Final answer:
An invasive species is characterized by its rapid reproduction in a new environment without natural predators, leading to significant ecological impact.
Step-by-step explanation:
The characteristic that defines an invasive species is their ability to reproduce rapidly in a new environment where they have been introduced, often by human actions. Unlike species that remain in their original environment, invasive species typically do not have natural predators and competitors in the new habitat, which allows them to out-compete native species for resources.
This reproductive advantage can lead to a dramatic population increase and can upset the ecological balance, making them a significant threat to biodiversity. This rapid reproduction can lead to a high number of individuals and eventually result in a threat to native species through competition for resources, predation, or disease.