The Continually Rewritten Biogeographical Map of Earth: The Spread of Non-native Species

On a contemporary Earth strongly shaped by human activity, the geographical distribution of life is changing at a pace rarely seen in the planet's history. Through human travel, trade, and environmental modification, numerous organisms are transported beyond their original natural boundaries and introduced into new regions where they may establish populations. Such organisms are known as non-native species. Their expansion in newly colonized environments has become an increasingly important subject in modern ecology and evolutionary biology. Biological invasions can reshape patterns of biodiversity, alter the functioning of ecosystems, modify interactions among species, and influence the relationship between human societies and natural resources.

When a species is transported beyond its original range into a new region, the establishment of a stable population is far from guaranteed. In most cases, the invasion process unfolds through a series of stages. The first involves transportation across geographical barriers and introduction into a new environment. The next stage concerns whether the species can survive and reproduce locally. The final stage involves expansion of the population into a wider area. Only when a species successfully establishes itself and begins to spread outward does the critical phase of biological invasion begin. This spread can be defined as the spatial expansion of a species within or around the area of introduction, where populations gradually move outward from the initial point of arrival.

Traditionally, researchers have often described this spread using the metaphor of ripples formed when a stone falls into water, with the species dispersing outward from a central origin in all directions. Real ecosystems, however, are far more complex than this simplified picture suggests. The speed and pattern of expansion of non-native species may vary greatly across regions and time. For example, the same species may expand at dramatically different rates on different continents. The house sparrow (Passer domesticus) has spread across Australia at rates exceeding 100 kilometres per year, while its expansion averages about 17 kilometres per year in North America and around 28 kilometres per year in Europe. Such differences reflect the influence of environmental conditions, population structure, introduction history, and human activities in each region.

From the perspective of ecological mechanisms, species spread involves more than simple movement. It is the combination of movement and successful population establishment. In biological terminology, the first process is often called dispersal, while the second involves the survival, growth, and reproduction of individuals in a new location. Spread only truly occurs when dispersing individuals successfully reproduce and form new populations. Many species may be transported to new regions, but if they fail to adapt to local conditions or cannot reproduce successfully, their dispersal will not translate into an actual expansion of their distribution.

The invasion process of non-native species（圖片來源：Haubrock PJ et al. (2025)，採用 CC BY 4.0 授權）

The spread of non-native species often follows a three-phase dynamic. Initially there is typically a slow lag phase, during which populations remain small and expansion proceeds slowly. As populations grow and become better adapted to local conditions, the rate of spread may accelerate rapidly, entering a phase of fast expansion. Eventually, when most suitable habitats have been occupied, the rate of expansion slows again as the invasion approaches a state of saturation. Although this three-phase model represents a theoretical generalization, similar patterns have been observed in many invasion events.

The mechanisms of species spread can broadly be divided into natural dispersal and human-mediated dispersal. Natural dispersal includes the species' own movement abilities, such as flying, swimming, or crawling, as well as passive transport by wind, water currents, or other organisms. Seeds, for example, may be carried by wind to new locations, while fish eggs may survive digestion after being consumed by waterbirds and later be expelled elsewhere (Lovas-Kiss Á et al. 2020). Some plants may attach to the fur or feathers of animals and be transported over long distances. These natural processes generally produce short- or medium-distance dispersal, although rare long-distance events across oceans or continents can sometimes occur.

Human activity, however, has become one of the most powerful drivers of species spread in the modern world. Global trade, transportation networks, and agricultural practices frequently transport organisms unintentionally to new regions. Ships' ballast water can carry large numbers of marine organisms, cargo and packaging materials may harbor insects or plant seeds, and horticulture and the pet trade often serve as sources of introduced species. In addition, human-built infrastructure such as canals, highways, and railways can create new pathways for biological movement, reconnecting ecosystems that were previously separated by geographical barriers.

Dispersal, colonization, gene flow, and genetic drift generated during the invasion process（圖片來源：Haubrock PJ et al. (2025)，採用 CC BY 4.0 授權）

Human-driven secondary introductions also contribute to the expansion of invasive species. Once a species has established a population in a new region, humans may inadvertently transport it again to other locations. These secondary introductions can create new population centres in different places, transforming what was originally a single invasion point into multiple sources of spread. As a result, previously unoccupied areas may experience sudden jump dispersal, producing a complex pattern in which several invasion fronts advance simultaneously.

Human activity can also increase the probability that introduced species successfully establish populations through a mechanism known as propagule pressure. When a species is repeatedly introduced into the same region, new individuals continually join the existing population, allowing population size to grow rapidly. A large number of introduced individuals reduces the risk of population extinction and increases genetic diversity, making it easier for the population to adapt to new environmental conditions. The mixing of individuals from different sources may also generate new genetic combinations, further enhancing adaptability and invasion potential.

Within the context of globalization, human activities have greatly increased the frequency of long-distance dispersal events. Many species that once relied only on wind, water currents, or animal carriers for short-distance dispersal can now travel hundreds or even thousands of kilometres within a short time through modern transportation networks. Airplanes, cargo ships, and freight trains enable frequent biological exchange between continents, accelerating dispersal far beyond the rates achievable through natural processes.

Human activity cannot simply be halted in the name of protecting natural ecosystems. Yet this does not mean that the process of species spread is beyond our influence. As global transportation, trade, and land use continue to reshape the movement of organisms, the central challenge lies in understanding these mechanisms and identifying potential risks before they occur. Only by clearly understanding the pathways, intensity, and ecological conditions associated with human-mediated species spread can we reduce the impacts of biological invasions without halting the functioning of human societies, allowing a more stable and enduring balance to develop between human activity and the natural world.

Note: Propagule pressure refers to the combined effect of the number of individuals introduced and the frequency of introduction events. For example, releasing twenty fish once results in lower propagule pressure than releasing ten fish on five separate occasions.

Author: Shui-Ye You

Reference:

1. Haubrock PJ et al. (2025). The spread of non-native species. Biol. Rev.

2. Lovas-Kiss Á et al. (2020). Experimental evidence of dispersal of invasive cyprinid eggs inside migratory waterfowl. PNAS.