GWD—13的一篇研究毛虫数量变化周期 中的一句话~

kevinxdx

For many years,viral disease had been reported in declining  populations of caterpillars,but population ecologists had usually considered viral disease to have contributed to the decline once it was underway rather than to have intiated it.
请问这里的once是什么意思呢？最好说一下once具体用法

kevinxdx

顶！！！