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.

这句话是什么意思啊？

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。

较低程度上鱼的离群至关重要，因为这个过程提供了新的基因来源和一种机制，通过这种机制，那些鱼群消失的地方能够得以重新繁荣起来。从意思上来讲（语法上略有区别），should=if，不过在这里多少有点even得味道。

问这句话的XDJM还真不少！

虚拟语气:如果那里的鱼消失

A: 如果那里的河流恢复

有时if引导的状语从句可以省略 if，而把从句中的动词 were, had 或 should 移到主语前面

这句话的意思是：描述环境退化如何能造成鲑鱼群的更甚于（或超越）纯数量上的减少的变化。