The Princeton Guide to Ecology

Biological invasions have been a feature of global ecology since the origin of life: plants and animals invaded the land from the sea, and chance dispersal events have occasionally allowed species to invade new continents, islands, or bodies of water. The current wave of biological invasions is qualitatively different from these prehistoric invasions because it is mediated by human activities. It is also quantitatively different because the frequency of invasions is orders of magnitude higher than background levels.

alien invasive species. An alien species that becomes established in an ecosystem and threatens native biological diversity or has other negative ecological and economic impacts.

alien (equivalently: nonnative, nonindigenous, foreign, exotic) species. A species, subspecies, or lower taxon occurring outside its natural range (past or present) and dispersal potential (i.e., outside the range it occupies naturally or could occupy without direct or indirect introduction or care by humans); includes any part, gamete, or propagule of such species that might survive and subsequently reproduce.

ecosystem services. The conditions and processes through which ecosystems, and the species that make them up, sustain and fulfill human life.

introduction. The movement, by human agency, of a species, subspecies, or lower taxon (including any part, gamete, or propagule that might survive and subsequently reproduce) outside its natural range (past or present). This movement can be either within a country or between countries.

Before mass migrations of humans across the globe, natural dispersal of plants and animals was restricted by geographic barriers such as oceans, mountain ranges, and deserts. These barriers to migration have been lowered by human activity. The current wave of biological invasions began at the end of the Quaternary glacial period as humans began to disperse across the globe. The process began in earnest with the age of exploration in the fifteenth century and subsequent European colonization of the New World. Biological invasions accelerated rapidly in the twentieth century with the advent of international shipping and aviation, the construction of highways, and the destruction of large swathes of natural habitat. Today, virtually every location on the planet, from mountain peaks to remote oceanic islands, has recently been invaded by species that originated elsewhere. Invasive species occur in all taxonomic groups, from mammals to fungi to viruses.

Charles Darwin, on his Beagle voyage, was perhaps the first scientist to observe and note the process of biological invasion. Many of these observations led to insights that were incorporated into The Origin of Species. Charles Elton’s seminal book, The Ecology of Invasions by Animals and Plants, was published in 1958 and is considered the classic text of invasion biology. The field of invasion biology burgeoned in the latter decades of the twentieth century and is now the subject of numerous books and several specialized journals.

In this review, I first examine which species invade which habitats and address how and why these

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