Imperialism is often understood through military conquest and political domination. However, the success of European colonial expansion involved a far more subtle and powerful force. The movement of people was inextricably linked to the movement of entire ecological systems, which provided colonists with powerful, invisible allies. This biological transfer helped European settlers not only survive but fundamentally reshape distant lands into versions of their homelands. The ease with which Europeans established lasting dominion in certain temperate zones was an ecological achievement, demonstrating that colonization was rooted as much in biology as in human ambition.
Defining the Concept and its Origin
The concept explaining this process is ecological imperialism, which describes the successful colonization of temperate regions by Europeans due to the simultaneous transfer of their associated plants, animals, and pathogens. Environmental historian Alfred Crosby formally introduced this theory in his 1986 book, Ecological Imperialism: The Biological Expansion of Europe, 900–1900. Crosby argued that the biological superiority of the European “portmanteau biota”—the collection of organisms carried by the colonists—gave them a decisive advantage over native ecosystems.
His work built upon his earlier study, The Columbian Exchange, detailing the intercontinental transfer of organisms following 1492. Ecological imperialism focused on why European populations thrived specifically in geographically similar temperate areas, such as North America, Australia, and New Zealand. These regions, known as the “Neo-Europes,” allowed European species to flourish and displace native life, enabling the establishment of European-style agriculture and society.
The Biological Agents of Transformation
The transformation of native landscapes was accomplished by a suite of organisms that worked together to prepare the land for European settlement. This portmanteau biota was composed of three main categories that facilitated the ecological takeover.
Flora
The first category included European crops and tenacious weeds that traveled with them, such as clover, dandelion, and common plantain. Native peoples often called the plantain “Englishman’s Foot.” These weeds excelled at rapid dispersal and outcompeted native grasses, while European crops like wheat, barley, and oats created familiar, productive agricultural fields.
Fauna
The second category involved domesticated livestock and associated pests that fundamentally altered the landscape. Animals such as cattle, pigs, sheep, and horses were introduced, often running feral and establishing massive populations. These Old World grazers quickly destroyed native vegetation through intensive feeding and trampling, allowing European grasses and weeds to take hold. Unintentional passengers, including black and brown rats, arrived on ships, becoming successful invasive pests that preyed on native bird populations.
Pathogens
The third and most devastating agents were the pathogens, or microbes, that caused “virgin soil epidemics” among indigenous populations. Diseases like smallpox, measles, and influenza were lethal to isolated native peoples, who lacked immunity. The resulting demographic collapse reduced the ability of native societies to resist colonization, opening the temperate regions for European takeover.
Case Studies of Environmental Takeover
New Zealand stands as a definitive example of ecological imperialism, where the unique, isolated ecology was rapidly altered by European introductions. Prior to contact, the islands lacked native terrestrial mammals, allowing bird species to evolve without predatory pressures. Settlers intentionally introduced massive numbers of sheep and cattle to convert the native forest into productive grassland. Imported grasses, better suited to intensive grazing than native flora, quickly became dominant.
The disruption was compounded by the arrival of unintentional and intentional invasive species. Stoats and ship rats, accidental hitchhikers and later deliberate introductions, decimated the naive native bird populations, many of which were flightless. Within decades, several endemic bird species, such as the huia and the laughing owl, faced extinction. The ecosystem was re-engineered to sustain a European-style economy based on pastoral farming.
Australia provides another clear case, particularly in its temperate southeastern and southwestern corners. The introduction of European sheep and cattle, ill-suited to the continent’s arid conditions, caused significant soil degradation and erosion. Settlers also brought European-style agriculture, leading to environmental impact in dry regions. Rabbits introduced for sport quickly exploded in population, becoming a destructive invasive species that competed directly with native marsupials for forage.
Enduring Consequences for Global Ecosystems
The historical process of ecological imperialism established a precedent for the global spread of invasive species, a legacy that continues to shape modern environmental challenges. The “portmanteau biota” of the colonial era set the stage for the current crisis of biological invasions, now recognized as one of the five main drivers of global biodiversity loss. Invasive non-native species compete with native species for resources, introduce new diseases, and alter habitats.
Invasive species are a major contributing factor in an estimated 40 percent of all endangered species listings worldwide. The historical movement of organisms has led to an ecological homogenization, replacing once-unique regional biotas with a more uniform, often European-derived, set of species. This pattern influences contemporary conservation efforts, which must focus substantial resources on managing and eradicating non-native species. Furthermore, the agricultural practices established during the imperial era continue to dominate global land use and resource distribution.