Are Humans an Invasive Species?

Martha Hoffman

As humans colonized the world they carried with them various organisms.  The reasons for shipping these organisms varied from the need for plants for agricultural use, to animals for food supply, to animals for companionship or labor.  Many of the species, however, made the journey without the knowledge of their human transporters.  For instance, germs and animal pests frequently stowed-away on board either ships or humans.  Upon arriving in the new land, these introduced organisms came up against the local ecosystem.  Some were unable to survive, others achieved a balance with the surrounding environment, while others flourished and spread.  This last group is made up of invasive species, which are non-native organisms that thrive in new regions and sap important resources from native life forms, and alter the ecosystem.  Some well known examples of invasive species in North America are rats and ferns.  These two species have done well because of a lack of predators and aggressive root systems, respectively.  It seems to me that while invasive species generally refers to organisms that have been transported to new worlds by humans, humans themselves ought to be classified as invasive species.  People, like many invasive organisms, have no real limit on growth (other than using up too many resources), and they substantially alter ecosystems that they inhabit.

            Currently, ferns are a problem in forests all over the Eastern United States.  They have begun to take over forests floors due to the overpopulation of deer.  As humans have spread, they have pushed away many of the carnivores that naturally hunt deer.  Deer, therefore, have been able to spread unhindered other than lack of space or food.  Besides annoying farmers, deer have substantially changed the ecosystems.  They graze on plants between a foot and five feet off the ground.  Therefore shrubs and bushes have been substantially depleted while trees and undergrowth (such as ferns) have thrived (American Fern Society).  Ferns, however, have posed an additional threat.  The roots of ferns grow by rhizomes which are a system of roots that extend from a horizontal stalk.  This method is highly aggressive and makes it difficult for other plants, including seedlings, to grow.   Also, ferns have spores which spread easily (  While ferns are not annoying to humans specifically, they do alter ecosystems and cause problems for other organisms. 

            Rats, on the other hand, are a huge annoyance to humans.  In particular, the black or European rat, “rattus rattus” is quite common, as is the Norway rat “rattus norvegicus” which comes from Northern Asia.  (Laboratory Rats) They were introduced at the beginning of the 17th century, along with several other types of rats, and quickly spread through American cities as they had in Europe before.  Rats carry diseases, eat food, and generally make life unpleasant for people.  They, like deer, lack predators here and, also like deer, have access to a large amount of food.  It has therefore been rather easy for them to flourish. 

            Germs have not typically been considered to be invasive species, however, I believe that they are.  Like rats they make life more difficult for humans, and when first introduced to a population germs can have drastic effects.  For instance, when Cortes landed in Mexico in 1519, his men carried smallpox with them (Diamond).  This quickly propagated through the population of Aztecs and killed almost half of them.  This entirely changed the ecosystem as Cortes’s army, and the Aztecs experienced it.  The natives were devastated, and the Spaniards quickly conquered them.  Another example of germs significantly changing the landscape of a population occurred in 1846 on an island, Faeroes.  A man from Denmark arrived who was infected with measles which quickly spread through the island and killed many of its’ inhabitants.  In both of these cases simple exposure to a new disease changes the human ecosystem drastically.  There are many more examples of this occurring such as Rome in 165, or the Plague in 1346, both due to expanded trade routes. 

            It is important to note that germs, plants, and animals have all been transported due to human activity.  Therefore, the fundamental mechanism in the development of invasive species is human involvement.  Without humans spreading out and traveling throughout the world, ecosystems would have remained the same, or gradually switched between different life forms and ways of doing things.  It would appear, then, that humans are the ultimate invasive species.  We, more than any other creature, influence change in the environment.  In addition to introducing novel species, we have other impacts as well.  Global warming is thought to be caused by us, but let us look at a smaller scale example.  In Australia, in 50,000bc humans arrived and changed the ecosystem drastically enough that they killed off a large percentage of large animal species. (Joyce)  Once upon a time there were giant kangaroos, giant birds, and giant sloths.  However, when humans got to Australia they hunted them, burned their habitat, and altered the environmental balance, shifting plant populations.  The majority of the megafauna in Australia died off very quickly.  Clearly humans are the most destructively invasive species of all.  We alter ecosystems through a variety of forces, and we are constantly taking over new lands and changing both the cultural and environmental landscape. 


American Fern Society.  25 Aug. 2005. The American Fern Society, Stephen McDaniel. 8 May 2006.

Diamond, Jared, "Ch. 11: Lethal gift of livestock," in "Guns, Germs, and Steel" W.W. Norton & Co, 1997, pp. 195-214.

Laboratory Rats. 2005. Canadian Council on Animal Care. 8 May 2006.

Joyce, Christopher.  Interview with Gifford Miller.  “Megafauna Extinctin” Morning Edition. National Public Radio. 8 Jan. 1999. 8 May 2006.

Return to ENVS2 homepage

Send message to Swarthmore College Environmental Studies

last updated 5/13/06