Some geologists assert that, as the island moved north due to the effects of continental drift, it submerged at various intervals, extinguishing its terrestrial flora and fauna. Botanists counter that some areas must have remained above sea-level, serving as refugia. Many members of the late Cretaceous – early Tertiary Gondwanan flora survived in New Caledonia's equable climate but were eliminated in Australia due to increasingly dry conditions. The isolation of New Caledonia was not absolute, given the rise and fall of sea level caused by the ebb and flow of ice ages. Land bridges or islands formed between New Caledonia and its neighbours, the Solomon Islands, Vanuatu, and Australia. Thus new species came to New Caledonia while "Gondwanan" species were able to penetrate the Pacific Islands region. Plants have limited "seed dispersal" mobility away from the parent plant and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors.