Humans are causing a massive shift in the ecology of the Earth. This is more obvious in some places than others, but even the most isolated ecosystems have been touched by the changes we’ve wrought in the atmosphere and ocean. This has caused the extinction of many species and is in the process of driving many more to an early demise. It is a grim picture, but not a complete one.
We have caused the extinction of many species, yes, but we are also causing the formation of new ones. There are many plant and animal species that wouldn’t exist without humanity and these new species are taking on ever more prominent roles in the new global ecosystem we’ve created. In a recent paper, Joseph Bull and colleagues have identified four major forces by which humans are promoting speciation: Relocation, domestication, hunting, and the creation of new ecosystems. Some of these new species benefit us. Some do not. Nevertheless, they cannot be discounted when considering humanity’s global impact.
Humans have a particular knack for moving species to places where they’ve never lived before. As discussed in our invasive species blog, some of these introductions are intentional, while others occur unknowingly. Regardless of the way they’re introduced, relocated species often undergo rapid evolution when they find themselves in a new environment, adapting to better suit the conditions of their new habitat. This can mean trouble for native species, if an invasive newcomer is able to out-compete the locals.
Australia makes for the perfect place to study the effects of species introduction in a closed ecosystem. European colonists brought a myriad of new species to Australia, and in the 150 years of their inhabitation, many of the plant species have already changed dramatically. We tend to think of evolution as something that happens very slowly, but when conditions change rapidly, evolution must become equally swift to compensate.
Throughout the past 11,000 years, humans have domesticated 269 species of plants and 474 species of animals. Not all domesticated plants and animals can be considered new species, but some are so unlike their wild counterparts that it is undeniable. Among the 40 most important crop plants, between 6 – 8 are considered new species. These plants have significant differences in form from their wild relatives and are unable to interbreed with their undomesticated counterparts (there may be a higher number of new species from domestication if a different definition of species is used).
We have changed species in dramatic ways through domestication and many of these domesticated plants and animals are in turn changing ecosystems dramatically. Cats have been domesticated for over 5,000 years, and introduced populations of domesticated cats are ravaging native animal populations where they are found. In these situations, not only do we see domestication at play, but relocation as well. Human alteration of species can cause cascading effects that are not fully realized for generations.
By hunting animals, humans have directly inserted themselves into the process of evolution. Natural selection was the primary mechanism proposed for evolution by Charles Darwin in On the Origin of Species. Under natural selection, organisms that are better adapted to the particular pressures of their environment survive and produce offspring that carry those adaptations. Hunting adds one more pressure to survival: Attractiveness to human hunters.
Animals that have traits that make them more appealing to hunters, like large body size or impressive horns, are killed at greater numbers than ones that hunters are less interested in. This means that the smaller animals with less interesting physical features are reproducing in a larger number. Elephants have been hunted for their tusks for so long that average tusk size is decreasing in many populations, even though this makes them less attractive to their own species and less adept at digging for water.
Commercial fishing often focuses on harvesting the largest fish, and heavily fished populations now reach adulthood and reproduce at smaller sizes. Many of the adaptations spurred by hunting are unfortunately detrimental to the species as a whole, except as a way to avoid their human predators. None of these hunting-driven evolutionary changes have been so great as to produce a new species yet, but rapid evolution is certainly taking place and speciation may soon follow.
Humans have caused significant changes in natural ecosystems, but they’ve also created entirely new ones, like cities. Cities pose a whole new set of challenges and opportunities for organisms, and some animals have adapted with vigor to their new surroundings. Many of the animals that have evolved to fill an urban niche are ones that we consider pests, like pigeons and rats. One of the most striking examples of urban speciation unfortunately comes from one of nature’s most pesky creatures: The mosquito.
When Londoners hid in the subway tunnels during the bombardments of World War 2, they noticed something. They were being bitten by mosquitoes even though they were underground. Mosquitoes need water to reproduce, and water there was in plenty in their subterranean surroundings. There was plenty of food too, with a steady supply of humans making their way through. In the seventy years that the London Underground mosquitoes have been isolated, they have transformed into a new species. Their breeding habits have changed so they are no longer compatible with aboveground mosquitoes and their genetic code shows new mutations as well. Within the course of a human lifetime, a new species of mosquito has evolved to pester us.
This swift evolution in human-altered environments can be seen all over the world. A new subspecies of raccoons is developing that stays awake during the day to eat the trash that is so abundant that it can support both a day and a night shift of scavenging. There are spider species that are so adapted to living in human houses that they die if transported outside. Whitetail deer love the forest edge, and our network of roadways has given them all new space to thrive while we have simultaneously hunted their predators to scarcity.
A lot of diversity is lost when an ecosystem changes swiftly, but the elimination of previous organisms makes way for the evolution of new.
Microorganisms tend to be ignored in favor of the plants and animals we can see and touch when considering extinction and speciation. There are undoubtedly microscopic species that go extinct when a larger species they co-evolved with die out, but these relationships are poorly documented. Humans have driven microorganism evolution by providing a multitude of new hosts for them to inhabit, but also by the addition of antibiotics. Microbes are rapidly evolving in response to antibiotics, and the result of their adaptations may render them invincible to modern antibiotics within our lifetime. Microbes reproduce much faster than any plant or animal and can respond with frightening rapidity to our efforts to eradicate them.
Extinction vs Speciation
Though species well-suited to a human-dominated planet are constantly evolving, that doesn’t mean we can ignore the ongoing extinction crisis. Many species that go extinct do so because their ecological niche was eliminated by human activity. With the niche no longer available, no new species can evolve to fill it. In a healthy ecosystem, there is a complex interplay between multitudes of different species. The disruption left behind when one species falls extinct isn’t easily cured by the introduction of another. Also, the species best adapted for human inhabitation, like rats and mosquitoes, are not necessarily the ones we most want to share our world with.
Even if humans drive the formation of new species, these new species won’t be enough to ecologically, culturally, or economically replace what was lost. Do we want to live in a world where the tigers roaming the forest are replaced by wild dogs prowling the streets? Perhaps not. The best we can do now is to learn to live with the species that we’ve inadvertently or intentionally created, and try to help those that are diminishing before they’re lost forever.
- Kate Dzikiewicz, Paul Griswold Howes Fellow