The global spread and increase of the human species is exceptional and seems unmatched in the animal kingdom. Some researchers suggest we resemble so-called invasive species. Does the label fit?
By Frank Götmark
(This a translation of a Swedish essay published in Svenska Dagbladet on 30 March, slightly modified)
Japanese oyster, raccoon dog, lupine – an increasing number of species in Sweden are being designated as invasive, and action programs and laws exist to combat them. In most cases, we ourselves have caused their spread here, through the plant trade, horticulture, long-distance transport and tourism. Several researchers have highlighted our species, Homo sapiens, as invasive on Earth. Are there good reasons for this, and can such a label be accepted? Or is our love for ourselves too strong?
Most people are aware of our African origins, but few know how humans conquered all continents in a relatively short time (about 70,000 years). In Scientific American 2015, Curtis Marean at Arizona State University described the human dispersal and increase under the title “The most invasive species of all”. Ecosystems changed dramatically when we eliminated our closest relatives and many mammals outside Africa. Group cooperation, linguistic communication, and the development of weapons facilitated an enormous expansion that no other Homo species managed to achieve.
There were barriers, such as oceans that were not easy to cross, but we developed good boats. 45,000 years ago, we had reached Australia, where hundreds of large marsupial and bird species disappeared. 13,000 years ago, we managed to reach North America via northeastern Siberia, where sea levels were lower due to the Ice Age (perhaps boats were used across the Pacific Ocean earlier). In a short time, we then made it all the way down to Tierra del Fuego in South America.
On the Eurasian continent, large mammal species became prey for our ancestors. Here, after the last ice age, the mammoth, woolly rhinoceros, giant deer, steppe bison, wild horse, aurochs and cave bear went extinct. The new Homo species was probably the cause, although climate change also had an effect. But these animals had previously survived several ice ages and interglacials. The spread and increase of Sapiens, the smart ape, was the key factor.
A spectacular megafauna also disappeared in North and South America. Here, alongside the saber-toothed tiger, lived the strange giant sloth Megatherium americanum, a vegetarian weighing up to 4 tons that could stand on two legs, with strong claws for grabbing trees and defense. There were also spherical glyptodonts of the same size as a rhinoceros, with thick armor and a long tail equipped with a spiked club for protection against enemies. Other species quickly extinguished by the Paleo-Indians included the sabertooth Smilodon, scimitar cat Homotherium and American cheetah Miracinonyx; three pronghorn antelope species; numerous horse Equus, tapir Tapirus and camel family species Camelidae; gomphotheres, mammoths and the American mastodon (all in the Proboscidea order); and the Aztlan rabbit and a giant beaver as big as a bear.
The enormous Amazon rainforest was also affected. Paleoecologists have examined the forest and found extensive human impact. In the article “Holocene Rain-Forest Wilderness: A Neotropical Perspective on Humans as an Exotic, Invasive Species”, researchers Robert Sanford and Sally Horn show that hunters and gatherers burned areas of the forest and cultivated corn and other crops. They also favored trees that produced nutritious large fruits, rearranging the tree composition. The Amazon is still species-rich and valuable for preserving a habitable climate, nature conservation and sustaining some remaining human tribes, but new, more modern Sapiens are on the move in the area.
Sanford and Horn described humans as invasive in their 2000 article. After their insightful work and that of Curtis Marean, only a few academics seem to have used the term, yet humanity has expanded enormously in numbers since we left the hunter-gatherer stage. We seem to turn a blind eye to our increase and its effects on the ecosystems on which we depend. “We don’t see the forest for all the trees,” the saying goes, but also relevant seems to be “We don’t see nature for all the people,” crowded together as we are in urban environments.
Anders B Johnsson writes in a new (Swedish) book, The Invasive Man (Den invasiva människan), that we may have only been 10,000 at “out of Africa”, but 3-4 million already when the ice retreated in Scandinavia 22,000 years ago. The global expansion led to a large population increase. But note that our population in Africa did not exterminate large mammals on that continent. The large animals had lived side by side with the (low-tech) Sapiens and developed much-needed wariness of humans. This was not the case in America or Australia, or on islands like Madagascar or New Zealand, where the arrival of humans was disastrous for fearless, easily hunted animals.
