今天在阅读一篇文章，文章中有一句是这样的：

It is nearly half a  century since biologists first asked Congress to re-introduce wolves into Yellowstone, so that they might usefully eat some of the elk then lumbering about in over-large herds.

翻译是这样的：近半个世纪前，生物学家第一次向国会提出重新在黄石引入狼群以便吃掉过度繁殖的麋鹿。

主要搞不懂 then 在这个句子中起什么作用或者是什么意思。