An alien species visiting Earth would find a planet clearly dominated by one species. It’s no secret that humanity has a major influence on all aspects of Earth processes – we’ve covered major portions of land surface in stone and asphalt; we’ve cleared natural landscapes and driven an ever-increasing number of species to extinction; we’ve filled the oceans with plastic and the atmosphere with greenhouse gases. This planet is very firmly in the grip of human activity.
But this legacy of environmental modification goes back further than we generally appreciate. Today’s issue of PNAS has a special section devoted to human evolution, and one paper outlines the modern understanding of the many ways our species has affected the physical world around us, and just how long we’ve been doing it. The fact is that our modern explorers – the intrepid adventurers of the past few centuries who discovered lands populated only by indigenous peoples or long-gone civilizations – were alien visitors to parts of the planet already dramatically transformed by human influence.
Here are some of the ways our species has been shaping natural environments since long before written history:
|If you do a Google Search for "Pristine Environment," you'll|
get results like this. But is there really such a thing?
Image from Wikimedia Commons.
Humans are predators. Intelligent, social, mobile, highly influential predators. Our species first appeared around 200,000 years ago, and by 12,000 years ago had spread to the distant corners of the globe. And everywhere we went, we impacted local fauna.
Fossils provide evidence for some very interesting effects human presence has had on ecosystems in the past. In some cases, pressure from humans encouraged the replacement of large, easy-to-hunt animals with ecosystems full of small, fast, hard-to-catch creatures like rabbits, monkeys, and rodents. In others, pressures from human predation caused certain species – including molluscs and turtles – to shrink in size over time.
In many cases, the largest animals suffered the most from human presence. The arrival of humans in many places in the world is associated with the extinction of megafauna – mammoths, ground sloths, and more – especially in places like Australia or South America, where local faunas had not evolved alongside humans. And like I discussed in another post, the extinction of megafauna brought along with it all sorts of ecosystem changes, including changes to plant dispersal, ecosystem structures, nutrient availability, and more.
The decline of large animals didn’t end with the Ice Age megafauna, unfortunately. Into the Bronze Age and Iron Age, overhunting and overfishing continued to cause the decline of animals in many parts of the world, and in most cases they have never recovered. This is a practice we continue to this day.
You don’t have to look any further than Florida Burmese pythons, Chestnut blight, or pretty much any science fiction story about alien invasions, to see that bringing a species to a place it doesn’t belong is often a bad idea. Humans have been doing this for a very long time.
As far back as 20,000 years ago, there is evidence of plant species translocated by humans. In some cases, such as crop plants, this is done intentionally. In others it is an accident, as in the case of weeds or rats. In both cases, the influx of new species can alter the existing ecosystem. There are parts of the world now where the local fauna has been largely replaced by sets of species that humans introduced. There are even species of weed-plants that have become so successful since being spread by people that paleontologists have trouble identifying where those species came from in the first place.
By far the most obvious massive intentional species introductions have been those of domesticated animals. Cattle, sheep, pigs, dogs, horses, chickens – these are animals that have spread from restricted native populations to worldwide domination through domestication. In fact, domesticated animals make up the vast majority of vertebrate biomass on the planet today.
Farming might just be humanity’s most powerful creation. And of course, with great power … well, you know where this is going.
The first thing agriculture requires is space to farm. To this end, humans have been cutting and burning down forests for tens of thousands of years. Fossil evidence shows that in some parts of the world, as far back as 40,000 years or more, the arrival of humans meant changes in fire activity. Over time, the increase of human populations throughout all parts of the world is followed by the replacement of native landscapes: in Europe, dense forests gave way to more open lands; in Africa, savannahs replaced rainforests; in China, shrublands were cleared for rice paddies.
These changes in vegetation come with a host of associated transformations. In addition to the obvious effects on local ecosystems, lower plant cover means less erosion, and in some places there’s evidence that local lake ecosystems were affected by the influx of eroded materials. There’s even evidence that the changing floral landscape affected climates: destruction of vegetation is linked to the rise in gases like carbon dioxide and methane, and agriculture in Tibet thousands of years ago may even have affected the regional monsoon system.
This is another trend that has continued into the present. Throughout the Bronze and Iron Ages, humans continued to replace forests with croplands. (Interestingly, several times throughout the history of Europe, forests showed periods of regrowth during plague outbreaks, when farmlands were temporarily abandoned). Many of these changes are still evident today, even in places no longer used for farming.
Islands are special places ecologically. Often cut off from continental influences, islands tend to have lots of unique species living in unique ecosystems. This makes them fascinating centers of rare and unusual species, but also makes them vulnerable to outside disturbances.
As humans spread around the globe, they brought their environmental engineering powers to islands as well, and most were changed drastically. Endemic species were often unable to cope with the influx of fires, crop plants, domesticated animals, and stowaways such as weeds, rats, and bacteria. The fossil records of dozens of islands in the Pacific, Caribbean, and Mediterranean show dramatic changes once humans arrive. Many indigenous species went extinct, others dwindled, and ecosystems shifted. Before human habitation, many islands were bad places for people to live, full of inedible plants and low populations of animal life, but humans transformed them upon arrival, and today many islands are dramatically different from what they were just millennia ago.
That pattern neatly describes human influence all over the globe, in fact. During our past several thousand years of gradual world domination, our species has transformed many dangerous, food-poor environments to human-friendly places where agriculture is productive and human habitation comes easy. We’ve replaced many diverse global ecosystems with a human-selected set of species – a short list of game animals, livestock, and edible plants.
Human influence on Earth’s environments goes back a long way, and has affected everything from fire regimes to surface processes to weather and climate. The authors of the PNAS paper point out the importance of this fact for discussions of ecological conservation. They go as far as to say the following, and I will leave you with this sentence from the article:
“’Pristine’ landscapes simply do not exist and, in most cases, have not existed for millennia. Most landscapes are palimpsests shaped by repeated episodes of human activity over multiple millennia.”
Boivin et al. 2016. Ecological consequences of human niche construction: Examining long-term anthropogenic shaping of global species distributions. PNAS [Link]
Schlutz and Lehmkuhl 2009. Holocene climatic change and the nomadic Anthropocene in Eastern Tibet: palynological and geomorphological results from the Nianbaoyeze Mountains. Quaternary Science Reviews [Link]