The term is now used by some writers in a more restricted sense than its initial usage. The earliest and most widespread concept among biologists is that of a species that has arrived in a specific geographic area from a different region (without further caveats). This is the forerunner of the term 'non-indigenous species', although it lacks the frequently invoked basis of the word 'introduced', which means different things to different writers. In this sense, the cattle egret (Bubulcus ibis), which arrived in North America by natural range expansion, the black rat (Rattus rattus), which is believed to have arrived as a hitchhiker aboard ships, and the kudzu vine (Pueraria lobata), which was introduced deliberately by humans, are all adventive species and have established populations. Common adventive species include herbivorous insects.
The later and more limited concept is that of a species that has arrived in a specific geographic area from a different region, but its population is not self-sustaining. Population numbers are only increased through re-introduction. After some time, an adventive species may become naturalized; or, some populations do not sustain themselves reproductively, but exist because of continued influx from elsewhere. Such a non-sustaining population, or the individuals within it, are said to be adventive. Cultivated plants are a major source of adventive populations. It is estimated that 10-20% of adventive species used in biological control programs eventually become naturalized.
We can readily see how this second (later) concept applies to cultivated plants. Those that grow within the confines of culture are ‘adventive’; those that grow outside those confines are ‘naturalized’. But the concept falls apart when applied to the far more numerous species of invertebrate animals and microorganisms: extremely few of these are cultured, and by the time they are detected in nature they tend to be established (‘naturalized’).