[求助]请NN解释一下在阅读里看到的一个很高级的语法现象

frankzhifei

 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.
            

It can be inferred from the passage that the occasional failure of some salmon to return to their natal streams in order to spawn provides a mechanism by which  an environmentally degraded stream could be recolonized by new salmon populations should the stream recover

上面的句子是下面的定位，但两句中都有should,不好理解这是什么语法现象，感觉是虚拟。

salvador

should here means if.