GWD 1 中一个句子的理解

jz5n3

GWD第一套题的第二个阅读题中有一句话看得有点迷糊，望高手指点
Low level 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.
这里“should the fish there disappear"修饰哪个主语？这个句子要怎样理解？
谢谢