Research has shown that domestic cats and their feral counterparts are capable of huge impacts to bird populations—one house cat and its offspring were purportedly all it took to push a bird that lived only on Stephens Island in New Zealand into extinction.
这句话中破折号后面的一句，one house cat and its offspring were purportedly all 的意思是呢？it took to ...在句子干什么成分？谢谢。
Research has shown that domestic cats and their feral counterparts are capable of huge impacts to bird populations—one house cat and its offspring were purportedly all that it took to push a bird that lived only on Stephens Island in New Zealand into extinction
主句主语：research
主句谓语：has shown
主句宾语：that domestic cats and their feral counterparts are capable of huge impacts to bird populations
同位语：one house cat and its offspring were purportedly all （省略关系代词that） it took to push a bird that lived only on Stephens Island in New Zealand into extinction
it took to ...为定语从句修饰前面先行词all
