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"