Background
The concept of mainland islands was pioneered in New Zealand and arose mainly from the particular circumstances of that country's history. For millions of years New Zealand was part of the supercontinent Gondwanaland, which included Australia, Africa, and South America, and shared the same flora and fauna. About 70 million years ago New Zealand became separated, earlier than Australia, South America and Antarctica. About five million years later dinosaurs became globally extinct leaving the way open to mammals to dominate - except in New Zealand where there were no land mammals (only 3 species of bats and seals). In the absence of mammals, birds became dominant. Evolutionary processes resulted in a unique assemblage of plants and animals, and New Zealand became a land dominated by birds. Without competition from browsing mammals, birds evolved to occupy niches that mammals occupied elsewhere. Threatened by few predators many birds had no need to fly and many species became flightless. Birds, reptiles, plants, insects, and bats, all evolved in the absence of mammals, and have little defence against alien species.
With human colonisation came many accidental or deliberate introductions of mammals and birds. These wrought havoc with native species and many became extinct, many others were reduced in range and number, with some teetering on extinction. Traditionally pacific rats (Rattus exulans), Norway rats (Rattus norveigucus), ship rats (Rattus rattus) cats, ferrets, stoats, and weasels were all considered to be the main culprits in the decline of native species of New Zealand birds, reptiles and insects. More recent information adds hedgehogs and mice to the list. These species have been introduced for a variety of reasons and some inadvertently. The effect remains the same: they have all contributed to the decline of native animals. Possums and deer did the same for the forest.
However, New Zealand also includes many offshore islands, some of which contained species rare or extinct on the mainland because introduced pests could not reach them.
Read more about this topic: Ecological Island
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