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Prehistoric megafauna were more susceptible to environmental pressures than smaller species


For close to a quarter of a million years the Earth was dominated by huge prehistoric beasts, including giant ground sloths, sabre-toothed tigers and woolly rhinos and mammoths.

Prehistoric megafauna were more susceptible to environmental pressures than smaller species
A new study has found that while the largest of prehistoric species, such as woolly mammoths, would have been
able to easily fend off predators and been top of the food chain, they would have particularly vulnerable
 to environmnetal pressures [Credit: Getty Images]
These species of megafauna grazed and hunted across the Americas during the Pleistocene period, surviving through multiple ice ages, before disappearing around 10,000 years ago.

A new study, however, adds weight to the idea that the beasts' great size was their downfall, and how small disruptions to the food chain of would have sent them careening towards extinction.

Scientists have put forward a number of reasons for the demise of the Pleistocene megafauna, including climate change, over hunting from humans and the spread of disease and even asteroid impact – or even a combination of these factors.

But the latest study paints a picture of these kings of the animal word teetering on the edge, and so unable to cope with their changing environment.

Researchers in Uruguay examined the relationship between the body sizes and food webs of South American megafauna, which would have dominated the continent thousands of years ago.

Prehistoric megafauna were more susceptible to environmental pressures than smaller species
Researchers in Uruguay examined the relationship between the body sizes and food webs of South American megafauna,
 like the sabre-toothed cat (smilodon), which would have dominated the continent thousands of years ago 
[Credit: Wallace63/WikiCommons]
They found that while the largest species would have been able to fend off predators, they would have particularly vulnerable to changes in their food supply.

Huge animals make a trade-off between size and energy requirements. While reaching a larger size has distinct advantages for dominating the environment and fending off predators, the additional mass requires substantially more fuel.

According to the researchers, the relationship between the largest Pleistocene animals and their food sources is comparable to modern large species.

For a modern day comparison, there are few animals which can challenge a fully grown African elephant, but they need between 75 to 150 kg (165–330 lbs) of food a day.

Prehistoric megafauna were more susceptible to environmental pressures than smaller species
The latest study paints a picture of these kings of the animal word, which included giants such as the woolly rhino, 
teetering on the edge, and so unable to cope with their changing environment 
[Credit: Getty Images]
When plotting a creature's body size against its position in the food chain, the researchers revealed a bell curve, with the outliers at either end.

What's more, living on the energetic breadline would have changed their behaviours too, as seen by giant ground sloths – which transitioned from being strict plant eaters to scavengers in order to meet their energy needs.

In a model of an animal's body size plotted against likelihood of being carnivorous, they found that the largest animals were less likely to be carnivores – owing to the huge energy requirements needed to keep them ticking.

The study indicates that these animals were not the fittest in evolutionary terms, as they were unable to adapt to changing environment.

Prehistoric megafauna were more susceptible to environmental pressures than smaller species
Living on the energetic breadline would have changed the behaviours of animals too, as seen by giant ground 
sloths – which transitioned from being strict plant eaters to scavengers in order to meet their energy needs 
[Credit: LadyofHats/WikiCommons]
Whether it was a loss of their favourite plants due to a shifting climate, having humans hunt their prey, or them directly, or falling to disease – the megafauna's size was so carefully balanced with their energy demands, couldn't keep up.

However, for smaller species which could adapt their energy needs and be flexible about what they ate, the changing environment provided new opportunities to expand into new habitats.

Dr. Matías Arim, a researcher at Universidad de la República in Uruguay and one of the researchers, told MailOnline: 'Being close to their energetic limit put this organisms into a fragile situation in which any shift in available energy, food, or changes in temperature could restrict their energetic support.

'Our results highlights that most of these large mammals were close to an energetic constrains, and consequently, were particularly vulnerable to changes on their environment. A context that could not be now ignored when other putative determinants of megafauna extinction are considered - such as human predation.'

Prehistoric megafauna were more susceptible to environmental pressures than smaller species
In a model of an animal's body size plotted against likelihood of being carnivorous, researchers found the Pleistocene 
megafauna were less likely to be carnivores – owing to the huge energy requirements needed to keep them ticking
[Credit: Getty Images]
Writing in the journal Biology Letters, the authors explained: 'Despite the unprecedented range in body size of Pleistocene mammals, a hump-shaped relationship between trophic position and body size was found, along with a food web structure that resembles that of modern faunas.

'The fact that the largest animals were close to energetic imbalance put these organisms in a delicate situation. Any shift in baseline conditions (e.g. resource availability) or the appearance of a novel predator able to hunt these organisms could lead this species into a highway to hell.'

Author: Ryan O'Hare | Source: Daily Mail [May 25, 2016]
TANN

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