally call ‘megafauna’ were first lost in Sahul — the super-continent formed by the connection of Australia and New Guinea during periods of low sea level. The causes of these extinctions have been heavily debated for decades within the scientific community.

Three potential drivers of these extinctions have been suggested. The first is climate change that assumes an increase in arid conditions that eventually became lethal to megafauna. The second proposed mechanism is that the early ancestors of Aboriginal people who either hunted megafauna species to extinction, or modified ecosystems to put the largest species at a disadvantage. The third and most nuanced proposed driver of extinction is the combination of the first two.

The primary scientific tools we scientists use to determine which of these proposed causes of extinction have the most support are dated fossil records from the extinct species themselves, as well as archaeological evidence from early Aboriginal people. Traditionally, the main way we use these data is to construct a timeline of when the last fossil of a species was preserved, and compare this to evidence indicating when people arrived. We can also reconstruct climate patterns back tens of thousands of years using models similar to the ones used today to predict future climates. Based on the comparison of all of these different timelines, we conclude that abrupt climate changes in the past were influential if they occurred at or immediately before a recorded extinction event. On the other hand, if megafauna extinctions occur immediately after humans are thought to have arrived, we attribute more weight to human arrival as a driver.

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Logbook of Australia’s ancient megafauna

20 11 2019


Australia is home to some of the most unique species worldwide, including egg-laying mammals, tree-climbing, desert-bouncing and and burrow-digging marsupials, and huge flightless birds. While these animals are fascinating, the creatures that used to roam Australia’s landscape thousands of years ago were even more remarkable — these included wombat-like beasts as big as rhinos, birds more than two metres tall, lizards more than seven metres long, and a marsupial lion as big as a leopard.

Just how and why these animals went extinct has been challenging scientists for decades. But examining dated fossil records is one of the primary ways we can look into the past. The ever-increasing number of fossils and the advances in dating techniques have produced a wealth of material we can use to reconstruct the long-lost past.

DiprotodonEven with these data, it has been a struggle to gather enough fossils for large-scale analyses because reports of these records are usually scattered across the scientific literature, with no standardised quality control to make them comparable to each other. Designing a way to standardise these records is therefore important to avoid misleading conclusions.

The FosSahul database was first established in 2016 to try to alleviate these problems — it gathered all the fossil specimens for large animals (excluding humans) from the Late Quaternary (up to ~ 1 million years before present) across the region known as ‘Sahul’, the combined super-continent that included New Guinea and Australia when sea levels were much lower than they are today.

While FosSahul was an important step, the database needed to be updated. First, the quality rating of the fossil dates in the original version was a little subjective and lacked transparency in some cases. This is because the database did not capture enough detail to be able to reproduce all the steps leading to a particular quality rating. Second, given that new fossils are discovered regularly, updates are necessary to include the latest research. Read the rest of this entry »

What is a ‘mass extinction’ and are we in one now?

13 11 2019

(reproduced from The Conversation)

For more than 3.5 billion years, living organisms have thrived, multiplied and diversified to occupy every ecosystem on Earth. The flip side to this explosion of new species is that species extinctions have also always been part of the evolutionary life cycle.

But these two processes are not always in step. When the loss of species rapidly outpaces the formation of new species, this balance can be tipped enough to elicit what are known as “mass extinction” events.

Read more: Climate change is killing off Earth’s little creatures

A mass extinction is usually defined as a loss of about three quarters of all species in existence across the entire Earth over a “short” geological period of time. Given the vast amount of time since life first evolved on the planet, “short” is defined as anything less than 2.8 million years.

Since at least the Cambrian period that began around 540 million years ago when the diversity of life first exploded into a vast array of forms, only five extinction events have definitively met these mass-extinction criteria.

These so-called “Big Five” have become part of the scientific benchmark to determine whether human beings have today created the conditions for a sixth mass extinction.

An ammonite fossil found on the Jurassic Coast in Devon. The fossil record can help us estimate prehistoric extinction rates. Corey Bradshaw, Author provided

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Cartoon guide to biodiversity loss LVII

6 11 2019

The sixth set of six biodiversity cartoons for 2019. See full stock of previous ‘Cartoon guide to biodiversity loss’ compendia here.

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