Range expansion may occur when:
- existing habitat that has not previously been colonised is reached
- appropriate conditions develop outside a species’ distribution, and the species colonises this new habitat
- appropriate habitat develops within a species’ range but in areas that were previously not occupied, and the species colonises this habitat.
More controversially, changes in the distribution of a species’ abundance (e.g. where the abundance of a species changes in different parts of its distribution in response to changes in conditions) might also be included here. In many instances, such changes in the distribution of a species’ abundance is the precursor to another form of range expansion.
Australian flying foxes (Pteropodidae, Pteropus spp.) are highly mobile species that exhibit all these forms of range expansion. Range expansions have occurred in several species. From the late 1800s to 2007, the black flying fox (P. alecto) expanded its southern range boundary polewards by 123 kilometres per decade, on average (Roberts et al. 2012). More recently, in the south of its range, the vulnerable grey-headed flying fox (P. poliocephalus) has established a permanently occupied breeding camp in Adelaide, some 500 kilometres from the nearest camp. In the north, newly established and permanently occupied breeding camps at Finch Hatton and Ingham are roughly 500 and 900 kilometres, respectively, outside the range boundaries. In each case, the camp is separated from the existing range, but is largely in an appropriate habitat.
Apparent expansion into adjacent but previously unoccupied habitat has been seen in the grey-headed flying fox, with apparently new camps established outside the species’ range on the Western Plains and in previously unoccupied habitat within the species range (e.g. Canberra and Tumut) during the past decade. However, examination of historical records indicates that the species was present in these areas (e.g. Wellington, Goulburn) in the 1800s, suggesting that range boundaries are highly dynamic across long timeframes as conditions vary.