The researchers also studied the process by which the aggressive species establish new colonies.

这里 which 引导的是定语从句 ,which 指代 the process ,which 作介词by 的宾语,在定语从句中,介词后的关系词只能 用 whom / which .\
又如,He bought a telescope through which he could study the sky .为什么不是 by the aggressive species which establish new colonies？
什么时候which可以放在先行词前面啊？