Complex sentence: New Zealand's flora and fauna evolved for 80 million years with no browsing or predatory mammals at all, so they have developed no natural defence mechanisms against introduced animals like possums, rats, mustelids and deer.
Split into simpler sentences: New Zealand's flora and fauna evolved for 80 million years with no browsing or predatory mammals. Consequently, the native birds, insects, and flora have developed no natural defence mechanisms against introduced animals like possums, rats, mustelids and deer.