Issues In Ecology Number 5 Spring 2000 Flgure 2-Many invaders occupy new ranges at an accelerating rate with pro nounced"lag"and"log"phases of pro 80 liferation and spread. This initial s rate of range occupation may be indis tinguishable from the rate of spread 600 displayed by non-invasive(but neverthe less non-indigenous)species in a new 400 range.thus hamperina the early identi fication of future invaders.Terrestrial most co monly illus 200 trate this pattern (.g.the spreado Opuntia aurantiaca in South Africa) (Moran and Zimmerman 1991 and sources [numbers 1-9]therein).By 1900 1930 1960 1990 contrast.invaders in other taxonomio Year groups may show no lag in range ex. pansion and rapidly occupy new range upon entry. ber of arrivals.But aiven the number of species spotted Hemisphere and elsewhere.The manifests from colum bus'second and subsequent voyages,for instance,indi Gib8gfleregardedaspoten commerce has growr sit or soon after arrival,immigrants occasionally survive meteorically since then,providing an opportunity for a to reproduce.Even then,their descendants may survive corresponding growth in biotic invasions.As a result for only a few generations before going extinct locally. these biotic invasions can be viewed as predominantly Again,however,some small fraction of these immigrant post-Columbian events.Put in perspective,the human-driven species do persist and become naturalized.At that point movement of organisms over the past 200 to 500 years their persist ence does not depend on recuring.frequen deliberate and accidental,undou y dwarfs in scope fre re-immigration from the native range.although a greate quency and impact the movement of organisms by natura number and frequency of new arrivals do raise the prob- forces in any 500-year period in the earth's history. ability that a species will establish permanently. The proportion of various types of organisms that Among the naturalized species that persist after have invaded as a result of accidental versus deliberate this extremely severe reductive process,a few will go movement clearly varies among taxonomic groups. to become invaders.An analogy yis often made be parasite all other biotic】 nva ed.Delibera hial int sions becaus emany importan t factors in disease epidem avenstead most commonly involed ology have direct parallels in the study of invasions.Be mentation or mutualists,such as mycorrhizal fung low we explore the epidemiology and underlying mecha- that form symbiotic relationships with the roots of nisms,which allow some species to become invaders. most plants Amona insects.some deliberate introductions have Humans as Dispersal Agents of Potential Invaders have erateertmr and de New Zealand. rity of invasive insects plant,animal,and microbial immigrations worldwide Introductions of marine invertebrates probably mir roughly tracks the rise in human transport and commerce ror insects.A few species have been deliberately in. Beginning around 1500,Europeans transported Old troduced,such as the Pacific oyster imported from World species to their new settlements in the Western Japan to washinaton state.but a arowing number of3 Issues in Ecology Number 5 Spring 2000 ber of arrivals. But, given the number of species spotted only once far beyond their native range, local extinction of immigrants soon after their arrival must be enormous. Despite such wholesale destruction either in tran￾sit or soon after arrival, immigrants occasionally survive to reproduce. Even then, their descendants may survive for only a few generations before going extinct locally. Again, however, some small fraction of these immigrant species do persist and become naturalized. At that point, their persistence does not depend on recurring, frequent re-immigration from the native range, although a greater number and frequency of new arrivals do raise the prob￾ability that a species will establish permanently. Among the naturalized species that persist after this extremely severe reductive process, a few will go on to become invaders. An analogy is often made between epidemics caused by parasites and all other biotic inva￾sions because many important factors in disease epidemi￾ology have direct parallels in the study of invasions. Be￾low we explore the epidemiology and underlying mecha￾nisms, which allow some species to become invaders. Humans as Dispersal Agents of Potential Invaders Humans have served as both accidental and de￾liberate dispersal agents for millennia, and the increase in plant, animal, and microbial immigrations worldwide roughly tracks the rise in human transport and commerce. Beginning around 1500, Europeans transported Old World species to their new settlements in the Western Hemisphere and elsewhere. The manifests from Colum￾bus’ second and subsequent voyages, for instance, indi￾cate deliberate transport of species regarded as poten￾tial crops and livestock. Global commerce has grown meteorically since then, providing an opportunity for a corresponding growth in biotic invasions. As a result, these biotic invasions can be viewed as predominantly post-Columbian events. Put in perspective, the human-driven movement of organisms over the past 200 to 500 years, deliberate and accidental, undoubtedly dwarfs in scope, fre￾quency and impact the movement of organisms by natural forces in any 500-year period in the earth’s history. The proportion of various types of organisms that have invaded as a result of accidental versus deliberate movement clearly varies among taxonomic groups. • Few, if any, invasive microorganisms have been delib￾erately introduced. Deliberate microbial introductions have instead most commonly involved yeasts for fer￾mentation or mutualists, such as mycorrhizal fungi that form symbiotic relationships with the roots of most plants. • Among insects, some deliberate introductions have had adverse consequences, including bumblebees in New Zealand. But the majority of invasive insects have probably been accidentally introduced. • Introductions of marine invertebrates probably mir￾ror insects. A few species have been deliberately in￾troduced, such as the Pacific oyster imported from Japan to Washington state, but a growing number of Figure 2 Figure 2 - Many invaders occupy new ranges at an accelerating rate with pro￾nounced “lag” and “log” phases of pro￾liferation and spread. This initial slow rate of range occupation may be indis￾tinguishable from the rate of spread displayed by non-invasive (but neverthe￾less non-indigenous) species in a new range, thus hampering the early identi￾fication of future invaders. Terrestrial plant invasions most commonly illus￾trate this pattern (e.g. the spread of Opuntia aurantiaca in South Africa) (Moran and Zimmerman 1991 and sources [numbers 1-9] therein). By contrast, invaders in other taxonomic groups may show no lag in range ex￾pansion and rapidly occupy new range upon entry