adventive

adj.not native and not fully established; locally or temporarily naturalized

"an adventive weed"

1. The adventive plant species began to spread rapidly in the new environment.
   这种外来植物开始在新环境中迅速传播。
2. Scientists are studying the impact of adventive insects on local ecosystems.
   科学家正在研究外来昆虫对当地生态系统的影响。