Austral Ecology (2004) 29 , 530–536 An Essay on Some Topics Concerning Invasive Species JAMES H. BROWN 1 * AND DOV F. SAX 2 1 Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA (Email: [email protected]) and 2 Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California, USA Abstract There is a tendency for both scientists and lay people to regard invading alien species as inherently ‘bad’and native species as inherently ‘good.’ Past invasions occurred commonly without human assistance. They rarelycaused large, lasting decreases in species richness or ecological damage. Current invasions provide opportunities forscientific study. They are unintentional, uncontrolled experiments, which can provide insights into attributes ofsuccessful colonists, relationships with native species, and impacts on the structure and function of ecologicalsystems. Key words: Biogeography, biological invasions, colonization, exotic species, invasive species, species richness. INTRODUCTION One of the senior author’s fondest memories is a drive withMarilyn and Barry Fox and Astrid Brown in 1990through Myall Lakes National Park in eastern New SouthWales, Australia. Barry was driving and Marilyn wassitting in the front passenger seat. Together they werepointing out objects of natural history interest. Periodicallywe would stop and get out to examine more closely some-thing special: a rare plant, bandicoot diggings, or an areathat had burned some known time ago. More frequently,however, we stopped so that Marilyn could wage war on anexotic plant. Seedlings and small saplings of Pinus radiata ,a tree native to North America, were growing along theroadway. Each time Marilyn spotted one, she wouldcommand Barry to pull over, charge out of the car, pull upthe offender by the roots, knock off the soil, and toss theremains into the road. At one point the requests to stopbecame so frequent – several times within 100 metres – thatBarry gently questioned the practicality of the exercise.Marilyn replied, ‘But Barry, it’s terribly invasive, andbesides, this is a National Park! This little vignette illustrates some characteristics ofexotic species. Not only were hundreds of plant andanimal species intentionally imported into distant partsof the world during the last few centuries, but thou-sands of additional species have colonized and spreaddespite efforts to prevent accidental introductions.Some regions have been much more severely invadedthan others. These include not only oceanic islands butalso the temperate habitats of the southern continents:Australia, southern Africa, and South America.Human-assisted invasions of alien species can beviewed as unintentional, uncontrolled experiments. Onone hand, they are the bane of agriculturists, conserv-ation biologists, and natural resource managers,because they sometimes cause enormous economiclosses and substantial short-term ecological changes.On the other hand, they should be viewed in thecontext of ‘natural’ colonization events that haveoccurred without human intervention throughout theEarth’s history. Studying the process and consequencesof the human-caused invasions can yield valuableinsights into the ecological and evolutionary processesthat generate and maintain biodiversity.There is no denying that the foreign invaders oftenelicit a visceral emotional response. There seems to besomething deep in our biological nature, related toxenophobia toward other humans, that colours ourview of alien plants and animals. There is a tendency totreat foreigners differently from natives: with distrust,dislike, even loathing. Coupled with this is a tendencyto view some prior condition as ‘pristine’ or mostnatural, and therefore the state that should be pre-served. Perhaps these are the reasons why people wouldadvocate extermination of a species just because it wasoriginally imported from some other place. This is theview, not only of lay people, but also of naturalists,including academically trained ecologists, evolutionarybiologists, and biogeographers. Recently in the UnitedStates, the Union of Concerned Scientists (2003)claimed that ‘the accelerating introduction and spreadof invasive species is among the most serious of threatsto global biodiversity’. What is there about exoticspecies that draws such strong responses from boththe environmentalist and scientific communities? Onone hand, we know that long periods of geographicalisolation have allowed the evolution of divergentendemic biotas on oceanic archipelagos such as Hawaiiand the Galapagos, island continents such as Australia,Madagascar, and New Caledonia, and other regions *Corresponding author.Accepted for publication August 2003.AN ESSAY ON INVASIVE SPECIES 531 such as southern South America and southern Africa.These biotas have been severely impacted by humanactivities. Many native endemic species have becomeextinct or are seriously endangered, and many alienspecies have become established. A cause–effectrelationship between these two phenomena has oftenbeen inferred, even though it is difficult to isolate theeffects of invasive species from other human impacts.Conversely we scientists know that the earth and itsbiota have always been very dynamic in space and time.The fossil record shows many episodes of massiveinvasions and extinctions as asteroids impacted, landmasses collided and drifted apart, seaways opened andclosed, sea levels rose and fell, and climates changed.The earth is currently experiencing yet another episodeof drastic change in environment and biodiversity. Thistime, however, instead of being triggered by asteroidimpacts, tectonic events, or glacial–interglacial cycles,the changes are being caused by the enormous impactsof our own invasive species. Modern humans arealtering climate, transforming habitats, connectingpreviously isolated lands or waters, exterminatingspecies, and transporting organisms. The rates andmagnitudes of these changes are among the largest thatthe earth has ever experienced. It should come as nosurprise, then, that there have been equally largeimpacts on the abundances, distributions and diver-sities of organisms.The purpose of the present essay is not to argue thatexotic species are ‘good’ so that their spread should befostered. It is not to suggest that modern humansshould let nature take its course and elect not tointervene in the dynamics of dispersal and