Around 3000 BC, according to Johnsson, we were 14 million, more than 3 times as many. In several areas, agriculture provided better food production and safer conditions in villages and small towns. The level of 50 million was passed around 1000 BC, and now the pace increased. On the way to 500 million people in the 17th century, some disasters occurred, such as the plague in the middle of the 14th century. But we only decreased temporarily, by about 80 million, and in 1750 we approached 800 million, a sharp increase in a short time.
At the beginning of the 19th century, humanity passed 1 billion. Now sanitary, medical and technological advances soon came and so, in 1960, we were 3 billion on the earth, an unimaginable increase. Europeans had colonized large parts of the “Third World” and increasingly used the ecosystems there for resource extraction and production. Nature at home had already been overexploited; for example, the forests were gone from large parts of Southern and Central Europe and Great Britain.
But our strongest impact on virtually all the world’s ecosystems and species came recently, from the 1970s until today. With 8.2 billion today, we are approaching a tripling of the population level in 1960! In 1968, Paul and Anne Ehrlich published their famous book The Population Bomb and followed this in 1990 with The Population Explosion, an appropriate title considering our numbers. The use of modern medicine, agriculture, fossil energy sources, technology and free trade explain our enormous increase. The eager anticipation among economists and politicians for “increased economic growth” characterizes the invasive human beings that Anders Johnsson describes in his book.
Populations of wild birds, mammals, amphibians, reptiles and fish species declined by an average of 73 percent between 1970 and 2020, according to the WWF. Our rampant exploitation, consumption and increasing pollution are reducing the numbers of many species. Regarding climate, the IPCC report from 2022 states; “Globally, GDP per capita and population growth remained the strongest drivers of CO2 emissions from fossil fuel combustion over the past decade (robust evidence, high agreement)”.
What is an invasive species?
An invasive species can be defined as an alien, non-native species that spreads and causes various forms of damage. Such species are desirable to regulate and, in the best case, eliminate from a country. But compared to our population growth they are a minor problem, at least in Sweden and many European countries. In North America and Australia, they are a larger problem. But again, they cause a lot less damage than Homo sapiens, who is in any case the cause of their spread.
Invasive species tend to appear near buildings and infrastructure; for example, on roadsides and other environments that are easily colonized, or in the sea via ballast in ships. It is often difficult to draw boundaries in time and space for invasive species. For example, in Sweden several species came in via seeds in agriculture during the 19th century and became common, such as certain weeds.
In other cases, we have deliberately introduced new species – the Contorta pine, planted in Sweden on an area equivalent to the size of the island Gotland in the Baltic, is an example. In addition, some native species are increasing and causing problems, such as the common rush in pastures, a species that in Sweden benefits from mild winters. Its negative effect (for humans) can easily be greater than, for example, the increase in exotic lupines, whose flowers are appreciated by many.
How consumption and the human ecological “footprint” should be reduced is discussed among some economists and politicians, but the potentially positive significance of slowed population growth and reduced world population is avoided by almost all economists, politicians, and environmental organizations. Today, population growth is strongest in Africa: the UN forecast is an increase from 1.4 billion today to 3.8 billion in 2100. The ongoing increase there is given incomprehensibly little attention in the media, despite the deplorable conditions for many women with large families and increasingly devastated ecosystems for everyone. Particularly south of the Sahara, birth rates are high (about 4.3 children per woman), which means even limiting population growth to the level predicted by the UN demands substantial increases in contraceptive use, and a major decrease in fertility.
How can high birth rates best be reduced in developing countries? Most population advocates, such as the American John Bongaarts, emphasize voluntary family planning programs and assistance for these if they are started by leaders in developing countries. A Swedish pioneer recently summarized lessons learned from these programs from 1960 and onwards (see “Interview with Carl Wahren” 2024). Mistakes occurred (in China, and India 1975-76) but in most developing countries the programs were voluntary and contributed to falling birth rates. It is desirable that developed nations’ foreign aid budgets for voluntary family planning increase, to facilitate greater contraceptive use in poorer countries. Instead, such aid budgets are being cut, drastically in the case of the United States. But African and other high-fertility countries also have opportunities: commentators suggest that in the current situation, countries should reconsider population programs, increase self-reliance, and reduce dependency on foreign donors.
Human numbers will likely peak this century. The question is whether we can rein ourselves in sooner rather than later and begin to reduce our economic demands on ecosystems in ways that would leave space for other species and allow restoration of damaged natural areas. It is way past time for the most invasive and destructive species to change course.
