Science

Deforestation ‘stressing animals out’, scientists warn


Some people may be panicking about our own species’ rampant destruction of the natural world, but we are not the only ones concerned.

The animals directly impacted by deforestation appear to be expressing more hormones associated with stress, according to bleak new research by American scientists.

Researchers who took samples of hormones which accumulate in fur found rodents and marsupials living in smaller remaining patches of South America’s Atlantic Forest are under more stress than those living in more intact areas of the forest.

The Atlantic Forest stretches along Brazil’s Atlantic coastline and into Argentina and Paraguay. It is one of the most diverse ecosystems on the planet, second only to the Amazon, but only a fraction of the original forest remains.

“We suspected that organisms in deforested areas would show higher levels of stress than animals in more pristine forests, and we found evidence that that’s true,” said Noe de la Sancha, a research associate at the Field Museum in Chicago, associate professor of Biology at Chicago State University, and co-author of the research.

“Small mammals, primarily rodents and little marsupials, tend to be more stressed out, or show more evidence that they have higher levels of stress hormones, in smaller forest patches than in larger forest patches.”

The researchers said the destruction of an animal’s habitat can drastically change its life. It results in less food and territory to go around, and the animal might find itself in more frequent contact with predators or in increased competition with other animals for resources.

Combined, these circumstances can add up to long-term stress.

“A lot of species, all over the world, but especially in the tropics, are understudied,” said Sarah Boyle, an associate professor of Biology and Chair of the Environmental Studies and Sciences Program at Rhodes College and the study’s lead author.

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“There is not a lot known about many of these animals in terms of even their baseline hormone levels.”

The researchers noted that the Atlantic Forest is often overshadowed by its neighbour, the Amazon, but is still South America’s second-largest forest.

It once covered about 463,000 square miles, an area bigger than California, Oregon, Washington, and Nevada put together.

Over the past 500 years, parts of the forest have been destroyed to make way for farmland and urban areas, and it remains vulnerable to logging and agricultural expansion, particularly soy production.

Today, less than one-third of the original forest remains.

It is home to around 20,000 species of plants and 450 tree species have been found in just one hectare, according to the WWF.

As well as thousands of species of birds, various rare mammals, reptiles and amphibians live in the forest including endangered jaguars, golden lion tamarins, woolly spider monkeys, maned three-toed sloths and red-tailed parrots.

The scientists said that stress isn’t necessarily a bad thing for animals and in small doses, can be life-saving.

“A stress response is normally trying to bring your body back into balance,” said David Kabelik, an associate professor of biology and chair of the neuroscience program at Rhodes College and one of the paper’s authors.

“If something perturbs you and it can cause you to be injured or die, the stress response mobilises energy to deal with that situation and bring things back into a normal state. It allows you to survive.”

This response could be triggered for instance if an animal encounters a predator, a flood of stress hormones can help give them the speed they need to run away, and then those hormone levels go back down to normal.

“But then these animals are placed into these small fragments of habitat where they’re experiencing elevated stress over prolonged periods, and that can lead to disease and dysregulation of various physiological mechanisms in the body,” he said.

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To assess the impacts on animals, the researchers focused on patches of forest in eastern Paraguay, which has been hit particularly hard by deforestation in the last century as the region was clear-cut for firewood, cattle farming, and soy.

To study the effects on the animals there, the researchers trapped 106 mammals from areas ranging from 2 acres to 1,200 hectares. Two acres is about the size of a rugby pitch, and 1,200 hectares is 4.63 square miles. The animals they analysed included five species of rodents and two species of marsupials.

The researchers took samples of the animals’ fur, since hormones accumulate in hair over periods of many days or weeks, and could present a clearer picture of the animals’ typical stress levels than the hormones present in a blood sample.

“Hormones change in the blood minute by minute, so that’s not really an accurate reflection of whether these animals are under long-term stress or whether they just happened to run away from a predator a minute ago,” said Dr Kabelik.

“We were trying to get at something that’s more of an indicator of longer term stress. Since glucocorticoid stress hormones get deposited into the fur over time, if you analyse these samples you can look at a longer term measure of their stress.”

In the lab the researchers ground the fur into a fine powder and extracted the hormones.

The team found that the animals from smaller patches of forest had higher levels of glucocorticoid stress hormones than animals from larger patches of forest.

“Our findings that animals in the small forest patches had higher glucocorticoid levels was not surprising, given the extent to which some of these forested areas have been heavily impacted by forest loss and fragmentation,” said Dr Boyle.

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“In particular, these findings are highly relevant for countries like Paraguay that currently show an accelerated rate of change in natural landscapes,” said Pastor Perez, a biologist at Universidad Nacional de Asuncion and another of the paper’s authors.

“In Paraguay, we are just beginning to document how the diversity of species that are being lost is distributed.

“However, this paper shows that we also have a lot to learn about how these species interact in these environments.”

The scientists also found that the methods of trapping the animals contributed to the amount of stress hormones present.

“It’s an important consideration that people have to understand when they’re doing these studies, that if they are live trapping the animals, that might be influencing the measured hormone levels,” said Dr Kabelik.

The study not only sheds light on how animals respond to deforestation, but it could also lead to a better understanding of the circumstances in which animals can pass diseases to humans, the researchers said.

“If you have lots of stressed out mammals, they can harbour viruses and other diseases, and there are more and more people living near these deforested patches that could potentially be in contact with these animals,” says de la Sancha.

“By destroying natural habitats, we’re potentially creating hotspots for zoonotic disease outbreaks.”

The results have implications far beyond South America’s Atlantic Forest.

“Big picture, this is really important because it could be applicable to forest remnants throughout the world,” said Dr de la Sancha.

“The tropics hold the highest diversity of organisms on the planet. Therefore, this has potential to impact the largest variety of living organisms on the planet, as more and more deforestation is happening. We’re going to see individuals and populations that tend to show higher levels of stress.”

The paper is published in Scientific Reports.



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