请教一个句子结构

specialcognac

请教一个句子结构
GWD中的一篇阅读，三文鱼回流的问题

Low levels of straying are crucial, since the process provides a source of novel genes and a mechanism  by which a location can be repopulated should the fish there disappear

大的结构没有问题，Low levels of straying are crucial, since the process provides A and B
但是红色的部分不太明白，a mechanism by which a location can be repopulated should the fish there disappear 请问怎么来划分这句从句？

望NN指点，谢谢了！

specialcognac

自己顶一下