Bronze winged pigeon Jeremy RingmaGeographic range size and extinction risk

Geographic range size (the size of a species' distribution) is often treated as a fixed attribute of a species for the purposes of calculating extinction risk. All else being equal, species occupying smaller geographic ranges are assumed to have a higher risk of extinction. However many species move around the landscape. Sometimes their movements involve relatively predictable to-and-fro migrations (migratory species). But sometimes they involve complex irregular movements, with these species often being referred to as nomads (nomadic species).

We recently modelled the distributions of many Australian nomadic bird species and found that at certain times their range contracts to a very small area making them much more vulnerable than had been previously realized (Runge et al, 2015). This has important implications for how we calculate their risk of extinction.

Nomads move in complex patterns, often associated with highly fluctuating resources such as seasonal fruiting or irregular desert rainfall. Movement strategies may be adjusted dynamically according to the prevailing conditions at each time and place.

For many of these species we have only a rudimentary understanding of where they spend their time, when and why those places are important, and what drives them to move around the landscape. These movements can lead to substantial temporary expansion and contraction of geographic ranges. Sometimes the size of the contraction can potentially pose an extinction risk. This is of particular concern here in Australia, where almost half of our bird species are migratory or nomadic.

Nomadic movements limit our ability to determine population dynamics and consequently our ability to estimate risk on that basis. Many migratory species can be surveyed annually because of predictable movements to and from breeding grounds, which allows reasonably accurate measurement of population change and extinction risk. However, for nomadic species when and where we monitor may dramatically influence our estimates of both population abundance and trend.

In our investigation we used a species distribution modelling approach to predict the distribution of 43 Australian nomadic bird species. By combining existing data from citizen-science projects with remotely sensed data from the time of each species record, we were able to map monthly distributions for these nomads over an 11 year period, even in species with only a few sightings.

We found that the distributions of many species expand and contract, and shift around the landscape throughout time, sometimes by one or two orders of magnitude. While many of these species have large ranges overall, at certain points in time (usually during periods of poor environmental conditions) they can be present in only small areas, making them vulnerable to threats in those places, whether through changing grazing regimes, loss of habitat to vegetation clearing or increases in feral predators. Indeed, all the species we examined exhibited significant bottlenecks during some period in terms of low areas of occupation.

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Image: Nomadic Bronze-winged pigeons. Photo: Jeremy Ringma